Pollution, income & more: sex ratio of babies skewed by many factors

What factors affect the ratio of baby boys to girls born to parents in a region? A study analysing half the American and the entire Swedish population offers clues.

Examining over 100 factors, the research shows that pollutants in the air and income levels may be at play. They found that mercury, chromium and aluminium pollution was linked to more boy babies while lead pollution increased the proportion of girls.

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A high number of fast-food restaurants and vacant buildings, both of which were labelled as measures of low-income levels, also had a statistically significant effect. Highly stressful events also had some impact on birth sex ratios in the study published in the journal Plos Computational Biology.

In contrast, factors such as season of birth, weather temperature, rates of violent crime and unemployment, showed no significant correlations. The team also cautioned that their findings demonstrate only correlations and not a direct causal relationship between the factors with the sex ratio at birth.