Are Pollution and Poverty Altering the Sex Ratio of Our Babies?

“There are a lot of myths about sex ratio and birth, but when you dig into the research, it turns out that everything that was tested on real data was done on relatively small samples [risking spurious correlations], and some statements are not founded in observations at all,” he said.

The new research, published in the journal PLOS Computational Biology, is the first investigation of numerous chemical pollutants and other environmental factors using large datasets from two continents. It used data on 150 million people in the US over eight years, and data on 9 million Swedish people over 30 years.

The factors that linked to significant changes, such as mercury pollution and proximity to industrial plants, saw the sex ratio shift by up to 3 percent. In a population of 1 million, that would mean 60,000 more girls than boys, or vice versa. Previous research on the impact on sex ratio of toxic pollutants called PCBs were inconclusive, but the new analysis found it significantly increased the number of boys.

The researchers also examined two highly stressful events in the US: Hurricane Katrina in 2005 and the Virginia Tech shootings in 2007. They only found a significant change in the sex ratio 34 weeks after the Virginia Tech shootings, with more girls born. To investigate this further, Rzhetsky said, studies would need to focus on those most affected by the events, rather than whole populations.

The size of the populations analyzed meant the study had strong statistical power, but the scientists noted they did not have data on the sex of stillborn babies and that the US population studied all had private medical insurance, and so were not fully representative of the entire population.

Gareth Nye, an expert in pregnancy at the University of Chester, UK, who was not part of the study team, said: “The thinking is that pollutants may influence the very early stage of fertilization, leading to one sex direction over another. [But] without cellular research, these will always be associations.”

“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,” he said. But he said that, while the populations studied were large, the number of US births was relatively small, with the total over 8 years less than the annual average of 3.8 million births: “This doesn’t necessarily mean [the study is] of lower quality, but does indicate the conclusions may not be as certain.”

Correction: An earlier version of this story ran an inaccurate byline. The writer is Damian Carrington, the Guardian’s environment editor.