Tit for tattling: Communication, cooperation, and how each could stabilize the other

– by Victor Vikram Odouard

Driving my Fiat 500 convertible through the Swiss countryside surrounding Geneva, a sign caught my eye. In large hand written letters, it read “CAiSSE” – “CHeCKOUT” in French. Behind the sign, a heap of pumpkins lay on a threadbare canvas blanket. But unlike most checkouts, no one was there. And unlike most self-checkouts, there were no cameras in sight. No, this little Hofladen relied on the honor system.

Why wasn’t there a horde of pumpkin bandits raiding our humble farmstand?

Scientists have proposed many mechanisms to explain the evolution of altruistic behavior, such as kin altruism, punishment, and direct & indirect reciprocity. These all share a common characteristic: they provide a second-order benefit to altruists, that compensates for the initial cost.

Most of these mechanisms require knowledge of who is cooperating (I will sometimes call altruistic behavior cooperation in this post). Sometimes that knowledge can come from observation, as is the case with reciprocity (I know who did me a favor in the past). But other mechanisms, like third party punishment and indirect reciprocity, require that knowledge of who has cooperated spread to those who were unlikely to observe the interactions in question. This requires a faithful communication system (read: gossip!) that accurately transmits the relevant information.

And here we arrive at the question we pose in this paper: why should we even have such a communication system, when it might be in people’s interests to lie?

The question echoes the original cooperation question – “why should people benefit society by acting altruistically when it can come at a personal cost?” Here, we simply replace “acting altruistically” with “communicating truthfully”. The parallel points to a solution – perhaps the same mechanism that keeps people cooperative can also keep people communicative.

In our paper, we analyze a model of indirect reciprocity to determine whether such a mechanism can exist. Imagine an agent, Ariel. She meets someone else, Blake, and she can make a choice: she can cooperate or defect. If Ariel cooperates, she pays a cost in order to produce a benefit for Blake. Let’s say she does cooperate. Afterwards, Blake can “gossip” about Ariel’s behavior, in this case, “Ariel good”, since she cooperated. In the next interaction, when Ariel meets Corina, and Corina is deciding whether to cooperate with Ariel, Corina is aware of Blake’s gossip – in this case, that Ariel cooperated. Blake’s gossiping thus produces Ariel’s reputation. Corina can then, for example, decide to cooperate with Ariel since she cooperated with Blake. This is indirect reciprocity.

The twist in our model is that the indirect reciprocity “twists back” on itself: instead of reputations being based solely on how agents act, they can also be based on how agents gossip. How does this work? Returning to our previous scenario, let’s say Ariel cooperates with Blake, but Blake nonetheless says that “Ariel bad”. Perhaps someone, Diana, saw the whole thing go down, and she communicates “Blake bad”, because he lied. In Blake’s next interaction, with Ed, Ed could therefore decide not to cooperate with Blake.

In this scenario, the machinery being used to maintain cooperation (accurate communication) is being co-opted to maintain itself! In our paper, we ask whether such a self-referential system can actually lead to a stable, high-cooperation state, and we found that yes, it could, in the case where (1) agents made occasional (but not too frequent) errors and (2) the norm punishes defection while nonetheless being self-consistent. Both of these conditions will require further explanation.

The first condition reflects an important fact about many complex systems, which is that heterogeneity can enhance stability: for instance, a virus often has a harder time sweeping through a population with many diverse immune responses to its infection, as it must circumvent many different types of defenses. Taking this one step further, if a population has been infected by many different sorts of viruses in the past, it can mount a diversity of different immune responses to any new pathogen – thus, a diversity of past “disturbances” (viruses, in this case), can make a population more robust by diversifying its immune defenses. This is exactly what we see in our model, where error introduces diversity by causing, for instance, some defections, or some untruthful gossip, where they wouldn’t usually occur. We found that this diversity of small-scale disturbances inoculates the system against more serious destabilizing forces.

As for the second condition, a norm in our setting is a determination of whether a given action is good or bad. A gossip system can encode such a norm: for instance, it might call cooperation and truthfulness “good” and defection and untruthfulness “bad”. Agents can then respond to those judgments: for instance, in one of the above examples, Blake was deemed “bad” for having lied about Ariel, and Diana therefore refused to cooperate with him. But a question arises: does the norm judge Diana’s action, which is in effect enforcing the norm (cooperating with those who followed the truthfulness norm, defecting with those who didn’t), as good? In this simple case, no – defection is considered bad. So Diana is judged as “bad” under the very norm that she is enforcing! This norm is therefore not self-consistent – a norm is self-consistent if it does reward the actions enforcing it. A self-consistent norm instead might deem cooperating with “good” agents good, and defecting with “bad” agents good, and everything else “bad”. This norm, called stern judging, is self-consistent, because it rewards the behaviors that enforce it. For instance, there are some societies that consider theft to be permissible if committed against people in bad standing in the community (for instance, people who unjustifiably stole from someone else). This is a self-consistent norm, as the punishment of individuals in bad standing is permissible under the norm’s own rules.

In our analysis, we find that maintenance of cooperation and communication requires such a self-consistent norm. This highlights that for decentralized norm enforcement, self-consistency is critical. If norm enforcement is centralized, self-consistency need not apply, as different rules may exist for judging the centralized body of enforcers and everyone else. For instance, most people judge killing to be bad, but in some countries, this judgment does not apply to a government doling out the death penalty. However, with decentralized norm enforcement, the people who enforce the norms are the people, and therefore no such distinction can exist. Self-consistency is thus needed.

Our study is fundamentally about whether a self-contained norm-enforcement system, not buttressed by anything external, can maintain high levels of cooperation. The external buttress that provoked our study was communication – most indirect reciprocity models assume an already-truthful communication system. But by noticing here that communicating truthfully is not so different from cooperating (in that both provide a public good), we hypothesized that the same indirect reciprocity mechanism enforcing cooperation could enforce truthful communication, without relying on a preexisting communication. The two conditions under which such a state was stable emphasize other important properties of a self-contained norm enforcement system. First, that it can “create its own diversity”, through error, allowing the system to function without being propped up by some external source of diversity. And second, that the norm is able to “reward its own enforcers”, by being self-consistent – this allows enforcement to occur without requiring a separate body of enforcers, operating under different rules.

