For the first time, scientists have found genetic variants that could influence when people have sex for the first time — and the age at which they have their first kids. While social and environmental factors remain important, this finding begins to outline the biological factors as well.

In the study, published today in the journal Nature Genetics, an international team of researchers analyzed genetic data from more than 125,000 participants, aged 40 to 69, from the UK Biobank, a national project that collected the DNA and personal information of half a million people in the UK from 2006 to 2010.

Scientists identified 38 gene variants that play a role in the age when people first have sex. Not all of them are specifically puberty-associated — several are known for influencing personality traits. Genes that made people more likely to take risks, for example, might also make them likelier to have sex at a younger age. In women, some of the genes also seemed to have an effect on when they had their first baby. The researchers then performed a statistical analysis of all the genetic variants and found that people who hit puberty earlier are more likely to also have sex earlier and have kids earlier.

Obviously, a person’s environment matters, too — social, economic, environmental, and cultural forces are well-studied as factors that contribute to early sexual initiation. The age at which a girl has her first period has dropped to 12.5 years in 1980 from 18 years in 1880, for instance, probably because of changes in diet and body size. Though the time at which a person reaches sexual maturity may not seem obviously significant, hitting puberty early has been linked to higher risks of certain illnesses, such as diabetes and heart disease. Looking at the genetic contributions to early puberty may help researchers untangle the biological component of these disease risks, if any exist.

"Early puberty timing seems to have adverse effects on diseases 30 to 40 years later," says Ken Ong, a pediatric endocrinologist at the University of Cambridge who co-authored the study. "Puberty timing and age at first sexual intercourse appear to be adverse for diabetes, heart disease, and a number of cancers, particularly reproductive hormone cancers."

There are other effects for early sexual initiation, too. People who hit puberty at a young age are less likely to get as far in school. Because the age when people become sexually mature and the age when people lose their virginity are genetically interconnected, today’s study implies that having sex at a young age could have consequences on your education and health. Though we already knew this, the study found a statistical correlation between genes and outcomes. For example, the researchers found that people who have genetic variants associated with sex at a younger age are less likely to get a college degree but more likely to be smokers. "This current work proved that [early puberty] has adverse consequences on much more immediate outcomes, such as risk-taking behaviors, such as age at sexual activity and also lower educational attainment," says Ong.

Not everyone was impressed with the study. Mary Hediger, who has researched adolescence for 35 years and recently retired from the National Institutes of Health, says the genetic explanation is definitely interesting, but it’s also long been known that earlier puberty is connected to earlier sexual activity and, often, earlier pregnancies. The study is certainly new in some sense, she says. But "we’ve known for a long time that menarche, particularly in girl, is a gateway to sexual activity. The prevalence of girls who had sex before they started their periods is very very low. And in most cases when they did have sex before menarche, it was not voluntary."

She also warns against studies that point to genetic factors influencing sexual behavior. Though genes are undoubtedly important, when it comes to losing your virginity and becoming pregnant, the social, economic, and cultural factors can’t be overlooked. "The kind of biological determinism makes me a bit uncomfortable," Hediger says. "It sort of gives you the impression — and a lot of these genetics studies sort of do that — that you’re more your biology than you are a product of your environment. You don’t want to give the impression that you’re doomed, your biology dooms you."

The study was quite large and extensive, says Peng Jin, a geneticist at Emory University School of Medicine. To confirm the findings from the UK Biobank, the researchers replicated them in more than 240,000 men and women from Iceland and over 20,000 American women of European-ancestry from the Women’s Genome Health Study. "I have to say, this is a quite impressive study," Jin says. "The number of individuals they’re examining is definitely very high, so I think that gives them more of a power to really identify the genetic factors associated with reproductive onset and success."

However, Jin says, all the genomes the researchers analyzed came from European people, or Americans with European ancestry. To further validate the studies, the researchers should replicate the findings in non-European populations. "It might be interesting to look at another culture," Jin says. People’s sexual behaviors are extremely complex; it’s impossible for now to say whether the genetic variations found in the study actually have impacts on when someone has sex for the first time or whether they’re going to go to college or not. "There’s only association," Jin says. "But whether there are genetic factors that are determining reproductive onset I think remains to be determined."

Ong says the point of the study was not to prove that the biological factors affecting puberty and sexual behaviors are more important than the environmental factors. Society and culture have definitely changed over the last century. "Despite these changes, we found that across those eras, the impact of genetic signals was constant," he says. The goal of the study is to deepen the understanding of what contributes to early puberty, early sex, and early pregnancies, so that public health officials can create "targeted and more effective approaches to health education and promotion of safer health-related behaviors," the study concludes.

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