The presence of pollutants in the air and water is associated with changes in the ratio of boys and girls born, a study has found. However, it is not clear whether these pollutants caused the change.


In some other species, environmental factors can affect the likelihood of offspring being male or female. For example, in many reptiles and fish, the incubation temperature determines the sex of the eggs.

In humans, there are many folk tales about what affects the sex of a baby, from what the expectant parents eat to how the baby is conceived. However, though there have been some small studies, not much is known for sure about what environmental factors can play a role in the baby’s sex.

The ratio of boys to girls, as assigned in the hospital at birth, is known as the sex ratio at birth (SRB). A higher SRB means more boys, and a lower SRB means more girls. In general, the SRB is slightly tilted in favour of boys, though it changes with geography and time.

Previous studies have used small datasets to look into whether pollutants, changes in weather or psychological stress could affect the SRB. In this research, a team of scientists led by Dr Andrey Rzhetsky of the University of Chicago compared records of 6 million births with environmental data from national databases. They looked at 3 million births in the USA between 2003 and 2011, and a further 3 million in Sweden from 1983 to 2013.

They found no link between the SRB and ambient temperature, seasons, unemployment rate or violent crime rate. However, some pollutants did affect the ratio, and in both directions. Iron and lead in soil brought the SRB down, whereas polychlorinated biphenyls (PCBs), mercury, carbon monoxide, aluminium in air, and chromium and arsenic in water all increased the SRB.

While the research can’t say for sure whether the pollutants are changing the ratio of boys to girls, the team say that they hope the results encourage policymakers to tackle environmental pollution.

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Reader Q&A: Are some women genetically predisposed to give birth to more boys or more girls?

Asked by: Phil Aspinall, Darlington

The effect is more obvious in men. In mammals, sperm carrying an X chromosome produce girls whereas sperm carrying a Y produce boys. So fathers with genetic defects on the X or Y tend to produce the opposite sex.

Studies reveal that in the US white fathers produce about 105 sons per 100 daughters, African American and American Indian fathers produce about 103, while older fathers produce more daughters. Other effects exist too, for example fathers with Hepatitis B have more boys.

In a sense nearly all women are predisposed to have more boys – the average sex ratio of 105 boys to 100 girls is influenced by partner choice, which will have a genetic component. So we might expect genetic effects in women too, albeit weak ones.

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Sara RigbyOnline staff writer, BBC Science Focus

Sara is the online staff writer at BBC Science Focus. She has an MPhys in mathematical physics and loves all things space, dinosaurs and dogs.