— by Jun-Hong Kim

Over 95% of placental mammals are polygynous, with females bearing sole responsibility for rearing offspring. This stems from the reproductive characteristics of placental mammals. Female placental mammals are the default egg producers and are responsible for pregnancy and lactation. In other words, the default reproductive costs borne by female placental mammals are significantly greater than those of males. Consequently, female placental mammals, who invest substantial resources in reproduction, are choosier in mating than males. Males, meanwhile, strive to avoid missing reproductive opportunities and devote themselves to status competition to secure females.

However, human mating patterns appear to be the exact opposite of this placental mammalian norm. Human males are just as selective as females when choosing mates. Humans are one of the very few placental mammal species in which mutual choice occurs. Nonetheless, some societies permit polygamy. Even in those societies, though, polygamy is far removed from the extreme polygamy seen in some placental mammals (e.g. one male – multifemale polygyny in gorillas). Only a few upper-class men have multiple wives, while most men are in monogamous relationships or remain bachelors.

The most plausible explanation for this is that the cost of reproduction borne by human males is as significant as that borne by females. If one sex incurs higher reproductive costs, that sex will be choosier and more discriminating. Traditionally, female reproductive costs of egg production, gestation, and lactation have given weight to female choices. The additional costs borne by human males are 1) paternal investment and 2) resources, such as food, land, money and cattle, which can be translated into fitness. These costs increased as male contributions grew throughout later human evolutionary history (i.e. from 2.5 million years ago to the present). Sexual dimorphism in body size decreased, and male contributions to childcare also increased once strategic hunting began after the emergence of the genus Homo. Particularly after the advent of agriculture, men controlled farmland and livestock, making the resources they owned even more crucial.

This study sought to explore the factors determining human mutual choice. The conditions favoring mutual mate choice—paternal investment and earning potential—vary across countries and groups. Other factors that likely influence the direction and strength of mate choice include the operational sex ratio (OSR), which is the sex ratio of males to females present in the mating market at any given time, and population density, or the rate at which mate encounter each other. These factors serve as predictor variables in a multivariate regression, with the size of the cosmetic industry as the outcome variable, which signifies the degree of male choice. When there is marked variation in paternal investment and resources, I expect women to compete for potential good providers and wealthy males.

Across all statistical models, the economic inequality (i.e. resource variation) and paternal investment variables were the dominant predictors of the size of the cosmetics industry. OSR and population density were not significantly associated with the size of the cosmetics industry, which is contrary to expectations. In numerical terms, each hour increase in paternal investment corresponds to a $2.17 increase in per capita spending on cosmetics. Also, each increase in the income inequality index and the social mobility index corresponds to a $0.29 and $0.36 increase in per capita spending on cosmetics, respectively. Typically, paternal investment is a stronger predictor of the size of the cosmetics industry than the two economic inequality variables. This fits with the expectation that male resource variation (i.e. resource earning potential and paternal investment) would generate selection for male choice.

To check for independence, the clustering patterns were examined in two-dimensional graphs. Most statistical tests used in cross-cultural analyses assume statistical independence of data points; however, countries, cultures, and languages are not independent because they share histories and environments. If points cluster by location (e.g. cultures from one part of the world tend to have similar values) or relationship (e.g. related languages tend to cluster together), then the p-value and slope may not be valid. A simple first step is to plot the values on a map and determine whether they exhibit distinct spatial patterns. As shown in the three scatter plots, there are no clustering patterns.

In a recent study on sexy selfies (sexualized self-portrait photographs), which is also a proxy for male choice, Blake et al. (2018) found that the frequency of sexy selfies was correlated with two different indices of economic inequality. Also, dowry as a payment by the bride’s family and kin, which is a proxy for female-female competition, is highest in stratified and nonpolygynous societies. However, the influence of paternal investment on male choice was ignored.

In some species, females sometimes have exaggerated traits when males are responsible for parenting, or when females have large variations in reproductive success. Male choice is predicted for species with paternal investment. That is, the more effort a male puts into paternal investment, the more selective he will be in mate choice. However, this prediction has not been thoroughly tested.