No-one knows why this is, but one theory is that with each malepregnancy, a woman's body forms an immune reaction to proteins that have a role in the development of the male brain.
Research has shown that teenage girls whose mothers took male hormones during pregnancy perform better in academic disciplines than other females, and are more likely to get into university.
Being male, testing positive for allergic sensitivity, having a mother with asthma, and having a mother who smoked during pregnancy were all associated with an increased risk for wheezing at age 5.