Romantic Love, Casual Sex, and Human Ecology

There are individual differences in personal preferences for romantic love (emotional investment in a relationship) and for short-term sexual relationships. These differences are related to personality traits, but also seem to be influenced by whether one grew up in a harsh environment. Some evolutionary theories have proposed that harsh environments foster a preference for short-term mating, while richer environments supposedly foster long-term committed relationships. However, evidence suggests that this view is highly inaccurate. People living in harsh environments do prefer long-term committed relationships (marriage) but do not strongly prefer either romantic love or short-term sexual relationships. On the other hand, people living in richer environments are more likely to prefer either or both romantic love and/or short-term sexual relationships. Hence, the influence of environment on mating preferences and perhaps on related personality traits is more complicated than a one-dimensional model would suggest.

Source: Wikimedia Commons

Although romantic love is a universal phenomenon, there are noticeable individual and cultural differences in how much and how intensely people experience feelings of being emotionally invested in a romantic partner (Schmitt et al., 2009). Additionally, people vary in how much they prefer to invest in long-term versus short-term relationships. One model that attempts to explain such differences is life history theory, which proposes that individuals vary along a fast/slow life history continuum involving different preferred reproductive strategies. Hence, fast strategies are associated with high mating effort, that is, sexual promiscuity, while slow strategies are associated with higher parental effort, that is, commitment to long-term monogamous relationships (Figueredo et al., 2006). A related model, differential-K theory, proposes that this applies to whole populations, such that entire ethnic groups tend to prefer either a faster or a slower strategy, depending on their ancestral environment (Dutton, van der Linden, & Lynn, 2016). Specifically, in harsh environments where life expectancy is relatively short and infant mortality is high, individuals should focus on having more offspring with less parental investment. On the other hand, in resource-rich environments, where life expectancy is longer and infant mortality is lower, individuals will focus on having fewer offspring and investing more intensively in each one. Hence, in these models, reproductive strategies fall along a one-dimensional continuum in which long-term versus short-term mating tend to be opposed. These reproductive strategies are each supposed to be associated with certain personality traits (Figueredo, Vásquez, Brumbach, & Schneider, 2004). Specifically, fast strategies are thought to be associated with antagonistic antisocial characteristics, whereas slow strategies are associated with cooperative prosocial traits.  

However, as I have noted in previous posts, empirical findings on cross-cultural differences in attitudes to short-term mating have contradicted the predictions of these theories. Specifically, people in resource-rich countries tend to have levels of sociosexuality, that is, more accepting attitudes towards sexual promiscuity and sex without commitment, than those in resource-poor countries with harsher environments (Schmitt & Shackelford, 2008). Even though the latter have higher rates of infant mortality and lower life expectancy, people seem to invest more in parenting effort than mating effort, even though life history and differential-K theory predict the opposite. One proposed explanation is that in harsh environments children are less likely to survive without biparental care, and so sexual promiscuity, which increases the odds that women will have to raise their offspring without the support of a partner, tends to have much more adverse consequences in harsh than in resource-rich environments. Furthermore, another study found that people in resource-rich countries tend to report experiencing higher levels of emotional investment in romantic relationships than those in poorer countries (Schmitt et al., 2009). One proposed explanation is that high levels of ecological stress during childhood discourage the development of emotional investment, whereas low stress environments allow emotional investment to develop more readily. Therefore, resource-rich environments tend to facilitate interest in both short-term mating and higher levels of romantic love/emotional investment. This contradicts the theory that short-term and long-term mating are opposite ends of a spectrum that are fostered by different environments.

Source: Wikimedia Commons

However, this raises a question about reproductive strategies in harsh environments. If such environments discourage short-term mating, but also reduce emotional investment, then what reproductive strategy do they foster? In resource poor environments, arranged marriages tend to be customary, and people are less likely to marry based on their romantic feelings. Hence, marriage, and therefore long-term mating, need not be based on emotional investment (Schmitt et al., 2009). For example, surveys in developing countries such as India and Pakistan have found that many people (about 50%) in Pakistan and India would be willing to marry without love, yet very few people (2-4%) in Japan or the US would be willing to do so (Schmitt et al., 2009). Hence, low stress environments may give people more freedom of choice to follow their personal preferences, such as marrying more for love rather than more material reasons, whereas high stress, harsh environments may constrain people’s mating preferences in the interest of survival.

