– by Rob Brooks, Khandis Blake & Lutz Fromhage

Choosing a mate involves a vast number of considerations. Personalities, looks, kindness, and attentiveness all come into play. So do status, wealth and material resources because mating partnerships are – at least in part – economic arrangements.

A couple shares much of their time, labour, and wealth, often investing in their joint social mobility and in raising children. Wherever and whenever money and status matter in heterosexual mating decisions, it is far more common for women to partner upward (a pattern called hypergyny) than it is for men. Across cultures and eras, hypergyny is common but its opposite (hyperandry) is vanishingly rare.

If economic considerations influence mating decisions, then it seems reasonable to expect the distribution of wealth to have important effects on mating patterns. For example, gaps in average income or wealth between women and men will likely alter the number of suitable partners a person might encounter. When women have few means of their own, then a majority of men could provide at least some upward mobility to any one of a majority of women.

That’s an argument we have encountered in verbal form, but when we wanted to make the argument in our work on the mating market consequences of inequality, we found a surprising lack of modelling. Moreover, thinking about the effects of income inequality within the sexes was even more sparse. If economic conditions like gender- and income inequality affect mating prospects, then that’s one more reason for them to also affect individual well-being, happiness, and political attitudes. So, in a recent Evolution and Human Behaviour paper, we modelled how inequalities between women and men, and inequalities within the sexes, might influence fitness prospects under hypergyny.

The model

We created a population of individuals (a million of each sex), assigning them ‘wealth’ according to specified sex-dependent distributions. Their ‘status’ is their rank-order in wealth within their sex, so we can compare what happens under various wealth distributions. Individuals go through a series of one-to-one encounters with random unpaired members of the opposite sex. If, in an encounter, the two individuals’ preference rules are met, they form a pair and don’t participate in any further encounters. The result that interested us was how sex and status influenced the chances of an individual pairing up within 100 encounters.

The simplest preference rule – Choice Rule 1 – considered an encounter a match if the man’s wealth was the same as or greater than the woman’s. Some might find men’s lack of agency and women’s hard-edged preferences somewhat affronting, so we tried a few other rules. Rule 2 only allowed a match if the man’s wealth exceeded the women’s but by no more than a specified ‘cap’. Rule 3 added a second trait (which we imaginatively called ‘X’) that men cared about in that they only paired with a woman whose X exceeded their own. And Rule 4 gave women and men both preferences for wealth and ‘X’. Rules 2 to 4 made the results of our modelling a little less stark, as one might expect, but not fundamentally different, so our paper focuses really on the original preference rule 1.

Inequality between the sexes

If the wealth distribution for women doesn’t differ from that for men, the result is simple and easily anticipated. Men of above average wealth always find a match, the poorest never do, and the success of those in between depends on where they fit in terms of ranked wealth (i.e. ‘status’). In the same way, women of below average wealth always find a man who meets their preference and the wealthiest never do. The result is so obvious it probably doesn’t need modelling, and the intuition is common among evolutionary scientists, at least as far back as Trivers and Willard’s 1973 paper about adaptive offspring sex ratio adjustment.

Things get a bit more interesting when we introduce a small sex difference in mean wealth. If men average one standard deviation more wealth than women, the chance of mating improves dramatically for below-average status men and above-average status women (remember the above-average men and below-average women were already successful in pairing).

We modelled the effects of sex differences in mean wealth for up to four standard deviations in each direction, and showed that the more men’s wealth exceeded women’s, the more likely poorer men and wealthier women were to find a mate. When women’s wealth exceeded men’s on average, it was the above-average men and the below-average women whose mating prospects declined.

These results provide some insight into the mating market consequences of gender gaps. Those consequences might alter how people approach mating, gender relations, and even voting. For example, Incel (involuntary celibate) men, many of whom are young and below average income, blame narrowing gender gaps for undermining their marriage and mating prospects. They are infamous for their often violent, misogynistic rage, much of it directed at movements like feminism and laws that limit gender inequities.

Income inequality

Next we turned our attention to within-sex income inequality by altering the standard deviation of the distribution of wealth within each sex. We began by manipulating inequality in the same way for women and men simultaneously. Reducing inequality improved the pairing success of the two groups that struggle most finding a partner under hypergyny: the poorest men and wealthiest women. These results match with the intuition that – under hypergyny – inequality leaves a larger number of poor men unable to find a mate, and likewise that very wealthy women will be unable to find a partner whose income exceeds theirs.

An increase in inequality, however, did not hit the poorest men and wealthiest women as hard as it hit those men who were just below average in wealth and those women who were just a bit wealthier than average. If our modelling holds up in real mating markets, it would suggest that mating markets might respond to inequality in ways that entail more than merely comparisons between wealthy and poor.

