Sure, people become sexually active under countless circumstances for countless reasons and at many different ages, but at least part of the timing appears to come down to our genes, Cambridge University researchers report in the journal Nature. Studying the genes and life histories of nearly 400,000 people, they conclude "there is a heritable component to age at first sex," reproductive aging expert John Perry tells the Guardian, "and the heritability is about 25%, so one-quarter nature, three-quarters nurture." Genes also appear to influence the age of the onset of puberty, first birth of a child, and even total number of offspring, reports the Telegraph. Perry's example: The genetic variant in CADM2 is associated with higher odds of risk-taking, earlier sex, and a greater number of children.
So what accounts for the other 75%? New Scientist reports that previous research has found teens are more likely to become sexually active younger if they are not religious, come from poorer families, and if their parents didn't get involved in their lives. But this study found that the earlier onset of puberty is associated with earlier sexual activity, notes Popular Science. In the mid-1800s, for instance, girls were getting their period at an average age of 18—a number that plummeted to age 12 by 1980. Earlier loss of virginity ricochets, adds a clinical epidemiologist, into "other consequences such as, all things being equal, earlier first birth, having more children, less likely to remain childless, and poorer educational outcomes." Future studies may look at non-Western countries (this one focused on the UK, Iceland, and US) to account for different cultures and attitudes about sex, one expert tells the Verge. (Earlier this year, South Africa was offering scholarships to virgins.)