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Are parents naturally biased towards their sons or daughters, depending on their conditions?

– by Valentin Thouzeau

According to Robert Trivers and Dan Willard’s hypothesis, in many species, parents in good condition should favour male offspring, while parents in poor condition should favour female offspring. Why? First, parents with more resources can support more offspring. Second, in polygynous species, males with more resources are more likely to have many offspring. Natural selection should therefore favour investment in male offspring when parents are in good condition, since their sons will have a chance to have many children in turn. Conversely, natural selection should favour investment in female offspring when parents are in poorer condition, since their daughters have a high chance of having children even if they do not have many resources (see Figure 1 for a schematic representation of this hypothesis). This prediction was tested in a numerous non-human species. For instance, in ungulates, the results showed that when a female’s partner can invest more resources in her offspring, she does indeed produce more sons.

Figure 1 – a) Example of a group in which males with many resources (represented by large circles) are likely to have more offspring than females with many resources, since they can have children with multiple females. b) Representation of the optimal choice in terms of investment by parents in a population represented in a. Parents with few resources should favour their female offspring, while parents with many resources should favour males. c) Prediction of the Trivers-Willard hypothesis arising from the choice in b. The sex ratio of offspring from parents with few resources should favour females, while it should favour males for parents with many resources.

But can this prediction be applied to humans? We often think we are above biology – yet some results seem to contradict this: one study showed that American millionaires have an average of 60% sons and 40% daughters! Does this prediction apply to how we treat our children? In another U.S. study, no difference was found between parents based on socioeconomic status in the amount of time they spend with their sons or daughters. So the results are uncertain.

At present, there are literally hundreds of studies testing the hypothesis in humans, and it is difficult to get a comprehensive view. Also, can we trust the proportion of results that support the Trivers-Willard hypothesis? It may be that researchers were more inclined to publish results that supported the hypothesis rather than those that invalidated it, which may have led to publication bias. We were surprised to find that results from studies involving many groups of animals were synthesised (this is the case, for example, for ungulates and non-human primates), but no synthesis of the work done in the human species had been undertaken until now. This is why we started this work.

We collected 87 studies reporting a total of 821 tests of the Trivers-Willard hypothesis. Although the majority of the samples were based in North America, the geographic coverage was considerable. The analysis of all these studies reveals that the results are largely compatible with the Trivers-Willard hypothesis and that there is no publication bias in this scientific literature that could alone explain the compatibility of the results with this hypothesis.

We then tested whether the hypothesis held true in the birth bias of boys and girls, in the investment bias that boys and girls receive after birth, or in both. We found that tests for both versions are equally prevalent in the scientific literature. However, the results indicate that birth bias is better supported than investment bias. Putting together the last 50 years of research on the Trivers-Willard hypothesis thus allows us to conclude that parents in good condition have, on average, more sons, while parents in poor condition have more daughters.

We have to keep in mind that even if the Trivers-Willard hypothesis is validated for the birth bias, this bias is very small (the correlation coefficient is 0.037). There is simply a very small additional probability of giving birth to sons when conditions are favourable, and to daughters when conditions are less favourable. Nevertheless, these results show how the theory of evolution leads to surprising predictions that allow us to discover unsuspected phenomen.

Read the original paper: Thouzeau, V., Bollée, J., Cristia, A., & Chevallier, C. (2023). Decades of Trivers-Willard research on humans: What conclusions can be drawn? Evolution and Human Behavior, 44(4), 324-331.

The reputational costs of retaliation: Why withdrawing cooperation is better than punishing a non-reciprocator

– by Sakura Arai

Two-person cooperation is ubiquitous in human society. You and your roommate take turns cooking dinner. You water your neighbors’ garden while they are on vacation, and in return, they feed your pet while you are away. But what if your partner fails to return the favor?

Inflicting a cost on those who fail to reciprocate—punishment—is one solution. Think of fines for littering, parking violations, or overdue books. When three or more people cooperate to achieve a common goal and share the resulting benefits (such as clean streets and libraries), punishment does sustain cooperation. But punishing can be counterproductive when two people are trading favors—especially in a biological market where there is competition for good cooperative partners.

Imagine how you could “punish” your partner. You could serve spoiled food to the roommate who keeps “forgetting” to make dinner, or salt your neighbors’ garden when they fail to feed your pet. But would these malicious actions change their mind and encourage them to start reciprocating again? To make matters worse, other people may think that you are a bad cooperator—and vengeful too. They may not want you as a cooperative partner.

There is an alternative to punishing: You can simply withdraw cooperation from your non-reciprocating partner. This communicates the same point—you don’t like the way you were treated. Plus, withdrawing may save your face. It’s possible that their excuses are true and they actually couldn’t return the favor due to injury, mistakes, or bad luck. By conveying the message without directly harming your partner, you may appear forgiving and even considerate.