Interestingly, both people’s short-term and long-term mating motivations are related to their individual personality traits. Specifically, cross-cultural research has found that high sociosexuality, that is, a strong interest in short-term mating and casual sex, tends to be associated with high extraversion and low agreeableness (Schmitt & Shackelford, 2008). This is in line with research suggesting that ‘dark’ traits, such as psychopathy and narcissism that are associated with low agreeableness tend to foster short-term mating (Jonason & Buss, 2012; Jonason, Foster, McCain, & Campbell, 2015). Emotional investment, which facilitates long-term relationships, tends to be related to high extraversion on the one hand, but to high agreeableness on the other (Schmitt et al., 2009). Hence, extraversion tends to foster interest in both short-term sexual liaisons as well as investment in long-term committed relationships. Some people may well be interested in both (as I discuss in a previous post), but differences in agreeableness tend to incline people to greater interest in one or the other. Extraversion is associated with reward-seeking and sociability, so high extraversion may increase a person’s drive to seek rewarding relationships in general. High agreeableness tends to be associated with greater sympathy for others, whereas low agreeableness is associated with greater selfishness and willingness to use others for one’s own gratification. Elsewhere, I have suggested that resource-rich environments may foster greater extraversion. For example, cross-cultural studies have generally found that people in Western countries tend to be more extraverted on average than people in Asian countries. It may also be that resource-rich environments give people more freedom to express their personalities generally. Hence, in such environments, people low in agreeableness may have more opportunities to pursue short-term sexual affairs, whereas those high in agreeableness find it easier to emotionally invest in romantic relationships. On the other hand, harsher environments may be more restrictive, and people may have fewer opportunities to follow their heart. Additionally, this suggests that environmental effects do not necessarily select for a unitary life history strategy, contrary to theories such as differential-K. That is, harsh environments do not necessarily foster interpersonal antagonism and low agreeableness, while resource-rich environments do not necessarily foster cooperative prosocial characteristics such as high agreeableness. Hence, it is more likely that different strategies can coexist in the same environment, and that resource-rich environments give people more scope to focus beyond immediate survival.

© Scott McGreal. Please do not reproduce without permission. Brief excerpts may be quoted as long as a link to the original article is provided.  


Dutton, E., van der Linden, D., & Lynn, R. (2016). Population differences in androgen levels: A test of the Differential K theory. Personality and Individual Differences, 90, 289-295. doi:

Figueredo, A. J., Vásquez, G., Brumbach, B. H., & Schneider, S. M. R. (2004). The heritability of life history strategy: The k‐factor, covitality, and personality. Social Biology, 51(3-4), 121-143. doi:10.1080/19485565.2004.9989090

Figueredo, A. J., Vásquez, G., Brumbach, B. H., Schneider, S. M. R., Sefcek, J. A., Tal, I. R., . . . Jacobs, W. J. (2006). Consilience and Life History Theory: From genes to brain to reproductive strategy. Developmental Review, 26(2), 243-275. doi:

Jonason, P. K., & Buss, D. M. (2012). Avoiding entangling commitments: Tactics for implementing a short-term mating strategy. Personality and Individual Differences, 52(5), 606-610. doi:

Jonason, P. K., Foster, J. D., McCain, J., & Campbell, W. K. (2015). Where birds flock to get together: The who, what, where, and why of mate searching. Personality and Individual Differences, 80(0), 76-84. doi:

Schmitt, D. P., & Shackelford, T. K. (2008). Big Five traits related to short-term mating: From personality to promiscuity across 46 nations. Evolutionary Psychology, 6(2), 246-282. doi:10.1177/147470490800600204

Schmitt, D. P., Youn, G., Bond, B., Brooks, S., Frye, H., Johnson, S., . . . Stoka, C. (2009). When will I feel love? The effects of culture, personality, and gender on the psychological tendency to love. Journal of Research in Personality, 43(5), 830-846. doi:


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