Night Owls Have More Sex, Morning Larks Have More Children
Separating mating success from reproductive fitness in humans
Posted Sep 09, 2019
They say that the early bird catches the worm. It gets more sex, too. Male songbirds spend much of their time singing to attract females, and the longer they work at it, the more chances they have to reproduce.
Among humans, however, it’s the night owls and not the morning larks that have more sexual encounters and more sex partners in their lifetime. In hunter-gatherer societies, males and females are segregated during the daylight hours, with the men hunting and the women gathering. Courtship occurs mainly at night around the campfire as couples flirt with each other before receding to the darkness for privacy. The late-night bar scene of modern times has deep evolutionary roots.
In the animal world, more sex generally means more offspring, because females are only receptive when they’re ovulating. However, humans also engage in sex for social purposes. So do people who have more sex also have more children? This is the question that German psychologist Ali Kasaeian and his colleagues at Tübingen University explored in an article they recently published in the journal Evolutionary Psychology.
The researchers begin with the well-documented relationship between frequency of sex and “chronotype.” This refers to an individual’s variation in alertness and activity during the day. Most of us can self-identify as either a “morning lark” who’s at their best in early hours of the day or as a “night owl” that’s at their peak in the evening.
Previous research has already shown that chronotype can be used to predict other personality traits. For instance, night owls are usually more extraverted and have more liberal views about casual sex. In contrast, morning larks are more introverted and usually prefer long-term monogamous relationships.
The researchers built on earlier studies by looking at two interrelated concepts, mating success, and reproductive fitness. Mating success is measured by a number of lifetime sex partners, while reproductive fitness is determined by the number of children you have. In the animal world, these two concepts are highly correlated, but this may not be the case in humans.
For this study, the researchers recruited over 1,800 participants, who responded to an online survey. Questions were intended to measure personality characteristics such as extraversion (how outgoing you are), chronotype (morning or evening orientation), sociosexuality (attitudes about casual sex), and long-term versus short-term orientation toward sexual relationships. Other questions included the age of first sexual intercourse, number of lifetime sex partners, number of children, and frequency of sex.
To a large extent, the results corroborated data from earlier studies. For instance, night owls tended to have an earlier age of first sexual intercourse, engage in more frequent sex, and report more sexual partners in their lifetime compared with morning larks. In contrast, morning larks were more likely to prefer a long-term orientation toward sexual relationships. They also reported a lower frequency of sex and fewer lifetime sex partners. But surprisingly, the morning larks also reported having more children than night owls.
Because the researchers took an evolutionary approach to this study, they found this last result surprising. As we’ve already seen, mating success and reproductive fitness are correlated among most animal species.
Humans, by contrast, mostly engage in sexual acts for recreation rather than procreation, and furthermore, they’ve learned effective methods for preventing pregnancy. In fact, almost all human sexual acts—whether casual or within a committed relationship—are undertaken for the purpose of social bonding rather than for producing offspring.
Kasaeian and colleagues note that the development of effective contraceptives since the 1970s has freed couples to engage in sex without fear of unwanted pregnancy. It’s certainly true that the invention of the pill was especially important for the liberation of women, who now have control over their reproductive processes.
However, people have been engaging in recreational sex since the dawn of our species—in fact, even our chimpanzee and bonobo cousins do so as well. But humans have also learned that there are ways to have sex without getting pregnant. For instance, women in hunter-gatherer societies typically breastfeed each child for up to four years. When coupled with a low-fat diet and vigorous lifestyle, breastfeeding keeps a woman from ovulating. In other words, she can have sex without fear of unwanted pregnancy until she has weaned her child.
Likewise, people with unrestricted attitudes toward casual sex are generally open to alternative forms of sexual behaviors. A longstanding form of contraceptive has been coitus interruptus, or withdrawing the penis from the vagina before ejaculation. Furthermore, people with unrestrained sociosexuality are generally more open to alternative activities, such as manual, oral, or anal sex, which do not lead to pregnancy. Clearly, those with an unrestricted sociosexual orientation seek to maximize mating success while minimizing reproductive fitness.
Chronotype is linked to reproductive strategies (long-term relationships and more kids versus short-term relationships and fewer kids). However, the ultimate source of morning versus evening orientation is unclear. To some extent, chronotype changes with age. Young children tend to be morning larks, while adolescents are usually night owls. And as we enter the workforce and start families, we shift back to a morning orientation willy-nilly.
Perhaps there’s an evolutionary component to chronotype. After all, adolescents are at the stage in their lives when they’re seeking mates, and so a night-owl lifestyle befits them. It could also be the case that we adjust to a morning or evening orientation depending on our life goals. Those who aspire to a short-term mating strategy can only be successful if they have an evening orientation. And in either case, parents and breadwinners do best by following the adage that the early bird catches the worm.
At any rate, the results of this study make perfect sense when we keep in mind that humans have learned how to separate sex from reproduction. Instead, we use sex to socially bond with others, both in short-term and long-term relationships. From this perspective, it’s no surprise at all that night owls have more sex, but morning larks have more children.
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Kasaeian, A. Weidenauer, C., Hautzinger, M., & Randler, C. (2019, July-Sept.). Reproductive success, relationship orientation, and sexual behavior in heterosexuals: Relationship with chronotype, sleep, and sex. Evolutionary Psychology, 1-10. DOI: 10.1177/1474704919859760