Genes May Time Loss of Virginity

March 31, 2009— -- Sexual precociousness is in our genes, new research suggests. A unique study of twins separated at birth finds a genetic link to the age at which a person first engages in sexual intercourse.

"It's not like there's a gene for having a sex at a certain date," says Nancy Segal, a psychologist at California State University in Fullerton who led the new study. Instead, heritable behavioural traits such as impulsivity could help determine when people first have sex, she says.

As genetic determinism goes, the new findings are modest. Segal's team found that genes explain a third of the differences in participants' age at first intercourse – which was, on average, a little over 19 years old. By comparison, roughly 80% of variations in height across a population can be explained by genes alone.

However, determining the extent to which sexual precociousness is inherited is trickier than making a similar calculation for height. A common family environment – whether it promotes or hinders early sex – could cause scientists to overestimate the effect of genes.

Social Effect

By studying 48 pairs of twins raised apart, as well as 23 individual twins, Segal's team sidesteps this confounding factor. "This gives us a pure estimate about how much genes affect behaviour," she says.

On the other hand, conservative social mores might delay a teen's first sexual experience, causing scientists to low-ball the effect of genes. Indeed, Segal's team noticed a less pronounced genetic effect among twins born before 1948, compared with those who came of age in the 1960s or later.

Other factors may also make the effects of genes harder to discern. For example, Segal's team also found that female participants who felt unhappy and unfulfilled in their home life were more likely to have sex at a younger age.

Essential function

As for the specific genes involved, another team previously found that a version of a gene encoding a receptor for the neurotransmitter dopamine is associated with age at first intercourse. Others have linked the same version of the gene – called DRD4 – to impulsive, risk-taking behaviour.

This might be one explanation for early sexual activity, says Joseph Rodgers, a psychologist at the University of Oklahoma in Norman. Yet unlike other risky behaviours, such as drug use or reckless driving, sex serves an essential biological function.

"For most of our evolutionary past, sexual behaviour hasn't been risk taking, it's about reproduction and success of the species," Rodgers says.