Read the original article: Odouard, V.V., & Price, M.H. (2023). Tit for tattling: cooperation, communication, and how each could stabilize the other. Evolution & Human Behavior, 44(4), 359-372.

Objectively Measured Facial Similarity Predicts Ratings of Facial Attractiveness in a Large Speed-Dating Dataset

– By Amy Zhao and Brendan Zietsch

Ariana Grande and Ethan Slater have been making headlines due to their alleged affair, which occurred while both parties were married to their respective partners. Another aspect of their relationship that hasn’t gone unnoticed is Ethan Slater’s striking resemblance to Ariana Grande’s brother, Frankie.

This is just one example of a couple who look like siblings. Many of you may even have your own stories of having mistaken a couple for being siblings, a phenomenon so common that it has led to the creation of a popular Instagram account called Siblings or Dating.

Despite all this anecdotal evidence, there has been mixed empirical evidence as to whether couples actually share facial similarities. But even if we assume that couples do have similar faces, it’s unclear why someone would even date another person who looks similar to themselves in the first instance.

Some researchers have proposed that couples might look similar because couples share the same lifestyles, diet, exercise, stressors, etc. and so, couples’ faces become more and more similar over time. However, there is mixed evidence to support this hypothesis.

Instead of couples’ faces converging over time, an alternative theory is that individuals prefer facially similar partners to begin with; that is, individuals find similar faces attractive. However, there is mixed evidence for this association too. The studies from which these mixed findings have been derived are flawed in several ways. Firstly, these studies have exclusively measured facial attractiveness through participants’ ratings of 2D images. These images consist of photographs of real-life participants, or they are computer generated/digitally morphed faces. There are several implications of using images to obtain attractiveness ratings. Firstly, humans have relied (until most recently) on face-to-face interactions to interact and form judgments about another person, especially another potential mate. And secondly, morphed faces can lack realistic features such as facial hair, skin texture, or head hair. Therefore it is unclear whether past findings can validly reflect real-life behaviours and preferences. In addition, facial traits (such as similarity, averageness, and masculinity) are often subjectively rated by participants. These facial trait ratings may be conflated with ratings of facial attractiveness and can be subject to other biases (e.g. the halo effect).

We conducted a large speed-dating experiment with 682 participants and there were 2285 speed-dating interactions. Participants speed-dated members of the opposite sex for 3 minutes at a time. After these interactions, participants rated their speed-dating partners on facial attractiveness, and kindness and understanding. We landmarked the facial photographs of all participants and calculated objective, data-driven scores for facial averageness, facial similarity, and facial masculinity to address the limitations of subjective ratings of such facial traits.

We found that participants rated partners who had geometrically similar faces to their own as more attractive, showing that individuals do in fact have a preference for facially similar opposite-sex individuals.

The question still remains, why do people even find facially similar people more attractive in the first case?

We found that people with similar faces also rated speed-dating partners as more kind and understanding, even when controlling for the ethnicity of the participant and partner. Our findings are in line with an alternative explanation where a preference for similarity is not driven by the attractiveness of similar faces per se, but that similar faces may signal kinship. While cues of kinship can decrease sexual desirability due to the costs of inbreeding, the same cues can increase positive social behaviour by promoting increased chances of survival via altruistic and cooperative behaviour from others (inclusive fitness benefits). There is also evidence that positive assortative mating (mating with someone who is similar) can result in net positive reproductive benefits, as a small degree of genetic relatedness is associated with higher fertility.

Sure, we have shown that individuals do prefer similar faces, but does this preference extend to real-life mate choice?

We’re not sure. Actual mate choice involves the mutual desire to form a romantic partnership, but these decisions are constrained by the mating market. For instance, individuals with high mate value are unlikely to pair with low mate value individuals. Given that not everyone can satisfy their preferences, individuals must make compromises. In addition, the effects we found in our study are small, and therefore these preferences may not carry through to realised mate choice. It would be a worthwhile future endeavour to investigate how the objective facial characteristics measured in our study relate to real couples.

Read the original article: Zhao, A.A.Z., Harrison, K., Holland, A., Wainwright, H.M., Cecatto, J.-M., Sidari, M.J., Lee, A.J., & Zietsch, B.P. (2023). Objectively measured facial traits predict in-person evaluations of facial attractiveness and prosociality in speed-dating partners. Evolution & Human Behavior, 44(4), 315-323.

The adaptive value of trivial giving: it signals one’s trustworthiness

– by Yuta Kawamura & Misato Inaba

In Japan, there exists a customary practice known as “oseibo”. This is a year-end gift given to a relative, colleague, boss, subordinate, or business partner with whom one shares a somewhat close relationship. In many cases, both parties exchange gifts of similar value that are not excessively costly. Typically, these gifts consist of food items that anyone can purchase (e.g., cooking oil). The exchange of oseibo between two parties rarely occurs just once; instead, it continues for several years or even longer.

Beyond the specific example of oseibo, such social exchanges are common in human society. People sometimes bestow gifts on each other or engage in food sharing with their partners. Nevertheless, these behaviors seem peculiar from the perspectives of direct reciprocity. According to the theory of direct reciprocity, for reciprocal relationship to be maintained, the benefits of cooperation (b) must outweigh the costs paid by the actor (c) (b/c > 1). In other words, social exchange must generate additional benefits. However, the mutual giving described above does not yield any further benefit (b/c = 1): regardless of whether gifts are exchanged or consumed separately, total gain for the two remains constant. This raises the question: why do people continue to exchange such seemingly meaningless gifts?

Our research team investigated the adaptive value of such trivial giving—small-stakes giving that does not yield any further benefit. Our hypothesis was that trivial giving functions as a signal of trustworthiness: that is, trivial giving serves as an indication that the actor is a cooperative person. Consider, for example, facing a natural disaster. To overcome the difficulties, prompt cooperation with neighbors becomes imperative. However, would you trust a neighbor whom you do not know at all? If you regularly show trustworthiness to each other by giving small gifts, you can enhance cooperation more expeditiously.

