Across the animal kingdom, it is very clearly not the case that males are always eagerly promiscuous and females are always choosy monogamists. As Robert Trivers (1972) pointed out some 40 years ago, which gender is choosier depends on a host of parental investment factors and sexual selection processes, and sometimes males are the choosier gender in a species (e.g., Mormon crickets, katydids, etc.). More than that, other features can affect sexual selection (e.g., local mortality, population density, etc.) and further influence gendered choosiness, so any kind of categorical, essentialist Mars-Venus distinction is hardly the right way to conceptualize animal sexualities. Evolutionary psychologists not only know this, they have been arguing this for decades (Kenrick et al., 1990; Symons, 1979).
That said, the fact that humans are mammals (with heavy obligatory investments of gestation in women) and because humans possess certain revealing patterns of sexually-selected attributes (e.g., men are larger, stronger, physically mature later, take greater risks, die much earlier, the list goes on), from a cross-species perspective is it likely that men probably possess design features that motivate them to seek and consent to indiscriminate sex more than women. Not a Mars-Venus type of gender divide mind you (with all men always promiscuous and all women always monogamous). Evolutionary scientists only expect men will, on average, seek and consent to casual sexual encounters more than women will. As all careful scientists acknowledge, evolved sex differences are rarely a simple manifestation of essential categories. Most evolved sex differences arise instead from degrees of hormone-related organizational effects on the brain, activational effects during adulthood, and sometimes direct genetic effects (McCarthy & Ball, 2011). Sex differences also undoubtedly result from processes of socialization and sex roles that are, themselves, derived products of our evolved biology (see Mealey, 2000; Wood & Eagly, 2007). Even though gendered sexual development is quite complicated, this doesn't nullify the ability of evolutionary psychologists to make accurate predictions about average-level behavioral sex differences in sexuality. And some of these sex differences are going to be remarkably robust (see).
OK, this part is important and many critics seem unable to understand it, so I'll repeat. The Trivers-based sex difference explanation often proffered by evolutionary psychologists does not expect that all men will always be eagerly seeking promiscuous sex, nor does it imply that all women always eschew causal mating opportunities. Just as not all men are taller than all women (and yet men are taller than women, on average), men are expected by evolutionary psychologists to be more eager than women for indiscriminate casual sex, but only on average. Factors such as a man's culture (including his religion...members of the Shakers religious sect don't have sex at all––no sex differences in promiscuity there!), his family history (unpredictable early childhood experiences tend to elicit more short-term mating; Simpson et al., in press), his phenotypic masculinity, his facial symmetry, his mate value, his testosterone level, his oxytocin receptor gene variants, his dopamine D4 receptor gene variants, and a host of other associative features likely impact his desire, ability, and general tendency to strategically pursue indiscriminate sex (see). Biological and social contexts matter for reproductive strategies in most species, this is certainly no less true for humans (yes, even male humans).
Evolutionary psychologists have been blazing trails in terms of researching the special contexts that matter most when it comes to casual sex in humans. Unfortunately, an especially troubling tendency among these Mars-Venus Straw Man propagators is that they either ignore or are entirely unfamiliar with the vast amount of evolutionary psychology research accumulated over the last 20 years. Let me provide some background.
Beginning in the early 1990s, much of the emphasis in evolutionary psychology theory and research turned to explaining sexuality differences within genders. The early work of David Buss, Doug Kenrick, Steve Gangestad, Jeff Simpson, and many others documented that women and men BOTH are designed for short-term mating (i.e., casual sex, brief affairs, one-night stands; Kenrick et al., 1990). What women and men want in their short-term mates, how they pursue them, the benefits they gain, all of these things are different from when the sexes strategically pursue long-term mates. In this view, all humans possess specialized adaptive designs for both brief sexual encounters and enduring romantic partnerships. Temporal context matters, and evolutionary psychologists were the first to point this out.
Crucially, the adaptive design of women's and men's short-term strategies is in many ways psychologically distinct (Jonason & Buss, 2012). Women can especially benefit in short-term mating from things like access to high quality genes (genes from men they couldn't access as long-term mates), access to protection and provisioning for herself and her offspring (such as in partible paternity cultures), using short-term mating to achieve long-term mating goals, and many other functions. Predictably, women who actively pursue short-term mating strategies tend to be somewhat choosy about the mate value qualities of their brief sexual partners, preferring to mate with men who are physically symmetrical, possess facial masculinity, and give off other cues to high testosterone levels (see Gangestad & Simpson, 2000).
