You may think having sex for the first time is a choice you make or a result of your environment, but that's just a part of it.
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Scientists are increasingly realizing that your genes also have something to do with your sexual behavior.
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Researchers at the Medical Research Council Epidemiology Unit at the University of Cambridge analyzed the genetic data of 59,357 men and 66,310 women between the ages of 40 and 69.
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Their study, published in the journal Nature Genetics, discovered 38 gene variants associated with age at first sexual intercourse, as well as when someone first has children and how many they'll have.
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The study found that the age when you first have sex is influenced by genes controlling the timing of puberty and physical maturity, as well as genes that contribute to personality types; 25% of variability was seen to be down to genetics.
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Other studies have found that puberty -- and sexual intercourse -- at an early age is also associated with performing badly at school.
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One gene, linked to irritability, was associated with people having sex later in life.
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Another gene linked to a risk-taking personality was associated with people having sex earlier in life -- and having a higher number of children.
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In the United States, about 43% of female and 42% of male teenagers between ages 15 and 19 have had sexual intercourse. But the decision comes down to a combination of the traits and genes identified with someone's environment. "People think a gene will cause a risk, but it doesn't," said John Perry, who led the research. "It just puts you at higher risk."