There is another perspective, however: people may not simply distinguish between trivial and nontrivial giving (i.e., cooperation). Despite receiving trivial gifts, some people might feel gratitude and try to reciprocate. Certain social emotion, such as empathy and gratitude, which may have evolved to facilitate cooperation, may malfunction even in the context of trivial giving.

To examine these competing hypotheses, it will be necessary to assess whether the frequency of trivial giving changes depending on the need to show trustworthiness. If trivial giving functions as a signal of trustworthiness, the behavior should be more likely to occur when there is a need to show that the actor is a cooperative person. We designed a newly modified Prisoner’s Dilemma (PD) experiment. The experiment linked two types of trials: trivial giving trials and cooperation trials. The cooperative trials followed the traditional PD structure. Participants were to decide whether to give 1,000 points to the partner, and if they did, the endowment was multiplied by three (i.e., b/c = 3). In trivial giving trials, however, participants were to decide whether to give relatively small stake (100 points) to the partner or keep it for themselves, and the total payoff remained the same regardless of their choice (i.e., b/c = 1). Participants repeated these trials with their assigned partners. The interaction was over with a one-twentieth probability.

In the real world, a combination of trivial giving and cooperation is commonly observed. Opportunities for trivial giving arise on a daily basis, while unforeseen challenges requiring cooperation to overcome them, such as natural disasters, occur less frequently. To simulate this scenario, the experiment was comprised of a high frequency of trivial giving trials (seven-eighths) and a low frequency of cooperation trials (one-eighths).

Additionally, we manipulated the need to show trustworthiness. In the experimental condition, participants were assigned the same partner for both trivial giving and cooperation trials. In the control condition, participants were assigned a different partner for each type of trial. Thus, while the payoff matrices were identical in both conditions, only in the experimental condition could participants show their trustworthiness in trivial giving trials toward the partner in a future cooperation trial.

The laboratory experiment involving eighty-four Japanese participants revealed that participants are more inclined to engage in trivial giving when they anticipate future opportunities for cooperation with their partners. This result suggests that trivial giving, seemingly meaningless acts, functions as a signal of trustworthiness. However, the experiment was constrained to specific contexts, leaving uncertainty over its applicability in the intricate real world. Considering the context of daily life, people can decide for themselves the magnitude of the signaling costs. Also, in some societies, people have the agency to easily exchange their partners. Further studies should integrate these real-world elements into experiments. Despite these limitations, our findings illuminate the significance of trivial giving, revealing it as a subtle yet powerful signal of trustworthiness that may underpin the fabric of human cooperation.

Read the original article: Kawamura, Y., & Inaba, M. (2023). Trivial giving as a signal of trustworthiness. Evolution & Human Behavior, 44(4), 332-338.

Matching effort to ensure fairness

– by Marcell Székely & John Michael

Imagine that you and your neighbor collaborate to clear away weeds from the common garden where strawberries grow. Assuming you and your neighbor spend about the same amount of time weeding, a reasonable expectation is that you would divide up the strawberries equally.  In contrast, if your neighbor has spent more time weeding than you have, you might think it reasonable that they should receive a greater share of the strawberries. Or if your neighbor spends less time than you weeding, you might think it reasonable to expect more strawberries for yourself.

This everyday example illustrates that we have a sense of fairness, which leads us to divide rewards of joint actions equitably among contributors. Moreover, it shows that this sense of fairness is not just a blind tendency to divide rewards equally but that it takes effort contributions into account. And indeed, recent research has confirmed that our intuitions about this hypothetical scenario are reflected in people’s actual behavior in controlled scenarios. For example, Frohlich and colleagues found that when participants were placed in one room and had to proofread a text to correct spelling errors for joint rewards, they divided their collectively earned rewards in proportional to individual effort costs. Even more strikingly, recent studies in developmental psychology have even shown that three-year-old children take effort costs into account to achieve a fair distribution of rewards. For example, Kanngiesser and Warneken found that three-year-old children kept more stickers after working more than their partner.

But now let’s tweak our hypothetical example: imagine that you and your neighbor partner are tidying up the stairwell together. Notice that in this version of the hypothetical scenario, there is no tangible reward that could be divided up at the end, so fairness cannot be assured through the division of rewards. So what would you do if you perceived that your neighbor was investing a higher level of effort than you? Or a lower level of effort?

In a new article, we describe a series of experiments which we conducted to investigate how people deal with such scenarios. In the experiments, we adapted an effort discounting task developed by Hartmann and colleagues. In the adapted paradigm, people are asked to make decisions about how much effort to invest (namely, by spending time pressing the spacebar repeatedly) for different monetary rewards. This makes it possible to quantify the costs people associate with different levels of effort and to investigate how people trade off those costs against monetary reward.

We turned this paradigm into a social task by letting participants observe their partner performing an effortful cognitive task in order to ‘unlock’ the effort discounting task. Previous research has shown that people spontaneously track others’ ongoing effort investments, and indeed do so accurately. Moreover, we were confident that participants would track the partner’s effort investment using these stimuli because this effort perception manipulation was successfully used in Székely and Michael and Chennells and Michael. Sometimes, participants were rewarded jointly, sometimes they were rewarded separately. This enabled us to test whether participants chose to invest different degrees of effort against monetary reward based on their relationship to the other person. In other words, it enabled us to investigate how social context modulates people’s decisions about how much effort different rewards are ‘worth.’

Our main finding was that people invested more effort when they were rewarded jointly with their partner than when they were rewarded separately. This provides evidence that when rewards cannot be divided to ensure fairness, people instead boost their effort investment as a means of ensuring fairness with a cooperation partner. In other words, the sense of fairness governs not only the distribution of rewards but the allocation of effort in joint actions.

This is important because, while most research on the evolution of the sense of fairness has focused on the division of rewards, a great many cooperative endeavors now, as in deep evolutionary history, do not produce any distributable rewards at all. One reason for this is that the success of cooperative endeavors is always at least to some extent uncertain. Like individual endeavors, they may not succeed because of exogenous factors or lack of competence. In addition, they may not succeed because the collaborative partner may encounter a tempting outside option and so may prematurely withdraw from the joint activity. For example, hunting and foraging in ancestral environments were uncertain endeavors, and they may not have yielded any rewards to distribute at all. Moreover, even if a joint activity yields a reward, this reward may not be divisible. For example, tidying up the stairwell or building a shelter together may not produce any divisible rewards. In such activities, there is nothing to distribute at the end, but it is still possible to distribute the effort costs. This may explain why our participants were willing to invest more effort in a joint action, and thereby to increase the potential rewards for themselves and their partner, when their partners had apparently invested a high degree of effort, even if the partner’s effort investment did not increase the potential rewards.

