To back up this belief, Perry’s team recently conducted a study in which they analyzed the genes of more than 125,000 people whose personal data are stored in the UK Biobank, a health resource containing blood, urine and saliva samples from 500,000 people ages 40 to 69 from across the country. Data on people’s lives and lifestyles, noted during interviews, are also stored in the Biobank.

The genes were analyzed and linked to reproductive milestones, such as the age when people first had sex, the age when they had their first child, how many children they had and, most important, when.

“We showed that 25% of variability was down to genetics,” Perry said. “So genes do play a significant role in your reproductive behavior.”

The remaining 75% was down to nurture, so it’s not all nature, but the study found 38 gene regions in the body’s genetic makeup that contributed to these life moments.