Men's short-term strategy is more indiscriminate than this, and appears to be designed to obtain larger numbers of partners (perhaps an obvious point for those passing Mike Mills' sex difference scientist test). And so again, all else equal, evolutionary scientists expect men will, on average, seek and consent to casual sexual encounters more than women will. Critics please take note here: Evolutionary psychologists are not declaring that women never ever seek short-term sexual encounters, evolutionary psychologists have been shouting at the top of their lungs that women are exquisitely designed for short-term mating! In more egalitarian cultures such as in Northern Europe where humans are able to pursue their short-term strategies more freely, short-term desires and behaviors are more prevalent among women (and men; Kennair et al., 2009). Even so, the expectation is that across most contexts when women actively pursue short-term mates they tend to do so in a more discriminating manner than men. As an oft-cited study by critics actually showed (see), even in hypothetical situations it takes Johnny Depp to evoke a Yes response from women to an offer of casual sex, for men the difference between saying Yes to a female stranger and Yes to Angelina Jolie is minor.
Given this more accurate and nuanced understanding of men's and women's evolved short-term sexualities, is there really all that much evidence that men and women differ, on average, in motivations for short-term sex? Any evidence beyond the seminal Clark and Hatfield (1989) research showing most men say Yes to sexual offers from strangers, whereas relatively few (if any) women say Yes? David Buss et al. (2011) reviewed numerous studies on this point, noting the following highly replicable findings are relevant for evaluating sex differences in the psychological design of short-term mating (to paraphrase): Men generally relax their level of selectivity when short-term mating, whereas women's standards, especially for "sexiness," go up when short-term mating. Men are more willing than women to engage in sex with partners outside of their long-term partnership, and when men have affairs they have them with larger numbers of sex partners. Men are overwhelmingly more likely to have sexual fantasies involving many short-term partners, and the content of men's pornography consumption, compared to women, contains themes of short-term sex with multiple partners. Men are more likely to pay for short-term sex, they express desires for larger numbers of sex partners over various time intervals, and they tend to seek sex sooner, after a briefer time delay, than women. Men are more likely than women to express "regret" about missed sexual opportunities, and men have more "unrestricted" sociosexual attitudes than women. Some of these findings have been replicated across dozens of samples in very large international studies, in representative samples of entire national populations, and in large scale meta-analyses, such as the robust sex differences reliably-observed in studies of extramarital sexual behavior and permissive attitudes toward casual sex (Lippa, 2009; Petersen & Hyde, 2010).
The status of support for evolutionary predictions about sex differences in strategies of short-mating is far from being in a "premature" state as some critics suggest (see). The accumulated empirical evidence on sex differences in short-term mating psychology is rather substantial. Well, that's maybe an understatement. Compared to the evidentiary status of most findings in sexual science, evidence of this sex difference could be described as mountainous. Himalayan, even. And the evolutionary psychological theories that explain these facts are not about Mars versus Venus. The Earth-bound theories of evolutionary psychologists are about differences in the psychological design of men's and women's short-term mating strategies (and long-term mating strategies), differences in sexual desires between genders and within genders (and even within the same person), differences in sexual diversity as beautifully lived across contexts of history, family, culture, and ecology (Gangestad, 2011).
Does all this evidence imply men and women are always morally justified in seeking out short-term mates? Not at all, that sort of conclusion would be falling into the naturalistic fallacy (see). Are there limitations, complexities, and problems with particular evolutionary explanations of human sexuality? Of course, that's the nature of Earthly sexual science––hypotheses often turn out to be wrong. Are there reasonable criticisms of evolutionary psychology? Sure, one is that evolutionary psychologists have probably paid too little attention to gene-culture co-evolution and the importance this may play across human populations (Bolhuis et al., 2011). But this Mars-Venus essentialist claptrap about evolutionary psychology as a whole––that's not only wrong, it's not even evolutionary psychology.
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