These findings also open up important new avenues of research on the evolution of cooperation: most of the empirical studies investigating the evolutionary origins and motivational mechanisms to cooperate have focused on the exchange of money, and their results are commonly used to support claims about humans’ willingness and motivation to cooperate more generally. While this focus on monetary rewards is justifiable, people often have to make decisions about how to divide up other resources as well as costs in the context of cooperative endeavors – and had to do so in our deep evolutionary past. It is therefore crucially important to extend the scope of our focus to resources other than monetary rewards. The current findings are an invitation to take up this important challenge.

The findings also provide new impulses for future research about whether and how the sense of fairness may be sensitive to specific features of cooperation partners, such as their competence. While we intentionally bracketed out questions about the relative competence of different actors for our study, parties to cooperative endeavors often contribute with different levels of competence. We look forward to the results of future research investigating how this is reflected in people’s decisions about the distribution of rewards and allocation of effort costs.

Read the original article: Székely, M., & Michael, J. (2023). In it together: evidence of a preference for the fair distribution of effort in joint action. Evolution & Human Behavior, 44(4), 339-348.

back of a woman with her right hand on the back of her neck

Do Body Scents Reveal Women’s Ovulatory Timing?

– by James R. Roney

In many nonhuman mammals, females emit clear cues of time periods when they are able to conceive: from large sexual swellings in some primates, to odors that reveal ovulatory timing. In humans, overt signaling of this kind appears absent, and scientists like Don Symons and Beverly Strassmann theorized that concealment of ovulatory timing may have facilitated the evolution of human pair bonding. Nonetheless, a series of studies have provided evidence that women smell more attractive near ovulation (reviewed here), raising the possibility that although the cues are more subtle than in nonhuman species, perhaps human ovulatory timing is not completely concealed. So, which is it, concealed or not?

In prior research, my lab used signal detection theory to test whether attractiveness shifts in women’s odors were substantial enough for perceivers to accurately diagnose ovulatory timing. The intuition is this: even if odors collected near ovulation smell better than odors collected at other times on average, is there still enough overlap in fertile vs. non-fertile odor attractiveness that perceivers cannot reliably categorize whether an odor sample was produced during the fertile window (the fertile window is defined as cycle days when conception is possible)? Our findings provided evidence for just such a conclusion: even for samples from the same woman, the signal detection analyses showed so much overlap in fertile vs. non-fertile sample ratings that attractiveness ratings could not diagnose fertile window odors with any appreciable accuracy.

A limitation of the above study was that its findings apply only to conscious perceptions of odor attractiveness. But adaptations designed to respond to odor cues of ovulatory timing do not have to produce subjective impressions of odor attractiveness. As long as those mechanisms respond to such cues in functional ways (for instance, by priming sexual desire or courtship efforts), then human ovulatory timing may be effectively unconcealed. And indeed, some prior research had provided evidence that men may respond to women’s ovulatory scents with increases in hormones and sexual thoughts. Perhaps, then, something really subtle is going on: men accurately diagnose ovulatory timing through subconscious hormonal and psychological responses.

We sought to test a replication of such hormonal and psychological responses in a recent study. Women collected both underarm and genital scent samples on 6 nights spaced five days apart over a period of 30 days. We chose 28 women odor donors with confirmed ovulation in their collection cycles and drew samples from three cycle regions from each woman: during the fertile window, before the fertile window (the follicular phase), and after the fertile window (the luteal phase). One hundred eighty-two male raters were randomly assigned to each smell one of the odor samples (or plain water as a control) and each odor sample was smelled by one man. We collected saliva samples before and after the smelling in order to test for reactive changes in men’s testosterone and cortisol, and we also collected a series of post-stimulus survey measures that assessed the priming of sexual concepts, attribution of sexual arousal to the women scent donors, sexual desire, and motivation to approach others.

The statistical tests most directly relevant to the question of concealed ovulation are those that compare responses to the fertile window stimuli to the responses to all other stimuli. None of those tests produced statistically significant results in this study. Thus, we found no evidence that men respond differently to ovulatory scent cues in terms of hormonal or psychological responses. Bayesian follow-up analyses also suggested strong evidence in favor of the null hypothesis for all dependent variables. These findings occurred despite our study generally having greater statistical power than prior studies that reported positive findings.

Our results differed from some reported in prior studies, and so further research on the question of adaptive responses to ovulatory scent cues could provide important additional evidence. Nonetheless, when considered together with our prior work using signal detection analyses to assess cycle phase shifts in scent attractiveness, the findings from this study suggest that body odors do not reveal women’s ovulatory timing.

Of course, scents are just one sensory modality through which ovulatory timing might be perceived. Cycle phase shifts in faces, voices, and behaviors also have some empirical support, although prior studies have not always rigorously determined ovulatory timing in such studies via use of biological markers (such as urinary luteinizing hormone tests, which were used in the research profiled in this blog post). Future research that both precisely determines ovulatory timing and applies signal detection analyses to any observed cycle phase shifts may lead to a more rigorous set of conclusions regarding the concealment of human ovulatory timing.

Read the original article: Roney, J. R., Mei, M., Grillot, R. L., & Emery Thompson, M. (2023). No effects of exposure to women’s fertile window body scents on men’s hormonal and psychological responses. Evolution and Human Behavior, 44(4), 305-314.

Language and Cooperation: It’s Complicated

– by Megan Bishop & Brian Lerch

Large-scale, advanced cooperative action that transverses familial lines is uniquely, exceptionally human. It lies in the foundation of our economic and political infrastructure, our corporate workflow, the small talk we make with the grocery cashier –evidence of our cooperative, pro-social nature is everywhere. Put simply, the evolution of human cooperation has bound us all to each other in a way that far transcends anything seen elsewhere in the animal kingdom. Such an incredible phenomenon begs a multitude of questions, but upon beginning the work leading up to my recent publication, The influence of language upon the evolution of cooperation, I became particularly enthralled by one: how?

