Twelve gene loci have been identified that are associated with the age at which we have our first child and how many – key measures of human reproductive behaviour.
Human reproductive behaviour is a highly complex activity that is thought to be influenced by personal choices, environmental factors, and social circumstances. The two key measures of reproductive behaviour are the age at which we have our first child (age at first birth, AFB) and the total number of children we have (number of children ever born, NEB). Both measures are frequently recorded for population demographics and are strongly linked to reproductive outcomes like human development, infertility, reproductive success or fitness, and risk of neuropsychiatric disorders.
Research into the genetic basis of human reproductive behaviour has been relatively scant though increased interest has grown in the last few years. A recent collaborative study in Nature Genetics involving 250 researchers and led by the University of Oxford was conducted to identify genetic areas, or loci, associated with human reproductive behaviour. A genome-wide analysis was conducted using 62 datasets from different studies containing information on AFB and NEB from 250,000 and 343,000 individuals of European ancestry, respectively. AFB and NEB values were calculated from direct self-reported data and indirectly from survey questions from both sexes related to information on their date of birth, date of birth of their child, pregnancy outcomes, and reproductive histories.
Researchers focused on genetic variations in loci across the human genome that determine our individual differences and whether these regions were associated to AFB and NEB. Twelve genetic loci, 10 of which were not previously known, were identified to be significantly associated with AFB and NEB. The identification of these loci are important as they are assumed to contain genes that have a role in human reproductive behaviour. From the study, a total of 24 genes were identified and are likely to be responsible for the effects of the loci on human reproductive behaviour.
The examination of women revealed loci with genetic variants related to postponing parenthood, including genetic information associated with later onset of their first menstruation and later menopause. Variants discovered in men were linked to when their voice first broke. These behaviours are key milestones in human reproduction, and the researchers speculate that one day it may be possible to use genetic-linked reproductive milestones to predict, for example, how late individuals can wait to have a child or if a woman is infertile.
If all 12 of the identified genetic loci were combined together, the researchers estimate that they could explain less than 1% of the variability in the age at which someone has their first child and 0.2% of the variability of the total number of children. This number may seem small at the moment, but as more genetic information is discovered in future studies, could rise to predict as much as 15 to 20% of the variance in AFB and NEB.
The results of this study are considered timely, given that modern industrialized countries have been gradually postponing AFB. In 1970, women had their first child at around age 24. In 2012, they had their first child around 29. While the discovery that genetic information is linked to reproductive behaviour can’t help predict the exact timing of an individual’s first born or the total number of children they will have, it has shed light on the genetic basis behind these behaviours and has implications in family planning and the prediction of fertility.
Written By: Fiona Wong, PhD