By comparing alternative evolutionary models, the International Sexuality Description Project marks the transition of evolutionary psychology to the next level of scientific maturation. The lack of final conclusions might partly be a result of the composition of the Sociosexual Orientation Inventory and the sampled populations. Our own data suggest that correcting for both gives further support to the strategic pluralism model.

“Sociosexuality from Argentina to Zimbabwe: A 48-nation study of sex, culture, and strategies of human mating” delivers on its title. By combining empiricism and careful hypothesis testing, it not only contributes to our current knowledge but also points the way to further advances.

Extending the findings of this work: Tribal peoples need study. Monogamy as marital institution and monogamy as sociosexual orientation must be separated. Sociosexuality must be considered as an aspect of somatic as well as reproductive effort; third-party interventions in sociosexuality need attention; and multiple sociosexual orientations, with frequency-dependent fitness payoffs equal at equilibrium, need to be modeled.

Men and women report having significantly different numbers of sexual partners, which is impossible in a large sample. Schmitt's target article is no exception. This focuses discussion on the nature of the samples, their heterogeneity, and the locale they are drawn from. Further, we query how humans determine, for example, sex ratio, in the context of large numbers.

Schmitt's study provides strong support for sexual strategies theory (Buss & Schmitt 1993) – that men and women both have evolved a complex menu of mating strategies, selectively deployed depending on personal, social, and ecological contexts. It also simultaneously refutes social structural theories founded on the core premise that women and men are sexually monomorphic in their psychology of human mating. Further progress depends on identifying evolved psychological design features sensitive to the costs and benefits of pursuing each strategy from the menu, which vary across mating milieus. These design features, like many well-documented mating adaptations, are likely to be highly sex-differentiated.

Schmitt finds that national sex ratios predict levels of sociosexuality, but how we should interpret this result is unclear for both methodological and conceptual reasons. We criticize aspects of Schmitt's theorizing and his analytic strategy, and suggest that some additional analyses of the data in hand might be illuminating.

Schmitt has equivocated about the underlying psychology of sociosexuality, but from the data presented in the target article, it would appear that he has drawn out the underlying cognitive architecture. In this commentary, I describe this architecture and discuss two emerging hypotheses about heterosexual and homosexual male sociosexuality.

Schmitt's findings provide little evidence that sex differences in sociosexuality are explained by evolved dispositions. These sex differences are better explained by an evolutionary account that treats the psychological attributes of women and men as emergent, given the biological attributes of the sexes, especially female reproductive capacity, and the economic and social structural aspects of societies.

Previous research has suggested that offspring sex ratio may be influenced by the actions of prenatal sex steroids, principally androgens. The relative length of the second (index finger) to the fourth digit (ring finger) has been reported to be a proxy to prenatal testosterone levels. This trait is sexually dimorphic, such that males display a significantly lower 2D:4D ratio (indicating higher testosterone exposure), and this dimorphism appears robust across different populations. We suggest that digit ratio (2D:4D) may form a useful marker to help explain variation in sex ratio and sociosexuality.

The target article effectively assesses multiple hypotheses for human sexuality, demonstrating support for a complex, integrated perspective. However, care must be taken when extrapolating human universal patterns from specific cultural subsets without appropriate ethnographic contexts. Although it makes a strong contribution to the investigation of human sexuality, the basal reliance on a reductionist perspective constrains the full efficacy of this research.

Schmitt recognized that research is needed to identify other factors associated with sex ratio and with sociosexuality that may explain cross-cultural variation in sexual behavior. One such factor may be the risk of sperm competition. Sperm competition theory may lead us to a more complete explanation of cultural variation in sexual behavior.

Although statistically significant correlations have been found among political, economic, and social indices, on the one hand, and measures of sociosexuality, on the other, it is likely that these correlations are second-order effects. Underpinning the reproductive freedom associated with higher sociosexuality are factors more closely related to biology, namely, easy access to safe, effective contraception and reproductive medical care.

The target article, which is part of a larger study, the International Sexuality Description Project (ISDP), seeks to explore cross-culturally aspects of human mating behavior on a global scale. However the nonrepresentation of large cultures restricts the depth of this study. The inferences drawn from such a sample must therefore remain limited despite the impressive sample sizes. In a larger context it raises thoughts on how partial disclosures may misrepresent the design of the larger study.

Operational sex ratio (OSR) is the correct sex ratio measure for predicting sociosexuality, but it is unclear whether this is the measure used. It would be valuable to know how OSR and sociosexuality correlate separately for males and females. The relationship between sociosexuality and OSR should also be examined with OSR measured at the local level of the mating market, where sex ratio must be having its psychological effects.

Evolutionary psychologists should go beyond research on individual differences in attitudes and focus more on detailed models of psychological mechanisms. We argue for complementing attitude research with agent-based computational modeling of mate choice. Agent-based models require detailed specification of individual choice mechanisms that can be evaluated in terms of both their psychological plausibility and the population-level outcomes they produce.

Across mammals, when fathers matter, as they did for hunter-gatherers, sex-similar pair-bonding mechanisms evolve. Attachment fertility theory can explain Schmitt's and other findings as resulting from a system of mechanisms affording pair-bonding in which promiscuous seeking is part. Departures from hunter-gatherer environments (e.g., early menarche, delayed marriage) can alter dating trajectories, thereby impacting mating outside of pair-bonds.

On the basis of a reinterpretation of the International Sexuality Description Project (ISDP) data, we suggest that findings are consistent with the view that human reproductive behaviour is largely under social control. Behaviours associated with a high Sociosexual Orientation Index (SOI) may be part of a progressive change in reproductive behaviour initiated by the dispersal of kin that occurs as societies modernize.

Although the search for universal human traits is necessarily the principle focus of researchers in evolutionary psychology, the habitual reliance on undergraduate students introduces profound doubts concerning resulting data. Furthermore, the absence of relevant data from foraging societies undermines claims of cross-cultural universality in this paper and in many others.

Shortcomings in the target article preclude adequate tests of developmental/attachment and strategic pluralism theories. Methodological problems include comparing college student attitudes with societal level indicators that may not reflect life conditions of college students. We show, through two principal components analyses, that multiple tests of the theories reduce to only two findings that cannot be interpreted as solid support for evolutionary hypotheses.

In this commentary, I consider Schmitt's cross-cultural investigation of sociosexuality from a comparative perspective. I argue that such a perspective lends support to an evolutionary explanation of a number of Schmitt's findings, including universal sex differences in sociosexuality and the sensitivity of mating behavior to contextual variables such as sex ratio.