Pursuing this broad question led me to another uniquely, exceptionally human development: advanced spoken language proficiency. The evolution of advanced human cooperation and language have been argued to have a reciprocal relationship, where the evolution of one drives the evolution of the other and vice versa. While the existence of large-scale cooperation assumes an ability to coordinate via complex language systems, few mathematical models examined the ways that language promotes cooperative action, despite the utility of mathematical models for analyzing how evolutionary mechanisms align with argued verbal logic.

Human cooperation has important roots in collaborative foraging, which is well modeled by a stag hunt game. Here, a group of early human foragers are presented with two options: individually hunt a hare with a guaranteed fitness benefit or risk the opportunity cost of the hare to collaborate with others in pursuit of hunting a stag. Upon the successful hunt of the stag, the fitness payoff would be evenly split amongst the group, with a much higher payoff than the hare. However, this depends on successful cooperation. Without successful cooperation, the stag hunt fails, leaving cooperators without the fitness benefit from the hare or the stag. Given the important evolutionary origins of collective human foraging, we build upon an existing stag-hunt model and include language in three various mechanisms to analyze its implications on early human cooperation.

First, we analyze an “enhancement” mechanism of language, where we assume better language systems allow for more complex coordination among cooperators, allowing them to pursue larger game with a greater fitness payoff. Here, our findings align with verbal arguments: more proficient language always makes conditions more favorable for cooperation. In our model, cooperation steadily evolves when language proficiency is better. However, this is limited by the proportion of cooperators in the population. Regardless of language proficiency, language cannot promote the evolution of cooperation with too few cooperators present.

Next, we studied an “assortment” mechanism of language, where we assume better language systems allow cooperators to communicate with, and therefore identify, other cooperators. In other words, with better language proficiencies, cooperators are more likely to join groups with other cooperators. Like our “enhancement” mechanism, our findings align with verbal arguments that better languages always favor the evolution of cooperation when enough cooperators are present in the population. However, this “assortment” mechanism is especially beneficial for cooperation when there is a high frequency of cooperators, leading to larger benefits of cooperation than is seen in the “enhancement” mechanism.

Finally, and most interestingly, is our “threshold” mechanism. Here, we assume better language proficiencies allow humans to achieve successful cooperation with fewer cooperators participating. This mechanism challenges the idea that better language proficiencies always favor the evolution of cooperation. We find that while better languages allow cooperation to evolve in populations with fewer cooperators, the overall fitness of individual cooperators decreases with better language. Upon reflection, this makes sense. Even when a smaller group can establish advanced complex coordination with improved communication, more defectors are able to “free-ride” off of the cooperators’ payoffs.

So, I answer my question of how? with an answer that frustrates answer-seeking scientists like myself, yet is universally true about our world: it’s complicated. It is likely a combination of these mechanisms that has influenced the evolution of human cooperation. For instance, perhaps the benefits of our assortment mechanism were slowed by the effects of our “threshold” mechanism. However, certain environments might not allow for the existence of one or more mechanisms, such as an environment with tightly constrained group membership would not enable positive assortment. The nuance between language and cooperation depends greatly on the mechanism of language. This model pokes and probes at our existing understanding and how language influences cooperation, unveiling that there is greater nuance than what is commonly assumed. My hope is that by challenging such a long-standing assumption, the scientific community continues to grow in our study and understanding of the fascinating, unique phenomenon we call human cooperation.

Read the original article: Bishop, M.E., & Lerch, B.A. (2023). The influence of language on the evolution of cooperation. Evolution & Human Behavior, 44(4), 349-358.

Putting the “Natural” in Natural Law

– by Carlton Patrick

When Clarence Thomas was nominated to serve on the United States Supreme Court, it was Joe Biden, then a Senator from Delaware, who chaired the Senate Judiciary Committee and oversaw the confirmation process. During the hearing, Biden questioned Thomas at length on his views on natural law, saying that while the discussion might “confound the people…whether someone applies natural law is of phenomenal significance, and there is not a single legal scholar you can question who would not tell you that is not true.”

To understand why Senator Biden was so invested in this line of questioning, it is helpful to consider the category of legal disputes that the philosopher Ronald Dworkin calls hard cases. These are instances where a judge must decide a novel controversy that isn’t determined by an existing statute or case law and so the judge must therefore make new law herself. There are countless variations of what principle the judge could use to decide the case. She could decide the case in a way that produces the most equitable or egalitarian result. She could make law that would maximize economic efficiency. Or she could make law that would emphasize conflict reduction, or safety maximization, for example.

But adherents to the natural law philosophy—a list that not only includes Justice Thomas, but also Aristotle, St. Thomas Aquinas, Hugo Grotius, and John Locke—posit that a true, objective set of moral principles exists out in the universe and that laws are only valid to the extent that they conform to this set of principles. Thus, in the face of a hard case, the judge cannot choose among orthogonal principles, and instead must make the positive (or man-made) law accord with the objective natural law that wafts through the ether. Otherwise, the law is simply not valid.

Where do we look to determine these objective principles of natural law? For a long time the dominant theory was that these principles emanate from God(s). Modern defenders of the natural law approach point to a variety sources including Gods, the universe, moral truths, and other metaphysical sources. Regardless of the source, though, most natural law proponents agree that there are certain universal values that are revealed to us through instinct and intuition, and that, as the legal philosopher Jonthan Crowe phrases it “we are characteristically disposed to pursue…and regard them as normatively worthwhile.” Justice Thomas phrases it this way: “the thesis of natural law is that human nature provides the key to how men ought to live their lives.”

What I argue in my article Evolution is the Source, and the Undoing, of Natural Law, is that there is danger in this sort of thinking. Rather than automatically deferring to our gut intuitions, we should investigate where they come from, and use that as the jumping off point for policy discussions about how to make law.

The natural law philosophy derives much of its momentum from the fact that our moral intuitions seem immutable. They are often widely shared, instinctual, robust to temporal and cultural differences. They emerge early in childhood, and are impervious to counter-argument. And it is true that legal codes from vastly different cultures and from different millennia have common features, lending credence to  Cicero’s assertion that the law is “not one thing at Rome, and another thing at Athens” but is instead “universal…unchanging and everlasting.”

