As the depths of the human genome are plumbed to increasing depths, many of the resultant findings are rather surprising. A study published this week in Nature Genetics describes genes that predict sexual milestones.

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Uncovering the social impacts of individual genes gives fascinating insight.

The age at which females reach menarche (first period) has become earlier over the years.

In 1880, menarche tended to occur around the age of 18, in the 1980s, this age had dropped to an average of 12.5.

The reasons behind this change are not yet fully understood; some wonder whether it might be due to improved nutrition and a larger body size.

Other scientists have argued that the shift in age could be due to a rise in chemicals released into the environment as industry expands.

Previous research has linked an earlier age of menarche to an earlier age at first sexual encounter and first birth, lower educational success and reductions in health. For this reason, understanding the factors that influence these timings is of significant interest.

Genome-wide association studies have already uncovered 123 genetic variants associated with the timing of menarche; these genes also play a role in the timing of puberty within males.

A new study, designed to build on these findings, was carried out at the at the Medical Research Council’s Epidemiology Unit at the University of Cambridge in the UK.

Using genetic data from more than 380,000 people, the team, led by Dr. Jon Perry, set out to further investigate the genetic variants involved in the timing of menarche and puberty. The aim was to understand whether the genes that promote early menarche have an impact on the age of first sexual intercourse, age at first birth, number of children and other life success-related parameters.

The team uncovered 38 gene variants that were connected to the age of first intercourse; many of these variants have previously been investigated and are known to play roles in brain development and neural connections. Dr. Perry says:

“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.”

One of the genes of note – CADM2 – is known to control brain cell connections and brain activity. According to Dr. Perry, they also found that CADM2 is “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.”

Historically, the age at first sexual intercourse and age at first birth have been considered to be influenced predominantly by peer and parent relationships, low parental monitoring, a lack of religious belief or a host of other social and cultural parameters.

This research shows that, perhaps surprisingly, individual genes also have an influence on the age at which we have our first sexual encounter. The researchers believe that these genes might influence “physical traits, such as puberty timing, or personality characteristics, such as risk-taking propensity,” both of which might play a role in decisions regarding sex.

Previous research, carried out by Dr. Perry and colleagues, linked earlier puberty age to an increased risk of diabetes, some cancers and heart disease. Dr. Ken Ong, lead author of the current study, says:

“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.”

The team hopes that by slowly unfurling the web of connections between genes, environment and sexual maturity, they will be able to inspire a more effective, targeted approach to interventions and the promotion of healthier behaviors.

Adding another factor to the mix, Medical News Today recently covered research showing that obesity produces an earlier start to puberty in girls but a delayed onset in boys.