A new study focussing on mothers of twins and singletons across 72 countries and monitoring over 18 million births has found that contrary to previous thinking, twin births are more likely in healthier mothers with healthier behaviours.
The study, by Professor Sonia Bhalotra at the Institute for Social and Economic Research with Professor Damian Clark at the Universidad de Santiago de Chile, is new evidence to challenge the idea that twins are random and has important policy implications showing the health of mothers is crucially important. The new study is also very relevant to other social scientists who use data on twin births and singleton births to examine other issues such as nature versus nurture.
Professor Bhalotra said:
“Twins have intrigued humankind for more than a century. Scientists use data on identical twins to assess the importance of nurture relative to nature. However our new research has found that the distribution of twins in the population is skewed in favour of healthier women with healthier behaviours which means they are more likely to spend more time on parenting (or nurturing) behaviours.
Many countries have implemented policies to encourage or discourage birth rate rises, so understanding how fertility influences child development or women’s careers is important to reviewing such policies. It has always been presumed that twin births are quasi-random, or independent of characteristics of the mother that influence the environment in which children are reared, or the mother’s preferences over labour supply.
We have found new population-level evidence that challenges this. Using 18,652,028 births in 72 countries, of which 539,544 (2.89%) are twins, we found that the likelihood of a twin birth varies systematically and substantially with the mother’s health and socio-economic condition. This research is meaningfully large, and widespread.
We can see the mother’s health and socio-economic state is important in both richer and poorer countries, and it holds for all available markers of maternal condition including health stocks and health conditions prior to pregnancy (height, body mass index, diabetes, hypertension, kidney disease), health-related behaviours in pregnancy (healthy diet, smoking, alcohol, drug-taking), exposure to stress in pregnancy, and availability of prenatal care. We also found the higher levels of education are also associated with higher levels of twin births, probably because education leads to uptake of new health-related information for women.
It is known that twin births are not strictly random, occurring more frequently among older mothers, at higher parity and in certain races and ethnicities, but as these variables are in practice observable, they can be adjusted for. Similarly, it is well-documented that women using artificial reproductive technologies (ART) are much more likely to give birth to twins and ART-use is recorded in many birth registries so, again, it can be controlled for and a conditional randomness assumption upheld. Our finding is potentially a major challenge because maternal condition is multi-dimensional and almost impossible to fully measure and adjust for. For instance, foetal health has been shown to be a function of whether pregnant women skip breakfast, whether they suffer bereavement in pregnancy, their exposure to air pollution, and a host of other such variables.
The underlying hypothesis is that twins are more demanding of maternal resources than singletons and so conditions that challenge maternal health, be they long-standing under-nutrition (marked by height) or pregnancy behaviours (like smoking), are more likely to result in miscarriage of twins.
Overall, our results imply that the distribution of twins in the population is skewed in favour of healthier women with healthier behaviours which are likely to be positively correlated with preferences for child quality, with parenting (or nurturing) behaviours and with women’s labour force participation.”
The twin instrument by Sonia Bhalotra and Damian Clarke – Research Paper, IZA Discussion Papers, 10405 – Dec 2016