But these phenomena can be explained without resorting to superstition. Thirty years of empirically validated evolutionary studies have provided us with a scientific framework for explaining that the reason we share so many moral intuitions, that they show up early in development, and that take on a deontological force is that they are part of our species-wide psychological architecture. They helped to navigate the conflicts inherent in the highly social world of our ancestors where censuring certain actions while praising others would have had significant social consequences. For example, our moral conscience—the intuition that personally taking an action would be the “right” or “wrong” thing to do—helps us accrue praise and reputational clout while avoiding condemnation and punishment. Likewise, our intuitions of justice facilitate the proactive condemnation (and reactive punishment) of certain behaviors by marking those behaviors as ‘wrong’ and attaching an intuitive degree of severity to their wrongness.

Cast in this light, the rubber-stamping of intuition into law urged by the natural law philosophy begins to take on a different view, and many will immediately recognize this as a version of the naturalistic fallacy. By automatically incorporating our moral intuitions into law, we are saying that simply because something evolved, it is per se good, and per se worth institutionalizing. But these intuitions—like all of our intuitions—exist not because they were passed down from a divine arbiter of truth, but instead because they likely granted some fitness advantage to their holder. And, as a result, these intuitions—like all of our intuitions—have the potential to lead us in a wayward direction.

Just as our folk intuitions of physics, economics, or probability can lead us to irrational, inaccurate, or counter-productive conclusions, our moral intuitions can be similarly flawed. They can be selfish, nepotistic, myopic, and capricious. We think the same action is more wrong when it’s done by someone outside of our political group, and less wrong when it’s one of our comrades. We are mortified at the thought of our neighbor’s child dying of hunger but are unmoved when it routinely happens to children living in a Cambodian slum. We flip-flop our moral positions when there is money or reputational benefit associated with doing so. In other words, when consistency in moral judgments runs up against the self-interest of the person making the moral judgment, it is self-interest that often prevails.

In evolutionary terms, these quirks of our moral intuitions are features, not bugs. They would have likely been quite advantageous in terms of navigating the types of recurrent conflicts of interests our ancestors faced. There are no doubt evolutionary advantages to favoring our political allies and our neighbors, or to endorsing different moral positions when doing so would result in a greater economic or coalitional position for us. But, I argue, the fact that our moral intuitions are skewed in these directions is reason enough to give us pause, and to require an argument why (as opposed to a presumption that) they should be used in lawmaking.

Read the original article: Patrick, C. (2023). Evolution is the source, and the undoing, of natural law. Evolution & Human Behavior, 44(3), 175-183.

This article is part of a special issue of Evolution & Human Behavior on Evolution, Justice and Law, edited by Debra Lieberman & Keelah Williams.

Stereotypes of criminality in the U.S. track ecology, not race – part of EHB Special Issue on evolution, justice and the law

– by Keelah Williams

In the United States, Americans stereotype Black individuals as being more likely to commit certain types of crime than White individuals. But is this really about race alone? Or might people sometimes use race to infer the kinds of environments others might inhabit and the behavioral predispositions those environments might incentivize?

In a paper recently published in Evolution and Human Behavior, I argue that beliefs about who is (or is not) likely to commit particular crimes are actually driven by inferences about the kinds of environments (“ecologies”) people come from. People from resource-poor and unpredictable ecologies are stereotyped as engaging in more criminal behavior than people from resource-rich and predictable ecologies. And because the legacy of racism means that race and ecology are intertwined in the U.S., American perceivers use race as a proxy for ecology: They tend to infer that Black people are more likely to dwell in “desperate” ecologies, and White people are more likely to dwell in “hopeful” ecologies.

In a series of three studies, I show that—in the absence of information about the ecology of the target individual—American perceivers stereotype Black targets as more likely to commit crimes such as drug possession, resisting arrest, and vehicle theft than are White targets. But when perceivers are provided with both race and ecology information about the targets, perceivers’ stereotypes track ecology but not race: White and Black targets from resource-poor and unpredictable environments are stereotyped as equally likely to commit those crimes. And, White and Black targets from resource-rich and predictable environments are stereotyped as equally unlikely to commit those crimes. These findings suggest that beliefs about others’ criminal propensities may not be driven solely by race per se but may instead reflect inferences about the behavioral disposition of people who come from hopeful or desperate ecologies.

These findings have practical implications for reducing racial bias in the criminal justice system. Previous work suggests that Black criminal offenders receive harsher punishments for the same crimes because they are seen as more dangerous and likely to reoffend than White criminal offenders. But if people’s beliefs about whether someone is predisposed to commit different crimes are driven by ecology rather than race, this suggests that race differences in legal outcomes could be reduced to the extent that targets are described as hailing from similar ecologies. (The appropriateness of using ecology information when making legal decisions is a separate question, however—especially because many cues to ecology may be outside of the person’s control.)

This paper was published as part of a Special Issue on Evolution, Justice, and the Law that I edited with Debra Lieberman. Evolutionary social science and the law are natural complements: The former seeks to understand behavior while the latter seeks to regulate behavior. As the articles in the Special Issue demonstrate, an evolutionary approach has immense value in the legal sphere because it has explanatory power, generates novel predictions for legally relevant behavior, and provides insights that inform normative debates in the law.

The articles in the Special Issue are grouped into five broad themes. The first theme—including papers by Carlton Patrick, Daniel Sznycer, Peter DeScioli, and Aaron Sell—considers the origins of moral judgments. Is there an objective set of moral principles independent of law? How do intuitions about justice arise? And should such universal intuitions guide lawmaking?

The second theme—including papers by Sarah Brosnan, Owen Jones, and Jeffrey Stake—addresses the topics of ownership and fairness. How do adaptations generate decisions to retain or relinquish material goods? What can the non-human primate literature tell us about the modern human sense of fairness and justice?

The third theme—including papers by Cass Sunstein and Keelah Williams—explores contextual factors that influence legal judgments. How does the prevalence of societal problems affect our categorization of what is problematic in the first place? And, as the beginning of this blog addresses, what best explains people’s propensity to infer criminality—ecology or race?

