Are women genetically predisposed to having babies earlier than other women? Perhaps, suggests a study recently published in the journal PLOS ONE.

An international team working on the SOCIOGENOME project out of the University of Oxford in the UK analyzed existing data on the molecular and quantitative genetics of 4,300 women living in the Netherlands. The project, led by professor Melinda Mills, is a five-year-long study of the sociogenetic influences of reproductive behavior. Mills hypothesizes modern society empowers women to "override natural selection, feeling the effects of lifestyle choices and social factors."

In addition to the women participating in Mills’ project, the present study includes separate data collected from 2,400 women participating in TwinsUK, another long-term (as well as the biggest) study on the cause, or set of causes. With both datasets, researchers found genes accounted for an estimated 15 percent of differences between when modern women have their first baby and 10 percent of differences in the number of children they have.

Researchers cited while previous studies have used similar datasets relating to twins or within the families, theirs is the first to use molecular genetic information of women who aren’t related to one another. The larger sample suggests natural selection “is not just a historical process.”

In a press release, Mills explained despite the idea younger generations should feel inclined to have children at an earlier age compared to women of the past, women are actually delaying having babies, or starting a family, due to social and environmental factors. Women do this in spite of the fact it becomes more difficult to get pregnant as they age.

“In the second half of the 20th century, women across many societies delayed starting a family,” Felix Tropf, lead study author from the University of Groningen in the Netherlands, added. “Although genes play a significant part, it seems wider social changes, such as an expansion of women in further education and work, as well as the availability of effective contraception are having a stronger effect on determining when women in modern societies have children.”

Researchers did acknowledge their datasets included narrow estimates, possibly a result of shared environmental factors in family designs. Thus, researchers need to replicate their data with even larger samples in order to “engage in a more rigorous examination of genetic effects, as well as gene-environment interplay.

Even so, the present data leaves researchers feeling confident they’re bound to find genetic variants associated with human fertility.

Source: Tropf FC, et al. Human Fertility, molecular genetics, and natural selection in modern societies. PLOS ONE. 2015.