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Researchers find genes for early sex

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(NBC News) - British researchers say they may have found a gene that influences people to lose their virginity early.

While no single genetic variation can completely affect behavior, the team of British researchers says the gene pattern they've found can account for a significant amount of the difference in when people have sex for the first time.

And the same genes are associated not only with risk-taking behavior, but with how many children people end up having and how grumpy they are, the team reports in the journal Nature Genetics.

Overall, they linked 38 different genes to age at first sexual intercourse.

"While social and cultural factors are clearly relevant, we show that age at first sexual intercourse is also influenced by genes which act on the timing of childhood physical maturity and by genes which contribute to our natural differences in personality types," said John Perry, a senior investigator at Britain's Medical Research Council who was involved in the study.

"We show that age at first sexual intercourse is also influenced by genes."

"One example is a genetic variant in CADM2, a gene that controls brain cell connections and brain activity, which we found was associated with a greater likelihood of having a risk-taking personality, and with an earlier age at first sexual intercourse and higher lifetime number of children."

The team did what's called a genome-wide association study — a giant trawl through all 20,000 genes in the human body.

They used three different databases: Britain's Biobank, where more than 120,000 men and women have given samples and also filled out detailed questionnaires; Iceland's population-wide genetic database of 241,000 people and nearly 21,000 women taking part in the U.S. Women's Genome Health Study.

Cultural influences have a fairly big impact on when and how people have sex for the first time, but this study shows genes also play a role and may help explain why so many young people defy these rules.

And genes may also regulate the basic biology that underlies sexual behavior, including the onset of puberty, the team said.

"Genetic variants associated with timing of puberty — which has decreased from an average age of 18 years in 1880 to 12.5 years in 1980 — in both men and women have been recently identified, and some studies have reported a correlation between puberty timing and age at first sex, suggesting that they may also be correlated at the genetic level," the team wrote.

"Earlier puberty timing, in both men and women, is associated with greater propensity for risk-taking behaviors, lower educational attainment, greater susceptibility to several adverse health outcomes and, in women, increased mortality," they added.

However, people who start having sex and babies younger end up having more children, and their genes end up spreading more widely, something called reproductive fitness, the team noted.

The researchers also said they may also have a scientific basis backing up parents who urge their teenagers to wait.

"We have already shown that early puberty and rapid childhood growth adversely affect disease risks in later life, but we have now shown that the same factors can have a negative effect at a much younger age, including earlier sexual intercourse and poorer education attainment," said the Medical Research Council's Dr. Ken Ong, who led the study.

Teens are known for their love of risky behavior.

The Centers for Disease Control and Prevention found in a 2014 survey that nearly 22 percent of U.S. teenagers admit to having ridden one or more times in a car or other vehicle driven by someone who had been drinking in the past month, 10 percent admit to having driven after drinking and nearly 88 percent said they skip wearing a helmet while biking all or most of the time.

"Certainly, the age at which a person first has sex is based on a lot more than the age at which they are first biologically capable."

And while the U.S. teen sex rate has dropped from 54 percent in 1991 to 46.8 percent now, only 59 percent said they used a condom the last time.

This helps explain the high U.S. teen pregnancy rate. But the CDC also finds that intervention helps - teen sex rates have dropped, as have rates of teen smoking and drinking. Studies show that teenagers do listen to their parents' advice when it comes to sex and birth control.

"While this study does find an association with genetic factors, it nonetheless supports the idea that the age at which a man or woman first has sex is overwhelmingly due to non-genetic factors, such as social or environmental context," said Dr. Alicia Smith, a psychiatrist at Emory University.

"Certainly, the age at which a person first has sex is based on a lot more than the age at which they are first biologically capable. It is also based on cultural and socioeconomic factors that are very difficult to account for in genetic studies," Smith added. 

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