The fourth theme—including papers by Michael Guttentag and Douglas Yarn—examines the extent to which law influences cooperative behavior. How are laws regarding resource distribution influenced by our evolved dispositions? When might extralegal channels of dispute resolution (e.g., dueling) promote cooperative relationships more than adjudication?

Finally, the fifth theme—including papers by David Buss, Kingsley Browne, and Martin Daly—touches on matters of sex and death. How do evolved sex differences affect perceptions of what is legally objectionable and the existence of disparities in the workplace? What role does inequality play in homicide rates? How can an evolutionary perspective help solve societal problems on a macro scale?

Altogether, the Special Issue offers a variety of papers that explore how our evolved psychology generates intuitions, preferences, and behaviors in the legal sphere. Debra and I hope you enjoy this Special Issue representing a burgeoning research program at the intersection of evolutionary science and law.

Read the original article here: Williams, K. (2023). Stereotypes of criminality in the U.S. track ecology, not race. Evolution & Human Behavior, 44(3), 255-263.

This is part of a special issue of Evolution & Human Behavior on Evolution, Justice & Law, edited by Keelah Williams & Debra Lieberman

On the origin of laws by natural selection

– by Peter DeScioli

Why do humans make so many laws? In a new article, I argue that the human ability to make laws evolved by natural selection. I hope to entice you with a few snapshots.

Laws may seem unlikely to come from evolution. There are so many laws, and they differ so much across societies. This variation shows that natural selection did not install a single code of laws in the human mind. We do not have ten commandments, or five or twenty, etched into our minds, or else we would see the same code of laws repeated in society after society.”

But does this mean that human evolution has little to tell us about the origin of laws? Not at all. To see why, just compare laws to tools. Humans make countless tools, and tools vary tremendously across societies. Yet, it is well-understood that humans evolved adaptations to make and use tools. The human mind does not have a fixed set of blueprints for ten or twenty tools. Rather, natural selection installed in the mind principles of physics and engineering that humans use to make new tools for new purposes. The same is true of laws: Humans evolved a creative power to make limitless laws by applying a stock of core principles. Humans are lawmakers like we are toolmakers.

With this in mind, we can dissect a sample of laws to find the common parts that humans use to make them. The parts of laws are ideas that we compose into messages, such as Thou shalt not kill, and Whosoever lieth with a beast shall surely be put to death (Exodus, 22:19). Some of the ideas that come up repeatedly include indefinite people such as thou and whosoever, actions such as kill, steal, and lie with, intentions such as wanting and planning, as well as truth, impartiality, possibility, necessity, punishment, and proportion.

Having laid out the parts of laws, we can then inspect them for clues about their evolutionary purpose. What is all this lawmaking for? How do humans use laws in evolutionary competition?

We first consider an apparent likeness between laws and threats. Threats are well-understood signals in games of conflict, and like laws, threats warn of punishment that is conditional on some event, such as If you come any closer, then I will punch you. However, on close inspection, we find that threats and laws differ in each of their parts. For instance, threats address a definite person while laws address an indefinite person, and threats are enforced by the threatener while laws are enforced by the whole community. Given these and other differences, we conclude that laws are not the same as threats, and thus cannot be explained by theories of threats.

Next, we compare laws to moral rules and find a close match:

Like laws, moral rules apply to everyone, they set a condition that is a specific action and requires evidence, they concern an infinite variety of actions, they are enforced by everyone with blame and punishment, and the punishment is proportionate. Moral rules match laws part for part. (DeScioli, 2023, p. 199)

Given this correspondence, we conclude that laws are produced by moral judgment. Moreover, I have previously argued that moral judgment evolved as a strategy for choosing sides in conflicts. Humans use the moral strategy to avoid some of the dangers of choosing sides by alliances and hierarchies. In the moral strategy, people choose sides by rules of action, like opposing a thief according to the rule, Thou shalt not steal, rather than choosing sides by alliances or hierarchy, such as supporting an ally or superior who stole someone’s goods. And since moral judgment is the source of laws, the parts of laws are also patterned by the same function: choosing sides in conflicts while avoiding the dangers of alliances and hierarchy.

Hence, we have found the origin of laws in our strategies for choosing sides. But we are not finished. The ability to make laws creates new games and spurs a multitude of strategies and counterstrategies:

The evolution of laws is not the end of the game but only the beginning. Once humans can compose laws to settle conflicts, the next problem is that they can make too many…. As prolific lawmakers, humans can easily produce a multiplicity of laws too numerous to remember and too discordant to apply. (p. 202)

To prevent a chaotic proliferation of laws, humans evolved the ability to debate which laws to accept into the community’s code and which to reject. Moreover, each person judges potential laws according to their own interests, so people inevitably disagree on many laws, giving rise to a battle to control the laws.

When many people disagree on a law, it is unstable and varies over time and across societies.… Locked in disagreement, the opponents struggle in a tug of war to control the law. (p. 202)

Last, I illustrate the competition to control the laws by examining common skirmishes over laws of violence, property, sex, factions, and power. In each case, we see how opponents struggle to enact opposite laws, meaning laws that would punish opposite sides in a conflict. We also encounter what I call paradoxical laws, such as Thou shalt not betray our faction. Paradoxical laws mimic the characteristic form of laws despite the fact that they undermine the rule of law in general, replacing it with the rule of faction or hierarchy.

The struggle to control the laws also damages science, because committed partisans condemn facts and ideas that could weaken the case for the laws they favor. Indeed, this may be why “the science of human evolution itself has long been accused of subverting the foundations of morality and society” (p. 207). In closing, I offer a defense against this charge:

In defense, it may help to point out that human evolution does not affirm a single code of laws but rather explains why humans make a great variety of laws and then fight to control them. (p. 207)

Read the original article here: DeScioli, P. (2023). On the origins of laws by natural selection. Evolution and Human Behavior, 44(3), 195-209.

This article is part of a special issue of Evolution & Human Behavior on Evolution, Justice and Law, edited by Debra Lieberman & Keelah Williams

What’s law got to do with it?

