An international research team, led by Professor Melinda Mills and colleagues at the University of Oxford and MRC Epidemiology Unit scientist Dr Felix Day, has discovered hundreds of genetic markers driving two of life’s most momentous milestones – the age at when people first have sex and become parents.

In an analysis of data from more than half a million UK Biobank participants published today in Nature Human Behaviour, the team linked 371 specific areas of our DNA, called genetic variants (known locations on chromosomes), 11 of which were sex-specific, to the timing of first sex and birth. These variants interact with environmental factors, such as socioeconomic status and when you were born, and are predictors of longevity and several later life diseases.

The researchers conducted a Genome-Wide Association Study (GWAS), a search across the entire human genome, to see if there is a relationship between reproductive behaviour and a particular genetic variant.  In the largest genomic study of the timing of reproductive timing conducted to date, they combined multiple data sources to examine age at first sex (N=387,338) and birth (N=542,901) in men and women. They then calculated a genetic score, with all genetic loci combined explaining around 5-6% of the variability in the average age at first sexual intercourse or having a first child.

Professor Mills, Director of the Leverhulme Centre for Demographic Science at the University of Oxford and Nuffield College, and the study’s first author says:

Our study has discovered hundreds additional genetic markers that shape this most fundamental part of our lives and has the potential for deeper understanding of infertility, later life disease and longevity.”

Several of the genetic variants were associated with genes that are involved in aspects of reproductive biology, including follicle stimulating hormone production, implantation, infertility, and spermatid differentiation, and others were implicated in risk seeking behaviour, sociability and anxiety. However the study found that these genetic traits are strongly moderated by social factors and the environment. Professor Mills adds:

We already knew that childhood socioeconomic circumstances or level of education were important predictors of the timing of reproduction. But we were intrigued to find literally not only hundreds of new genetic variants, but also uncover a relationship with substance abuse, personality traits such as openness and self-control, ADHD and even predictive of some diseases and longevity .

We demonstrated that it is a combination of genetics, social predictors and the environment that drives early or late reproductive onset. It was incredible to see that the genetics underlying early sex and fertility were related to behavioural disinhibition, like ADHD, but also addiction and early smoking. Or those genetically prone to postpone sex or first birth had better later life health outcomes and longevity, related to a higher socioeconomic status in during childhood.”

Dr Day notes:

We found that the genetic factors driving reproductive behaviour are strongly related to later life diseases such as Type 2 diabetes and cardiovascular disease. It is exciting that the genetics underlying these reproductive behaviours may help us understand later life disease.”

Professor Mills concludes:

Starting your sexual journey early is rooted in childhood inequality but also has links with health problems such as cervical cancer and depression. We found particularly strong links between early sexual debut, ADHD and substance abuse, such as early age at smoking. We hope our findings lead to better understanding of teenage mental and sexual health, infertility, later life disease and treatments to help.”

• Full paper: Identification of 371 genetic variants for age at first sex and birth linked to externalising behaviour. Melinda C. Mills et al. Nature Human Behaviour. 01 July 2021.
• Frequently Asked Questions