Parents, when you have "the talk" with your kids, keep these new study findings in mind: Genetic makeup may help determine when people lose their virginity.
The study results matter because people who have sex and babies earlier appear to fare worse educationally and have poorer physical and mental health, said study co-author Ken Ong.
"Genes influence the ages of first intercourse and first birth by acting on both personality and physical characteristics," said Ong. He is an epidemiologist with the University of Cambridge in England, who analyzed genetic data of hundreds of thousands of people.
Researchers have previously linked the start of puberty to genetic factors, but Ong said it wasn't known how genes may affect the timing of first sex and when females first give birth. The new study finds a "moderate genetic component" for both, although not a direct cause-and-effect relationship.
"We already know that the timing of puberty is under clear genetic influence, as well as environmental factors like diet and weight," said Dr. Tim Spector, professor of genetic epidemiology and director of twin research at Kings College London in England. "Our increasing childhood obesity problems are making puberty increasingly early," said Spector, who wasn't involved in the study.
The start of puberty decreased from an average of 18 years in 1880 to 12.5 years in 1980, according to background notes with the study.
Wouldn't earlier puberty automatically lead to earlier sex and earlier birth? It's not that simple, the researchers suggested.
Decades ago, when puberty occurred years later, people tended to have first sex later than now but give birth earlier, Ong explained. "In recent years, there is -- on average -- nearly 10 years between first sexual intercourse and having your first child," Ong said.
In the new study, Ong and colleagues examined genetic data and other records of nearly 400,000 people in the United Kingdom, the United States and Iceland. The investigators linked several genetic variations to age at first intercourse, and found some linked to age at first birth, number of children, age of puberty and willingness to take risks.
Spector said the study "convincingly confirms" previous research suggesting that genes affect the age you lose your virginity, although they aren't the ultimate deciders.
"Twin studies from our group and others have also shown that age of first sexual intercourse as well as number of partners is also around 50 percent heritable," he said.
There's also a "clear link" between genes that promote early sexual activity and genes that promote risk-taking, Spector added. "These genes are likely to have an effect throughout life," he said.
What do the study findings mean in day-to-day life? "The main message is that our teenage behaviors are a mix of our genes and environment," Spector said.
Ong agreed, adding that your fate isn't entirely dictated by your genes. Working to delay early puberty could stave off some problems created by early sex and early birth, he said. He believes the study results "strengthen the argument to find safe and effective ways to avoid children starting puberty at very young ages."
But David Karasik, a genetic researcher with Israel's Bar-Ilan University, cautioned that it's still not clear why the age of puberty has fallen over the past century. The question, he said, is whether it has to do with your environment or your genes -- with their activity affected by diet and lifestyles -- or both?
Whatever the case, some people are genetically predisposed to reproduce earlier, Karasik said, and sex education might fail in those people. Then again, social norms can affect the choices people make. It's a matter of "nature versus nurture," he said, "as in every human trait."
The study findings were published in the April 18 online edition of Nature Genetics.