The proportion of males to females born is often slightly skewed in favor of males, according to the World Health Organization. While hypothetically, fertilized human eggs are about equally likely to have XX or XY chromosomes, many social, biological, and environmental factors potentially influence whether fertilization results in a live birth.
In a paper published in PLOS Computational Biology on December 2, a team of researchers report combing through data on 150 million people in the US over eight years, and data on 9 million Swedish people over 30 years, in an attempt to tease out the potential influence of some of these factors. They find significant statistical correlations between various environmental pollutants, as well as stressful events, and the sex ratio of babies born.
The team scanned datasets and recorded health statistics from the two countries, including information on the birth date, geographic distribution, and biological mothers of the babies born in the time ranges specified. Measurements of environmental pollutants were provided by the US Environmental Protection Agency, which organized the data by US county, and the Swedish Meteorological and Hydrological Institute and Statistics.
Based on analyses of correlations between environmental pollutants on the sex ratio of births, the scientists reported that airborne and waterborne pollutants such as aluminum, chromium, and mercury were associated with a higher proportion of male babies born. Lead seemed to show the opposite correlation, with higher levels of lead in soil correlated with a lower proportion of male babies born. Further analysis tested potential links between stressful events such as Hurricane Katrina and the Virginia Tech shooting in the US and the sex ratio, and found that only the shooting was associated significantly with a changed sex ratio—in this case, more females born than males 34 weeks after the event. No statistically significant links were found between sex ratio and other factors like temperature and crime.
University of Chicago computational geneticist and study coauthor Andrey Rzhetsky tells The Guardian “This is a list of suspects to investigate, and all the suspects have some credible evidence, but we’re very far from conviction.” Such evidence would include work on human cells and animal models. According to the authors, the limitations of the study include a lack of data on stillbirths and the use of a study population limited to people who used private medical insurance, who were not representative of the entire US population.
Gareth Nye, a University of Chester physiologist with a specialization in pregnancy who was not involved in the study, agrees in the Guardian that “without cellular research, these findings will always be associations.” He adds, however, that “There is no doubt that pollutants play roles in health and disease and that this form of computational research has a role in helping us understand why.”