By Martin Fieder

 

Evolutionary biology has always been interested in the question, which individual traits influence how many children we have, and how much each of these traits contributes to it. As nearly every trait has to some extent a genetical basis, together with the progeny also the genes that are associated with any favorable trait are spreading – in evolutionary terms “the trait is under positive selection”.

 

It is meanwhile known that several traits influence how many children a person has, at which age a person becomes mother or father, or what the chances are that a person finds a mate. It is also established that the effects of those traits may differ whether the person is a woman or a man. Moreover, for some of these traits their genetical basis in terms of so-called polygenic risk scores has been determined, which enables us to estimate the liability of an individual for a certain trait. What has been largely neglected so far is the question, how much each trait contributes (in statistical terms how much variance is explained) to the number of children or the probability of finding a mate/being ever married. We therefore aimed to explore that issue and analyzed how much each single trait contributes to the individual variance of number of children and ever being married. As we used the Wisconsin Longitudinal study (WLS) for that analysis, ever being married is a rather good proxy for mating in general because participants in that data set are born between 1937 and 1940, as marriage was very common at that time in the US.

 

Among these analyzed traits are religious intensity (as it is known for its pro-fertile effect in both men and women), facial attractiveness as well as social status because social status in terms of income is also known to foster number of children in men though not in women. Our analyses showed that among men, this positive effect of social status on a man’s number of children can simply be explained by mating. Being married was by far the most important factor of having children in this data set. And the factor that contributed most to the fact, whether or not a person was ever married or not, was income (~18% in men and ~7% in women). The direction of that effect, however, differed between men and women. In men, higher wages were associated with a higher chance of ever being married. Accordingly, female preference for high social status in terms of high income predicted a man’s chances of marriage and hence, his number of children. In women, on the other hand, higher wages were associated with a lower chance of being married. We do not have a final explanation for the negative effect of income on ever being married in women, but likely the problem of combining family issues and career plays a role.

 

The effect of education, another indicator of social status, on marriage and reproduction was less straightforward. In both men and women of our data set, higher education predicted a lower number of children, which is in line with the literature and, due to postponing of reproduction, is particularly expected in women. Interestingly, in men but not in women, the genetic predisposition towards higher education (polygenetic score for general cognitive ability) had a small positive effect on the number of children. A more detailed analysis revealed, that in men, a higher predisposition for education/general cognitive ability was associated with higher income, which in turn predicted a higher number of children.

 

“Our data show that in men, social status in terms of income has by far the strongest effect on marriage and as a result on reproduction as well. This is a finding which is clearly in line with evolutionary assumptions, predicting that women should prefer men of higher social status for their own and the sake of their progeny”

 

The effect of all other analyzed traits on the number of children or ever being married was much weaker. Only facial attractiveness explained 6% of ever being married in women and 2% in men, which, however, is still lower than expected as facial attractiveness has always been reported to be an important determinant for mating.

 

Our data show that in men, social status in terms of income has by far the strongest effect on marriage and as a result on reproduction as well. This is a finding which is clearly in line with evolutionary assumptions, predicting that women should prefer men of higher social status for their own and the sake of their progeny. However, as income is a very recent indicator of social status we are not able to make any final conclusions on ongoing selection pressures in modern societies. On the basis of our data, we can just argue that men in modern societies seem to be under pressure to “earn money” if they aim to find a mate and to have children.

 

Read the paper: Contemporary selection pressures in modern societies? Which factors best explain variance in human reproduction and mating?

By Dimitris Xygalatas

 

What do peacocks and humans have in common? I suppose there may be other possible answers to this question, but one feature they do share is the use of extravagant, apparently senseless traits as a means of transmitting important information. When such transmission is beneficial to both senders and receivers, those traits will be favored by the forces of selection, even if at first glance they appear to be wasteful. In a nutshell, this is the core claim of costly signaling theory.

 

In the case of the peacock, the signal is built into its body. Its disproportionately long, iridescent tail is calorically expensive to produce, maintain, and carry around. It reduces agility, adds drag during flight, and makes its bearer a flashing target for predators. But as Darwin already noted, there is a method to this evolutionary madness. Although the tail itself may have no utility, the effort that goes into growing and maintaining it can still pay off. As only a fit peacock could afford to carry such a costly ornament and still be alive, peahens use this ornament as a reliable indicator of the male’s prowess. By selectively mating with males that carry extravagant tails, they are investing in good genes for their offspring, inadvertently pushing the males to grow ever more grandiose tails.

