The ratio of baby boys to girls born to millions of parents has changed due to pollutants and indicators of poverty.
A study of half the US population and the entire Swedish population found that pollution such as mercury, chromium and aluminum correlated with more boys being born, while lead pollution increased the proportion of girls. The sex ratio may have been affected by proximity to farming.
Measures of deprivation, such as a high number of fast food restaurants and vacant buildings, were also linked to statistically significant changes in sex ratios, as were indicators of stress, including road deaths and the Virginia Tech shootings.
The season of birth, weather temperature, rates of violent crime and unemployment did not show significant correlations.
The research only shows correlations between factors and sex ratios at birth. Future work to examine the effects of chemicals on human cells or animal models in the laboratory would be needed to show causality.
The University of Chicago led the research and said that all the suspects have some credible evidence, but they are far from conviction.
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Air pollution may be damaging every organ in the body.
Air pollution may be damaging every organ and cell in the body.
Half of the embryo should be girls and half boys at conception. Sex ratios can be skewed due to hormonal factors when more female embryos are terminated.
There are many suggested factors, such as stress or something in the environment. It makes sense that it can go up and down because of the differences between male and female embryos. They have different hormones.
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, and some statements are not founded in observations at all.
The first investigation of chemical pollutants and other environmental factors using large datasets from two continents was published in the journal. It used data on 150 million people in the US over eight years and 9 million people in Sweden over 30 years.
Sex ratio shifted by up to 3% due to factors such as mercury pollution and proximity to industrial plants. In a population of 1 million, that would mean 60,000 more girls than boys. The impact on sex ratio of toxic pollutants called PCBs was found to increase the number of boys.
The researchers looked at two high stress events in the US, the Virginia Tech shootings and Hurricane Katrina. The sex ratio went up after the Virginia Tech shootings, with more girls born. To investigate this further, studies would need to focus on those most affected by the events.
The size of the populations analysed 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.
The University of Chester's expert in pregnancy, who was not part of the study team, said that pollutants may influence the very early stage of fertilisation, leading to one sex direction over another.
Computational research has a role in helping us understand why pollutants play a role in health and disease. The number of US births was relatively small, with the total over 8 years less than the annual average of 3.8m births.