– by Michael Guttentag

Law has played a pivotal role in the emergence of human sociality. If this conjecture is correct many questions naturally follow. When and why did the turn to law emerge? How could law have played a role in the distant past if powerful sovereigns are a comparatively recent development? What distinctive contributions does the legal system make to the maintenance of social order?

In Evolutionary psychology and resource-sharing laws I explore these and other questions by considering laws that serve a particular purpose – sharing resources. Resource-sharing laws are laws that encourage cooperation and discourage competition.  One simple example of a resource-sharing law is a law that limits private property rights, because such a law will reduce the incentive to compete rather than cooperate for scarce resources.

Concepts of law

A first step in addressing questions about law and the emergence of human sociality is to note that there are a multitude of ideas about what law is. One concept of law views the law as the commands issued by a sovereign and backed by a credible threat of punishment. This concept of law was popularized by John Austin in the Nineteenth Century, and suggests that law is a relatively modern affair, contingent on the existence of a sovereign powerful enough to command obedience (Austin, 1863, The province of jurisprudence and uses of the study of jurisprudence).

An alternative conceptualization of what law is comes from H. L. A. Hart and his 1960 masterwork “The Concept of Law” (Hart, 1960, The concept of law). In Hart’s account law’s uniqueness comes from the combination of two distinctive features: normativity and a two-tiered rule structure. According to Hart, unlike commands obeyed under threat of force, laws are obeyed because people to at least some degree accept the law as describing how people ought to behave. This is the normative aspect of law.

The second defining feature of law according to Hart is the way legal systems are constructed. Unlike other normative systems, the law contains both rules and rules about rules, or, in Hart’s terminology, both primary rules and secondary rules. Primary rules delineate when people “are required to do or abstain from certain actions, whether they wish to or not,” and include “restrictions on the free use of violence, theft, and deception,” and “various positive duties to perform services or make contributions to the common life.” Secondary rules describe how to “introduce new rules of the primary type, extinguish or modify old ones, or in various ways determine their incidence or control their operations.”

The combination of primary and secondary rules, law’s two-tiered structure, creates a generative system for establishing normative obligations much as language provides a nuanced, flexible, and generative way for humans to communicate. The possibility that law has played a pivotal role in the emergence of human sociality makes sense if one begins from Hart’s view as to what makes law a distinctive normative system.

Outlawing polygyny

One historically important resource-sharing law is the polygyny prohibition. Studying this prohibition highlights several ways laws contribute to the maintenance of a cooperative society. The polygyny prohibition is typical of a resource-sharing law in that it works by reducing the gains that can be realized by engaging in competitive behavior, in this case reducing the benefits granted to those who secure additional mating partners. Laws prohibiting polygyny are among the first recorded laws, dating back to ancient Athens, and illustrate several of the benefits of using a legal system to share resources. First, the public process of defining that a rule is a law makes reversing course more difficult. In this way the law can make commitments to arrangements that discourage competition such as the polygyny prohibition more credible. Second, the presence of a law in the historical record provides a meme that is easy to replicate. The family law provisions adopted by Solon around 600 B.C.E. have provided a template for millennia of one way a society can modify marital practices to reduce competition for mates.

Origins of a law instinct or norm psychology

The use of legal rules to facilitate resource sharing can also provide insight into how an ability to use this kind of generative normative system (a law instinct or norm psychology) may have emerged. Big game hunting, a formative practice in shaping human behavior, presents the kinds of resource-sharing opportunities and challenges that laws designed to share resources are particularly well-attuned to address. Successful big game hunting requires balancing individual and cooperative interests. A legal regime limiting the private property claims of individual hunters could encourage cooperation and might be quite helpful in this context. Moreover, our ancestors’ ability to “solve” the meat-sharing problem by defining what constitutes communal property and what constitutes private property might have provided a pathway to develop a broader tendency to use a generative normative system to “solve” resource-sharing problems.

Divvying up transaction surplus

Another insight from exploring resource-sharing laws as a window into law’s roles in the maintenance of a cooperative society is that this line of inquiry helps to explain aspects of the nuanced relationship between prosociality and transactions with strangers in market transactions. There are well-established competing hypotheses about the link between participation in market transactions and prosociality. As Albert Hirschman observed, on the one hand, there is the “doux commerce” thesis that commerce is “a civilizing agent of considerable power and range.” On the other hand, there are concerns that “capitalist society … exhibits a pronounced proclivity toward undermining the moral foundations of any society” (Hirschman, 1982, Rival interpretations of market society: Civilizing, destructive, or feeble?)

Largely absent from this debate is the question of how parties to market transactions divvy up any gains from trade. Gains from trade present the kinds of resource-sharing challenges that resource-sharing laws are designed to address. This insight generates testable hypotheses. The ability to successfully divvy up transaction surplus might moderate the relationship between participating in market transactions and prosociality. If this is correct, then the relationship between prosocial attitudes and participation in market transactions should vary depending on market structure. Where there is one price at which all transactions take place, there is less of a need for cooperation because the market price provides a ready-made mechanism with which to allocate transaction surplus. In contrast, in a market characterized by a series of independent bilateral negotiations the challenge of sharing transaction surplus in a fair and efficient manner needs to be addressed each time a transaction is consummated. As a result, at least some amount of prosociality may be helpful or even generated when participating in a bilateral negotiation.

Work by Armin Falk and Nora Szech shows how an experiment could shed light on this hypothesis by exploring whether market structure moderates the relationship between prosociality and participation in market transactions. Falk and Szech did not find a statistically significant difference on their measure of moral values between treatments in which transactions were executed by bilateral trade and in which transactions took place at a pre-set price. Their work does, however, suggest ways to further explore the hypothesis that divvying up transaction surplus explains at least some part of the nuanced relationship between prosociality and participating in market transactions.

These ideas and others in the article Evolutionary psychology and resource-sharing laws show how considering resource-sharing laws in the context of evolutionary psychology generates a variety of insights as to how and why law may have played a pivotal role in the emergence of human sociality.

Read the article: Guttentag, M. (2023). Evolutionary psychology and resource-sharing laws. Evolution and Human Behavior, 44(3), 264-271.

This article is part of a special issue of Evolution & Human Behavior on Evolution, Justice, and the Law, edited by Debra Lieberman & Keelah Williams.