— 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.

– by Kaitlyn Harper

As evolutionary psychologists, we love a good story about sexual selection. They usually go like this: humans have evolved to prefer a given trait (e.g. intelligence, facial symmetry, waist-to-hip ratio) because it offers an evolutionary benefit, either directly or indirectly. Over time, people who prefer and therefore mate with partners who have these advantageous traits are more likely to pass on their genes and as a result these preferences become more and more common. Trait confers fitness benefit to partners. Corresponding mate preference evolves. Everyone goes home happy. But there’s a key assumption in this narrative that’s often taken for granted: that people actually end up with partners who match their preferences. And that’s where things start to get messy.

Empirical research paints a more complicated picture. Across many studies, the link between what people say they want in a partner and who they actually choose tends to be surprisingly weak. In many cases, preferences simply don’t map onto real-world mate choices. This inconsistency creates a fundamental tension: how can mate preferences be driven by sexual selection if they don’t consistently guide mate choice in reality? One suggested approach to reconcile this inconsistency is to consider mate choice as a multivariate process. That is, people aren’t picking partners based on a single trait, they’re juggling many preferences at once, making trade-offs in the process. This means that any single preference might only have a small effect on mate choice in reality, swamped by other competing criteria. It’s a tidy solution: mate preferences which have been observed in isolation can still exist, but diluted effect sizes could explain the lack of observed association between preferences and partner outcomes in more realistic settings. But this raises another important question: if the effect of each preference on partner choice is so small, are they still visible to selection?

To test the multivariate limits of the classic sexual selection narrative, we built an agent-based model that simulates a population of individuals who choose mates based on heritable traits and preferences and pass on these genes over 100 generations. We asked an overlooked but crucial question: what happens to mate preference evolution when individuals are choosing partners not just based on one or two preferences, but up to ten? Does increasing multivariate trade-offs in mate choice weaken the evolution of preferences? We ran the model under two scenarios – one where preferences evolve through indirect selection (i.e. fitness benefits are conferred through good genes passed on to offspring), and one where we added direct selection (i.e. fitness benefits are also conferred immediately through fertility benefits).

The results, which we unpack further in our full article, revealed that as the number of preferences used in the model increased, the evolution of those preferences slowed dramatically. With just one or two preferences, the model worked much like classic sexual selection theory predicts, where preferences and traits evolved hand-in-hand. But once we asked agents to consider many preferences, things started to break down. Trait evolution was largely unaffected, but preference evolution hit a wall. In our models, once mate choice involved ten preferences, indirect selection barely moved the needle of preference evolution. Direct selection held up better, proving more robust under the multivariate constraints.

So, what do our findings mean for the way we think about human mate preference evolution? They suggest that the heavily relied on approach, where each mate preference is assumed to have evolved by offering unique indirect fitness benefits, needs to be reconsidered given the many human mate preferences that have been observed. That’s not to say that these observed preferences aren’t real, or that they didn’t evolve – it just means the standard one trait, one fitness benefit, one preference approach may not hold up in the context of multivariate mate choice. Instead, we may need to consider alternative frameworks: perhaps preferences reflect mostly direct benefits, function as redundant signals, or arise from cognitive mechanisms. One idea is that only a few core preferences – such as for health, status, or resources – are truly evolved, while the traits we find attractive are indicators of these broader qualities that we learn implicitly via associative learning (e.g. finding clear skin attractive because we’ve subconsciously learned it is a signal of health).

Ultimately, our findings invite a rethinking of the classic sexual selection story, urging us to zoom out from individual preferences and develop more comprehensive models that reflect the multivariate nature of mate choice.

Read the original paper here: Harper, Kaitlyn T., & Zietsch, Brendan P. (2025) Multivariate mate choice constrains mate preference evolution. Evolution and Human Behavior, 46(4), 106694.