In group cooperation, withdrawing cooperation has disadvantages that do not exist in two-person cooperation. Withdrawing cooperation from a free rider simultaneously penalizes members of the group who are good contributors. It may also entail abandoning the entire group project and the benefits that come with it. Neither is the case in two-person cooperation, especially when alternative partners are available.

In two-person cooperation, a non-reciprocator can be sanctioned by withdrawing cooperation or by punishing. And your reputation may suffer if you continue to do favors for partners who do not reciprocate. You may appear to be a pushover and easy to take advantage of. Then your partner will certainly keep exploiting you, and so might other people. In terms of reputation, you may be better off sanctioning than just continuing to help.

So, for your reputation, what should you do when your partner fails to reciprocate? We asked over 400 US residents, as a third-party observer, what they would think of someone who took one of the three responses: punish, withdraw cooperation, or neither (keep cooperating).

Here’s a short scenario we presented to participants: Imagine two people, Alex and Casey, interacting with each other through an economic game. There are two roles in this game: giver and receiver. The giver is given $5 and then decides either to share $5 with the receiver or take $5 from the receiver. We told them that Alex and Casey played this game for 3 rounds (without knowing how many rounds there would be). In round 1, Alex was the giver and Casey was the receiver; Alex gave Casey $5. In round 2, Casey became the giver and gave Alex $0. Namely, Alex cooperated with Casey in round 1, but Casey did not reciprocate in round 2.

In round 3, Alex became the giver again. Participants learned that Alex made one of three responses:

  • Punish: Alex took $5 from Casey
  • Withdraw cooperation: Alex gave $0 to Casey
  • No sanction (keep cooperating): Alex gave $5 to Casey.

We then asked participants to rate Alex on 24 adjectives: cooperative, generous, aggressive, vengeful, incompetent, gullible, etc. The goal was to see what reputations (plural intended) they inferred from each response. Did people see Alex as mean and vengeful? How generous and trustworthy did Alex appear? Did she seem gullible or exploitable?

Withdrawing cooperation always had better reputational consequences than punishing. When punishing Casey’s failure to reciprocate, Alex was evaluated as less cooperative—an average of related adjectives such as generous, trustworthy, likable, considerate—and more vengeful—mean, aggressive, unforgiving—than when withdrawing. Moreover, people found the punisher less preferable as a potential cooperation partner than the withdrawer.

What inferences did people make when Alex did not sanction Casey at all?  Participants thought she was highly cooperative and desirable as a partner. But she was also seen as easier to exploit—more exploitable, gullible, incompetent—than when she withdrew cooperation or engaged in restorative punishment (thus recouping her investment in Casey). These two negative sanctions were equally effective ways for Alex to enhance her reputation as difficult to exploit.

Restorative and costly punishment were different, however. In a follow-up study, punishment was costly: To inflict a $5 cost on Casey, Alex had to pay $5. This made her seem as exploitable as the non-sanctioner. Both lost an extra $5: The punisher paid to retaliate and the non-sanctioner paid to keep helping Casey, a partner who took without giving in return. People inferred Alex was easier to take advantage of in both cases, compared to simply withdrawing further cooperation.

A reputation as more difficult to exploit may prevent others from mistreating you. But this reputation can be gained by withdrawing cooperation or restorative punishment: negative sanctions that do not entail extra costs for you. And both kinds of punishment—restorative and costly—produce reputational costs: Compared to withdrawing cooperation, punishing makes you appear more vengeful, less cooperative, and less desirable as a partner. So, in two-person cooperation, withdrawing cooperation may be the best option when your partner fails to return the favor. Our studies show that investigating reputations—multiple aspects of reputation—can shed new light on the functions of motivations to sanction those who don’t give back.

Read the article here: Arai, S., Tooby, J., & Cosmides, L. (2023). Why punish cheaters? Those who withdraw cooperation enjoy better reputations than punishers, but both are viewed as difficult to exploit. Evolution and Human Behavior, 44(1), 50-59.

Call for submissions: 2nd Biennial Heterodoxy in Psychology Conference at Chapman University

The Heterodoxy in Psychology Conference provides an interactive forum for the exchange of research and ideas that are ideologically and intellectually heterodox in the science, teaching, and practice of psychology and closely allied disciplines.

Conference information:

Dates: January 9-12, 2020

Location: Chapman University & Doubletree Hotel, Orange, CA

Abstract Submissions: Due by September 30th

Competitive student travel grants are also available!


For more details about the conference, including information about submitting paper or poster proposals: