Sexual promiscuity refers to mating with more than one partner in a relatively short-time period (e.g., within one estrus cycle). Promiscuous individuals may or may not exhibit long-term social bond(s) with one (or more) partner(s).
Under some conditions, mating with multiple partners is adaptive. Evolutionary perspectives emphasize adaptive psychological sex differences involved in producing promiscuous sexual behavior. In addition to such sex-differentiated and species-typical psychological adaptations underlying promiscuous sexual behavior, evolutionary perspectives point to individual and socio-ecological variation to explain individual-level and population-level differences in sexual psychology and behavior. We briefly review some theory and evidence for context-dependent adaptations designed for sexually promiscuous behavior. We refer to “sexual unrestrictedness” and sociosexuality (see “Definition” above) as proxies for sexual promiscuity.
Parental Investment, Operational Sex Ratio, and Promiscuity
Trivers’ (1972) parental investment (PI) theory predicts that the sex with higher obligatory PI (e.g., expensive eggs, internal fertilization, gestation, lactation in mammals) will be choosier about mates (i.e., sexually restricted or lower sociosexuality). The sex with lower obligatory PI has more to gain from seeking multiple mates (i.e., sexually unrestricted or higher sociosexuality) due to high reproductive potential. Major predictions of the theory are confirmed by the pattern of sex differences found in sex-role reversed species such as polyandrous wattled jacanas where males invest more than females in parenting and the females are more sexually competitive. It’s also been confirmed experimentally. In the Australian katydid Kawanaphila, variation in food supply over the course of a single breeding season can flexibly shift the intra-sexual competitiveness and choosiness of the sexes by changing the ratio of males who can provide a nuptial gift (given as PI to females) compared to females seeking males (i.e., operational sex ratio). When food is scarce, it is difficult for males to produce their large spermatophore gift. When food is experimentally made abundant, operational sex ratio can become male-biased. Males then compete over access to females and females are choosy about males (Gwynne and Simmons 1990).
Confirming related predictions in humans, in a large cross-national comparison across 48 nations, there were significant sex differences in sociosexuality across all nations, and there was a moderate negative correlation between sex ratios and average levels of sociosexuality. In other words, when there were relatively fewer men, there was higher sociosexuality or more promiscuity. When there are more men, there is lower promiscuity (Schmitt 2005). This supports the evolutionary hypothesis that sexual promiscuity is a behaviorally flexible adaptation (in both sexes) expressed under some conditions (but less often in women). Theoretically, when men are rare (and thus more reproductively valuable than when in the majority), women have fewer options and intra-sexually compete more; men can more easily fulfill their mate preferences for variety, and over evolutionary time, men who did not shift their behavior toward promiscuity under these circumstances would have been outreproduced by those that did. These sex ratio effects on promiscuity evidently result primarily from changes in men’s promiscuity (Schact and Mulder 2015).
Costs and Benefits of Personality Traits Related to Promiscuity
Nettle (2011) argues each of the Big Five personality traits is associated with both evolutionary advantages and disadvantages. Whether personality traits such as sexual unrestrictedness and extraversion are primarily beneficial or costly for a given individual could depend on variation in (1) socio-ecological conditions and/or (2) other individual characteristics, which may indirectly influence the development of a trait (e.g., due to a gene-environment correlation, extraversion and sociosexuality could be reactively heritable, condition-dependent adaptations partly calibrated by experiences associated with one’s physical attractiveness (Lukaszewski and Roney 2011; Lukaszewski and von Rueden 2015). Consider the benefits and costs of extraversion. Nettle (2005) found extraversion is associated with higher number of lifetime sexual partners, extra-pair copulations in men, and likelihood of leaving one relationship for another one in women. Others have found that extraversion is positively associated with sociosexuality (Wright and Reise 1997). Extraversion also relates to sensation-seeking and could increase risk of disease and accidents (Nettle 2005). Openness has also been positively associated with sociosexuality (Wright and Reise 1997). But, openness is also associated with decreased disgust sensitivity (Druschel and Sherman 1999), and since one function of disgust sensitivity is to avoid infectious disease transmission, openness could also be costly under conditions where there is high risk for transmission of disease.
Parental investment theory predicts there are reproductive benefits to sexual unrestrictedness to men (under many conditions). And there’s abundant evidence men have specialized psychological adaptations for seeking sexual variety (e.g., Buss 1998). Similarly, there’s abundant evidence some women have a variety of adaptively designed mate preference shifts to promote short-term mating with men exhibiting traits thought to signal good genes (e.g., facial symmetry, masculinity) during peak fertility (Gildersleeve et al. 2014) and even evidence these mate preference shifts are predictable by individual difference variables related to reproductive strategies (early vs. late age of first menstrual period) (Durante et al. 2012). There are also reproductive costs to unrestricted sociosexuality in both sexes (e.g., disease risk), which may produce adaptive variation in sociosexuality. For example, Thornhill et al. (2010) predicted and confirmed that, across nations, unrestricted sociosexuality is more prevalent under socio-ecological conditions in which the threat of human disease transmission is lower (though the relation with female sociosexuality scores only was significant after controlling for other variables). In addition, extraversion, openness to experience, and individualistic values (e.g., increased gender equality) rather than collectivistic values were higher where infectious disease risk was lower. These findings confirmed predictions from parasite-stress theory of sociality and values.
However, Hill et al. (2015) found women high in perceived vulnerability to illness (i.e., history of vulnerability to disease) who they experimentally primed with disease threat exhibited increased desire for sexual partner variety (i.e., higher sociosexuality), supporting the hypothesis that, under high disease stress, such individuals should adaptively choose to increase the genetic diversity of their offspring by increasing preferences for sexual variety. These findings seemingly contradict predictions of parasite-stress theory as Thornhill et al. (2010) reported disease threat decreases women’s sociosexuality (and associated personality traits). But, Hill et al.’s (2015) results may potentially be reconciled with parasite-stress theory because only those women high in disease vulnerability responded to disease threat primes with increased preference for sexual variety. So, the findings could possibly suggest that only women in poor personal phenotypic health condition (those highly vulnerable to early extrinsic mortality) adaptively increase desire for sexual variety in response to high disease threat, whereas healthier women may use other strategies (e.g., restricted sociosexuality, collectivistic values) to effectively avoid disease. Regardless, both evolutionary theories explain (women’s) sociosexuality as context-dependent adaptations.
Promiscuity in the form of sexual infidelity by a long-term partner has other costs (e.g., paternity uncertainty, desertion costs, withdrawing of resources, and retaliation by mate) that may explain why promiscuity is often moralized. Price et al. (2014) argued that where male investment in offspring is particularly important (and when women are more economically dependent on their male partner), anti-promiscuity moral beliefs function to increase paternity certainty. When paternal investment is more important, wasting paternal resources on another man’s genetic offspring is even more costly. Similarly, desertion by an investing partner is more costly as well. So, anti-promiscuity morality would emerge from evolved sexual psychology interacting with economic and social conditions (Durante et al. 2012).
Evolutionary perspectives on personality and promiscuity inherently emphasize how organism’s genes adaptively influence personality but also that gene’s respond to personal and socio-ecological conditions epigenetically. This entry reviewed some ideas that place adaptations for sexual promiscuity and desire for sexual variety (within each sex) in a situationalized adaptive context.
- Nettle, D. (2011). Evolutionary perspectives on the five-factor model of personality. In D. M. Buss & P. H. Hawley (Eds.), The evolution of personality and individual differences (pp. 5–28). New York: Oxford University Press.Google Scholar
- Trivers, R. (1972). Parental investment and sexual selection (Vol. 136, p. 179). Cambridge, MA: Biological Laboratories, Harvard University.Google Scholar