That becomes even more true when we manipulate the inequality in one sex alone, while keeping it constant in the other. Those details are probably beyond a readable blog post, but the most intriguing finding is that a small shift in status can make a big difference between whether inequality suits or harms one’s mating interests..

Where to from here?

Evolutionary behavioural scientists have long understood that mating markets can have profound effects on a variety of behaviours. This enterprise has been most successful, to date, in the study of biased sex ratios and their flow-on effects. Nonetheless, economic inequalities are known to be associated with behaviours that are related to mating, as documented in Martin Daly and Margo Wilson’s extensive research on homicide rates. Indeed, our recent studies of self-sexualisation on social media, and the incidence of Incels posting on Twitter, drew to our attention the need to model how inequalities affect mating markets.

Our model shows that the effects of inequalities on mating interests are never as simple as favouring one sex at the expense of the other. It is our hope that our paper stimulates both theoretic and empirical exploration of how economic inequalities between and within the sexes shape mating markets and a variety of human behaviours.

Read the original paper here

Brooks, R.C., Blake, K., & Fromhage, L. (2022). Effects of gender inequality and wealth inequality on within-sex mating competition under hypergyny. Evolution and Human Behavior, 43(6), 501-509.

– by Lei Fan

Most people would rather touch flowers than wet rags, eat fresh foods than rotten ones, and drink water from one’s own bottle than from a stranger’s. These preferences seem to have straightforward functions: they protect us from the microbes that cause infectious diseases. Researchers suggest that we humans – like many other animals – have evolved some ability to detect and avoid pathogens – that is, we have a behavioral immune system.

A popular hypothesis in the behavioral immune system literature suggests that negative attitudes toward people with different skin color, ethnical origins, or religions – that is, outgroup members – serve pathogen-neutralizing functions. The line of thinking goes as follows: different groups live in different areas, and those areas have different pathogens. One group carries pathogens that they have acquired some immunity against, but that another group has not acquired immunity against. Hence, the average outgroup individual is more likely to carry novel pathogens than the average ingroup individual is, which makes these intergroup interactions pose higher infectious disease threats. Historical events illustrate the most extreme disease consequences of such interactions. For example, the introduction of measles and smallpox from Europeans to Native Americans led to a 57% reduction in effective population size (Lindo et al., 2016). Consistent with these ideas, many studies have reported that intergroup biases are stronger in areas with higher pathogen threats (e.g., Letendre et al., 2010, O’Shea et al., 2019), after research participants have been primed to feel vulnerable to disease (e.g., Faulkner et al., 2004, Klavina et al., 2011), and among individuals who report greater motivations to avoid infection (e.g., Aarøe et al., 2017; Clifford et al., 2022). However, other findings haven’t been straightforwardly consistent with the idea that outgroups are, in general, interpreted as pathogen threats (e.g., Ji et al., 2019, Karinen et al., 2019, van Leeuwen and Petersen, 2018).

Inspired by the test that van Leeuwen and Petersen (2018) conducted, we aimed to test a foundational aspect of the hypothesis that the behavioral immune system interprets outgroup members as more of a pathogen threat than ingroup members: that people should feel less comfortable with microbe-sharing contact with ethnic outgroup targets than ethnic ingroup targets.

This paper was a registered report – a format in which our hypotheses, methods, and analysis plan were peer-reviewed before we collected the data and knew the study’s outcomes. We recruited participants who were either White UK residents or East-Asian Chinese residents. Each participant saw one of 40 faces, which were either East Asian or White. Some participants saw a face that was unmodified; others saw a face modified to have an infection symptom (shingles); and others saw a face modified to be wearing a facemask. Participants completed 7 items in which they reported how comfortable they would be with indirect contacts with the target, such as using drinking from the same bottle as the target, and sitting next to the target while the target was coughing and sneezing.

Our results revealed that contact comfort was lower for targets modified to have symptoms of infection relative to targets that didn’t have these symptoms. And people who scored higher on an instrument assessing the tendency to be disgusted by things that contain pathogens (e.g., dog feces) reported lower contact comfort. So, we had some evidence that the contact comfort items worked as intended. But were people less comfortable with contact with outgroup members than ingroup members? No – there was no interaction between participant ethnicity and target ethnicity. The presence or absence of infection cues (or a facemask) did not change this (lack of) effect.

