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.
Suppose male births are less common than female
- 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
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
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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