A unique study of twins separated at birth suggests that genes may help determine the age at which a person first engages in sexual intercourse.

Psychologist Nancy Segal of California State University in Fullerton, USA, looked at 48 pairs of twins who were separated at birth as well as 23 individual twins to see how genes influenced their sexual maturity. Using this methodology, the researchers were able to avoid the influence of a common family environment, which might have led them to overestimate the effect of genes.

Lead researcher, Nancy Segal said:

It’s not like there’s a gene for having a sex at a certain date. Instead, heritable behavioural traits such as impulsivity could help determine when people first have sex.

In comparing the twins sexual histories, Segal had each of the participants take a sexual life history interview, composed of a sexual meaning survey, a sexual life history timeline, and a sexual behaviour questionnaire.

Segal’s team found that genes explained 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.

It might be argued that conservative social mores might delay a teen’s first sexual experience, causing scientists to underestimate the effect of genes, and indeed, the research team acknowledge 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 found that female participants who felt unhappy and unfulfilled in their home life were more likely to have sex at a younger age.

As for the specific genes involved, another team of researchers had 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.

The study “Age at first intercourse in twins reared apart: Genetic influence and life history events” is published in the journal Personality and Individual Differences.

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Posted by Jonathan in Biology, Psychology

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