 

The same logic can also apply to behavioral traits. In humans, some of the best-known examples come from the domain of ritual.  Signaling theories of ritual argue that costly ceremonies help solve important coordination problems: for any group that requires cooperation among its members, information on people’s levels of commitment is crucial. But declaring one’s loyalty to the group is cheap. Paying a hefty membership price provides more compelling evidence of commitment to the group and its values. Actions speak louder than words.

 

There is mounting evidence in support of this view. For instance, empirical research shows that people who participate in costly rituals are more committed to group causes. This is not lost on community members, who perceive participation as a signal of prosocial qualities. However, unlike the indexical nature of the peacock’s tail (what you see is what you get), human signals are embedded in complex sociocultural contexts.

 

For the sake of simplicity, formal models of signaling typically assume that the intensity of the signal varies while all else being equal. But in reality, all else is never equal. Social inequalities may influence default perceptions of a sender’s quality, impacting the signal’s cost-benefit ratio. While previous research treated this variation as noise, we designed a study to quantify it.

 

Our study, reported in Evolution and Human Behavior, took place in the island of Mauritius during the Thaipusam Kavadi, a Tamil Hindu festival held in honor of Lord Murugan. Performed by millions of Hindus in India and around thew world, it involves several days of activities that culminate with a pilgrimage to the god’s temple. Before embarking on the day-long procession under the scorching tropical sun, devotees have their bodies pierced with metal needles, hooks, and skewers. They also carry large portable shrines called kavadi. Those are built on a wooden, bamboo, or metal frame supported on the shoulders and are decorated with flowers and peacock feathers (in serendipitous but seemingly emblematic fashion, the peacock is Murugan’s symbol). The word kavadi, which in Tamil means “burden”, is what gives this ritual its name – aptly, as those structures can often weigh as much as a person.

 

In this setting, we recruited 80 adult participants of various ages. We measured the size of the kavadis they carried and the number of piercings they endured. After they completed their pilgrimage, we also asked them questions about their religious beliefs and practices and gave them a survey to assess their socioeconomic status.

 

We found great variation in both the intensity and form of signaling: the number of piercings ranged from zero to 600, and kavadis varied from petite to gargantuan. The festival provides different signaling opportunities and, as it turns out, participants harness them according to their means. High-status individuals carried larger and more flamboyant kavadis — for those at the top third of the socioeconomic ladder, almost three times larger than those at the bottom third.

 

Why would that be? Quite simply, because they could afford to. Building and decorating a kavadi can be expensive: the most elaborate structures can cost the equivalent of three months’ salary of an unskilled worker. And those larger kavadis are not necessarily heavier (at least not proportionally), as the affluent can purchase better and more lightweight materials (e.g. aluminum in place of iron), which allows them to amplify their signals even further.

 

“During the ritual, low-status individuals on average put almost four times more needles into their bodies compared to the upper classes. Lacking the material capital to compete with the elites, those of low status turned to whatever other means were available to them: quite literally, their own blood, sweat, and tears.”

 

On the other hand, there was a negative relationship between socioeconomic status and frequency of participation: across their lifespan, low status individuals carried the kavadi over four times more often. But it was not just a matter of frequency: during the ritual, low-status individuals on average put almost four times more needles into their bodies compared to the upper classes. Lacking the material capital to compete with the elites, those of low status turned to whatever other means were available to them: quite literally, their own blood, sweat, and tears.

 

It is worth noting that across all these metrics, there was a large effect of gender. While most women had only one piercing, the median number for men was 79. Men also participated in the ritual more times over their lifetime and carried kavadis that were on average over twice as large. Unsurprisingly, then, the associations with socioeconomic status were even stronger when we looked at the male-only portion of our sample.

 

While the physically demanding aspects of the festival are mostly a male affair, this does not mean that women do not engage in commitment signaling — far from it. For instance, they are more likely to fall into trance, which can be a hard-to-fake signal of devotion. Moreover, women play key roles in parts of the ritual that are seemingly peripheral but socially important nonetheless, such as hosting and organizing elaborate family feasts at the conclusion of the festival — a laborious and time-consuming activity.

 

Our study was the first to quantify the multi-layered costs of an extreme ritual and their variation along socioeconomic lines. Our findings are in support of a costly signaling view of ritual. Needless to say, cultural practices are complex and cannot be reduced to monocausal explanations. Our study has merely scratched the surface of one of the most widely performed rituals in the world.

 

 

Read the paper: Social inequality and signaling in a costly ritual