Hence, do we interpret ethnic (outgroup) membership as diagnostic of infectious disease risk? Results of the current study suggest that no, we do not. But other results from previous studies seem still providing a lot of evidence supporting the idea that the behavioral immune system relates to intergroup attitudes in some way, such as those mentioned earlier. Thus, the main take-home message from our paper is, the story behind these mechanisms is probably way more complex than group membership being treated as solely indicative of pathogen threat.

by Anastasia Makhanova

The COVID-19 pandemic has profoundly affected people’s psychology and behavior. At the grocery store, for example, people might avoid busy aisles. If a stranger is coughing, people may preemptively step away and walk in another direction. People may altogether avoid grocery stores—and the store’s other patrons—and order grocery delivery to their door. Although the pandemic has kicked some of these behaviors into overdrive, the motivation to avoid pathogens is not new. Indeed, pathogens have been a recurrent and pernicious threat throughout the evolutionary past and that pressure shaped adaptive strategies that function to minimize pathogen threat.

Some strategies, as in the examples above, involve vigilance to environmental cues—including other people—that are associated with increased likelihood of contagion. People can detect signs of illness in faces, smells, and posture. Heightened pathogen avoidance motives, however, also lead to a more conservative threshold about whether a cue is linked to pathogen threat. That is, people who typically are more (vs. less) vigilant to pathogens and people who are in the moment reminded about the dangers of pathogens tend to avoid others who belong to groups only stereotypically associated with illness. Indeed, pathogen concern has been linked to age-based prejudice, weight-based prejudice, and—most consistently—people who belong to a different ethnic or racial group.

Why does heightened pathogen avoidance lead to greater prejudice against people from ethnic and racial outgroups? On the one hand, myriad historic accounts detail terrible consequences of pathogen transmission between two groups during initial contact, such as when the smallpox virus, brought over by the Spaniards, decimated the Aztec population. On the other hand, in the evolutionary past, contact with other racial and ethnic groups was likely so infrequent that there would have been insufficient pressure for the development of a specific adaptation to avoid outgroups in response to an illness outbreak. Nevertheless, it is plausible that—when pathogen avoidance is heightened—people would become especially wary of outgroups.

Although numerous studies show that heightened pathogen avoidance motives are linked to bias against people from racial and ethnic outgroups, it is unclear whether this pattern of results is specifically due to group membership (i.e., this person is an outgroup member and not an ingroup member) or another appraisal of the person. We conducted a study to identify the unique influence of group membership on social avoidance under the pressure of pathogen threat.

To isolate group membership, we relied on a method from social psychology called the Minimal Group Paradigm developed by Henri Tajfel and his colleagues in the early 1970s. This paradigm involves creating novel, arbitrary groups called minimal groups that are not connected people’s existing social identities. When people are placed into one of two minimal groups, they immediately develop a favorable attitude toward that group and a relatively unfavorable attitude about the outgroup. We hypothesized that, if heightened pathogen avoidance is associated with prejudice against racial and ethnic outgroups because of their outgroup status, then heightened pathogen avoidance would be associated with greater prejudice against a minimal outgroup.

First, to create minimal groups, we asked participants to complete a personality questionnaire. Without scoring their answers, we told participants that they were either “Green” or “Orange” personality type at random. Next, we used an experimental manipulation to heighten pathogen avoidance for some of our participants under the guise of being interested in how personality affected perceptions of photographs. Participants were randomly assigned to one of three groups: pathogen threat, non-pathogen threat, and control. Each watched a slideshow of images depicting people who were sick, anxiety-inducing scenes such as tornadoes, or furniture, respectively. Then, we asked participants to tell us about their initial intuitions about whether they would like other people who were part of their minimal ingroup (i.e., Orange if the participant was assigned to Orange) and other people who were part of their minimal outgroup (i.e., Green if the participant was assigned to Orange). Given the classic findings that people tend to like their ingroup and dislike their outgroup, we hypothesized that people in the pathogen threat condition (relative to those in the other two conditions) would show a greater divide: they would rate the minimal ingroup even more positively and the minimal outgroup even more negatively.

Unfortunately, we did not find support for our hypothesis. There were no differences between the three conditions in terms of the relative preference for the minimal ingroup versus the minimal outgroup. We did, however, replicate the classic findings: everyone evaluated the minimal ingroup more positively than the minimal outgroup.

We conducted exploratory analyses to examine whether people who were predisposed to heightened pathogen avoidance (i.e., people who reported higher trait levels of pathogen disgust) reacted differently to the experimental manipulation than those who were low in pathogen avoidance. Although the pattern of results in those analyses was largely inconclusive, we did find that people predisposed to heightened pathogen avoidance reacted more strongly to the Minimal Group Paradigm as a whole. That is, the difference between the ingroup and outgroup evaluations was greater for people high in pathogen disgust compared to those low in pathogen disgust. This latter finding, although exploratory, does suggest that some aspects of people’s pathogen avoidance psychology may be specifically attuned to information about group boundaries.

Generally, our findings show that, when people are faced with a situation that increases their concern about pathogen threat, they are unlikely to be reacting only to whether someone is an ingroup or outgroup member. In such situations, other appraisals are likely driving the observed biases against racial and ethnic outgroups.