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I wonder why there are more boys than girls in different parts of the world?
Question Date: 2016-08-31
Answer 1:

Thanks for your great question! Given you are wondering why there are more boys than girls in some places, it seems you already know that a 1:1 ratio of boys to girls is thought to be the ideal equilibrium maintained in human populations from an evolutionary standpoint. This concept is known as Fisher’s Principle. To understand its reasoning, consider this scenario presented by the evolutionary biologist W.D. Hamilton:

Suppose male births are less common than female births:
- A male has better mating prospects than a female and can therefore expect to have more offspring.
- Parents genetically disposed to producing males would then have more offspring, and the genes for male-producing tendencies would spread.
- Eventually, male births would become more common than female births.
- But as the 1:1 ratio is approached, the advantage of producing males disappears.
- Males and females now have equal mating prospects, and so a 1:1 equilibrium is maintained.

However, this principal considers the advantage of being male or female during reproductive years. For a 1:1 sex ratio to be achieved in reproductive humans (aged, say, between 15 and 50), an evolutionary stable sex ratio at birth like the one Hamilton described assumes an equal probability of males and females surviving to a reproductive age.

It turns out that the probability of death for men is higher than women for reasons like war casualties (many more soldiers are male than female), more aggressive/dangerous/violent behavior, and a higher likelihood of experiencing life-threatening health complications like heart attacks and strokes. Consequently, the ratio of males to females tends to decrease as the age of a population increases.

Therefore, for an approximately 1:1 sex ratio to be achieved in a population of reproductive adults, more boys must be born than girls. That is why the sex ratio for the worldwide population is 1.01 (101 boys for every 100 girls), but the worldwide sex ratio at birth is 1.06 (106 boys for every 100 girls).

Your question alluded to the fact that sex ratios vary by country, ethnicity, cultural values around gender, socio-economic status, the environment (e.g., climate or pollution), and many other factors. While these factors can be correlated with different sex ratios, it is difficult for scientists to determine which factors actually cause differences in sex ratios based on census data alone. Current research focused on when and how the human body alters the likelihood of giving birth to a male vs. a female will hopefully provide clues to the underlying causes of shifts in sex ratios.

Fisher, R.A. 1930. The genetical theory of natural selection, Clarendon Press, Oxford
Hamilton, W.D. 1967. Extraordinary sex ratios. Science, 156: 477-488.
Grech, V., C. Savona-Ventura, C, P. Vassallo-Agius. 2002. Unexplained differences in sex ratiosat birth in Europe and North America". BMJ, NCBI/National Institutes of Health, 324: 1010–1.

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