For the first time, scientists have identified areas of DNA—specifically, 12—associated with reproductive habits, in this case the age when men and women have their first kid and how many kids they have. Reporting in the journal Nature Genetics, researchers at the University of Oxford analyzed more than half a million men and women in the Netherlands and Sweden and found what they are calling a “biological basis for reproductive behavior,” though they acknowledge social and environmental factors reign supreme. Still, “we now know where to find the DNA areas linked to reproductive behavior,” lead author Melinda Mills says in a statement. “For example, we found that women with DNA variants for postponing parenthood also have bits of DNA code associated with later onset of menstruation and later menopause.”

As such, the findings could one day have implications for fertility and help doctors advise women on how long they can wait based on these DNA variants. But when taken together, those variants “predict less than 1% of the timing … and of the number of children [men and women] have in the course of their lifetime.” The paper—co-authored by more than 250 sociologists, biologists, and geneticists—acknowledges that the figure seems “extremely small,” but it notes the variants can predict a woman’s likelihood of never having children. The findings are being modestly hailed by one of the researchers as a “small piece to understanding this very large jigsaw puzzle.” (This world-renowned chef has been accused of pregnancy discrimination.)

The genetic architecture of human reproductive behavior—age at first birth (AFB) and number of children ever born (NEB)—has a strong relationship with fitness, human development, infertility and risk of neuropsychiatric disorders. However, very few genetic loci have been identified, and the underlying mechanisms of AFB and NEB are poorly understood. We report a large genome-wide association study of both sexes including 251,151 individuals for AFB and 343,072 individuals for NEB. We identified 12 independent loci that are significantly associated with AFB and/or NEB in a SNP-based genome-wide association study and 4 additional loci associated in a gene-based effort. These loci harbor genes that are likely to have a role, either directly or by affecting non-local gene expression, in human reproduction and infertility, thereby increasing understanding of these complex traits.