Typically, there are more male babies born than females, with the global average lying at 105 boys born for every 100 girls. Although more males are born almost universally, there are some incidences of higher number of females being born too. But why is this, and how does it happen?
Some places in the world seem to have a particularly high share of girls. Although more boys are still born in places, there are significantly more girls than average. Rwanda for example, is the country with the lowest male-female birth ratio, with just 101 boys born for every girl. Similar figures also exist in other Sub-Saharan countries such as the Ivory Coast (102) and Togo (102) (Livingston: 2013).
But why is this? One reason for more females being born than males in these countries may be related to the practice of polygamy, or having multiple wives. With fewer women being sexually available for men, a more practical way to ensure the passing on of genes would be to produce more female offspring than male.
Another theory is that women living in harsh conditions with fewer resources suppress the survival of boys in the womb. As male fetuses are more likely to be miscarried or stillborn than female fetuses and, once born, tend to suffer from more fatal diseases, and take more mortal risks than females, their chance of making it to breeding age is less (MinuteEarth: 2014). This means that a more secure way to ensure the passing of genes to future generations would again be via producing girls, and not boys.
However, there may be other explanations for a higher likelihood of bearing female. In industrialised countries for example, the ratio of males to females being born has been decreasing since the 1950’s. For a time, it was presumed that older parents had reduced chances of producing sons. Although there may be some relationship between age and the sex of children born, a study conducted in Denmark found that 51.6% of children born to fathers younger than 25 were male, whereas 51% of children born to fathers over 40 were male. Thus, it is unlikely that the declining male-female ratio of births in industrialized countries is solely reliant on this factor (Weisskopf: 2004).
Perhaps a larger factor influencing the birth ratio of males to females in these countries are chemicals. For example, studies have found that prospective mothers taking clomiphene citrate (Clomid) for infertility bore male babies only 48.5% of the time. Meanwhile, a study looking at workers producing 1,2-dibromo-3-chloropropane (DBCP), a chemical that kills worms in agriculture, saw even larger decreases in the number of male babies born. While exposure to the chemical suppressed sperm quality to the point of being unable to pass offspring, after exposure to the substance ended, 36 children were born to 44 of the workers. Yet, out of these 36 children, only 10 were boys, meaning that the male-female birth ratio here was just 27.8% (Davis: 1998).
Such incidences of chemical exposures seem to have affected the birth of boys on larger scales too. For example, similar decreases in the male-female birth ratio occurred from fathers’ exposure to dioxin and dioxin-like chemicals after an explosion in an herbicide factory in Seveso, Italy, in 1976, and a contamination of rice oil used in Taiwan. For fathers who were under 19 during the incident in Seveso, their male-female birth ratio decreased to just 38.2%, while for fathers exposed to the rice oil contamination aged under 20, experienced a male-female birth ratio of just 45.8%. This means that chemical exposure, especially at a young age, seems to have a large influence on the incidence of boys or girls being born. The reasons for this however require further research.
To conclude, many factors influence whether male or female babies are born. Although the natural bias may be towards more males being born, under certain environmental conditions, such as strained resources or chemical interference with male gonads, more females may be born as they are more likely to survive to breeding age than males. The exact mechanisms behind this phenomenon however require further research.
Livingston, Gretchen: Pew Research Center
Weisskopf, Marc: Scientific American
Davis, Devra: JAMA