Casual sex and the paths to parenthood
– by By Dmitry Grigoryev, Albina Gallyamova, and Elizaveta Komyaginskaya
If someone is more open to casual sex, would you expect them to have more children? The prediction has intuitive appeal: more partners could mean more opportunities for reproduction. In our recent study in Evolution and Human Behavior, however, people who were more open to uncommitted sex than others in their country tended to report fewer children. Understanding this result requires separating several ideas that are often bundled together: life history trade-offs, mating effort, demographic context, and the pathways through which sexual relationships ultimately become births.
Life history theory starts from a general problem: organisms have limited time and resources, so investment in growth, survival, mating, and parenting involves trade-offs. In parts of the psychological life history literature, these trade-offs have often been organized along a fast–slow continuum, with earlier reproduction, greater mating effort, and openness to casual sex treated as features of a relatively “fast” strategy. But this way of grouping the traits together has become increasingly debated. Recent theoretical and empirical work has questioned whether mating-versus-parenting and offspring quantity-versus-quality are really expressions of the same underlying continuum, and studies do not consistently show that unrestricted sociosexuality goes together with earlier or greater reproduction. Whether these characteristics actually fit together is therefore an empirical question.
Working with an international consortium, we investigated that question using a preregistered analysis plan and data from 80,047 heterosexual adults across 92 countries. We examined sociosexuality: individual differences in openness to sex without commitment. Three questions assessed comfort with casual sex, sexual fantasies about someone just met, and the number of partners with whom respondents had had sex without seeking a committed relationship. We analyzed attitudes and desire together, separately from reported behavior. These questions did not directly measure the time or energy invested in finding partners or raising children.
We also constructed a country-level demographic index combining total fertility, adolescent fertility, and mortality before age 40. Higher scores described relatively faster demographic regimes, with higher fertility, more adolescent births, and greater early mortality. This index characterized national demographic conditions; it did not assign a life history strategy to each respondent. Participants separately reported how many children they had.
Because respondents were nested within countries, we took a multilevel approach and separated two questions that are easy to confuse. One asks whether people who are more sexually unrestricted than others in their own country have more children. The other asks whether countries whose respondents report higher average sociosexuality also have respondents with more children. These comparisons capture different sources of variation—and, as it turned out, they gave different answers. Within countries, people who were more sociosexually unrestricted than the average person in their country reported fewer children. This negative within-country association appeared both for attitudes and desire and for reported behavior. It remained after accounting for age and national reproductive timing, as well as national wealth, education, gender equality, and historical kinship institutions, such as extended-family residence and marriage rules. The associations were modest and stronger among women than men. They also persisted in analyses restricted to parents or respondents aged 40 and older.
A comparison across countries told a different story: countries where respondents reported more uncommitted sexual behavior also tended to have respondents with more children. We did not find the same country-level pattern for attitudes and desires. This does not mean that more sexually unrestricted individuals in those countries had more children. Within the same country, the individual-level association still ran in the opposite direction: more unrestricted individuals tended to report fewer children. This contrast is exactly why separating levels of analysis is so important.
People living in faster demographic regimes also reported more children after country-level adjustment. This is partly unsurprising since fertility itself contributes to our demographic index, but the broader pattern is still informative. In our paper, we treat these regimes as reproductive contexts rather than as psychological strategies carried by every individual: local reproductive timing, kin networks, norms, and institutions may all help make earlier and higher fertility more common and easier to sustain. Faster regimes initially appeared associated with lower sociosexuality, but that association was no longer statistically clear after adjustment for national wealth, education, gender equality, and kinship institutions. Country context also mattered for one specific part of sociosexuality: behavior. In faster demographic regimes, the negative association between a person’s unrestricted sexual behavior and number of children became weaker, especially among men. In other words, unrestricted behavior did not become a fertility advantage overall; rather, it was less strongly associated with having fewer children. One possible interpretation is that where reproduction begins earlier and sexual behavior, partnership, and childbearing remain more tightly coupled, mating behavior has a more direct route to births. Attitudes and desire showed little evidence of this contextual variation, and the behavioral variation was less clear in the older and parent-only samples.
The picture changed most when we took relationship status into account. Accounting for whether respondents were single, married, or in other relationship situations reduced the negative association for attitudes and desire by almost 90%; it was no longer statistically clear. The behavioral association weakened much less and remained negative. Relationships therefore deserve close attention in explaining how sexual dispositions and childbearing become connected.
Why might relationships matter so much? There are at least two directions of influence. Openness to casual sex may be related to the formation and stability of partnerships through which parenthood often occurs. But entering a stable partnership or becoming a parent may also change sexual interests and opportunities. Reliable partners and relatives can help sustain the prolonged demands of raising human children, so stable pair bonds don’t have to mean having fewer children; in some circumstances, they may make several births easier to support. And all of this depends on the surrounding social environment: access to сontraception, expectations about marriage, and available family support may further alter how sexual relationships are realized into births. Our cross-sectional data cannot tell us which of these pathways produced the associations we observed. Beyond the limits of the cross-sectional design, several features of the data also place some limits on how broadly these results can be interpreted. The data was self-reported, and respondents were about 31 years old on average, so many had not completed childbearing. About two-thirds were women. The response scale combined five or more children into one category. Country sample sizes were unequal, and cultural differences in willingness to report sexual experiences could affect comparisons. We left offspring survival and later reproduction unmeasured, so number of children at the time of the survey is therefore only an incomplete measure of lifetime reproductive success.
Even with those limitations, our results are difficult to reconcile with one simple story: that greater openness to casual sex is a straightforward marker of a single “fast” strategy that should also lead to more children. The multilevel approach shows where the pieces come apart. Faster demographic regimes are associated with higher fertility, while within those same societies, more sexually unrestricted individuals tend to report fewer children. They leave open how the underlying dispositions evolved. A useful next step is to follow people—and ideally couples—through partnership formation, first births, and subsequent births to see when and how mating dispositions are actually converted into parenthood. That may be a more useful question than asking whether sociosexuality is simply “fast” or “slow.”
