Study unravels genetic markers involved in life’s momentous milestones

A team of Oxford researchers collaborated with international scholars and Cambridge researchers to successfully identify hundreds of genetic markers that are involved in two of life’s most momentous milestones—the age at which people first have sex and become parents.

The researchers linked 371 specific areas in the DNA, known as genetic variants (known locations on chromosomes), 11 of which were sex-specific, to the timing of first sex and birth. The variants interacted with environmental factors, such as socioeconomic status and time of birth, and are predictors of later life disease and longevity. The study was published in Nature Human Behaviour.

The scientists analyzed the entire human genome by conducting a Genome-Wide Association Study (GWAS) to figure out the relationship between reproductive behavior and a particular genetic variant.

The researchers combined multiple data sources to examine age at first sex (N = 387,338) and birth (N = 542,901) in men and women. Then, they estimated a genetic score with all genetic loci combined explaining around 5%–6% of the variability in the average age at sexual debut or having a first child.

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

Melinda Mills, Professor and Director, Leverhulme Centre for Demographic Science, University of Oxford and Nuffield College

Mills is the first author of this study. The genetic signals are induced by not only environmental and social factors but also by reproductive biology. The findings are related to implantation, follicle-stimulating hormone, spermatid differentiation, and infertility.

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 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.”

Melinda Mills, Professor and Director, Leverhulme Centre for Demographic Science, University of Oxford and Nuffield College

“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 behavioral dis-inhibition, like ADHD, but also addiction and early smoking,” added Mills.

Mills continued, “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.”

Genetic factors that drive reproductive behavior are strongly linked to later life diseases such as cardiovascular disease and type 2 diabetes.

“It is exciting that the genetics underlying these reproductive behaviors may help us understand later life disease,” stated Mills.

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 a better understanding of teenage mental and sexual health, infertility, later life disease, and treatments to help.”

Melinda Mills, Professor and Director, Leverhulme Centre for Demographic Science, University of Oxford and Nuffield College