Why There's a Boy for Every Girl - the 1:1 Sex Ratio

Most animals have a sex ratio that is very close to 50% males and 50% females (a 1:1 sex ratio).  Why is that?  The simple answer was hinted at by Charles Darwin about 150 years ago and then formalized by theorist Ronald A. Fisher in the mid 20th Century. 

Lets imagine an animal population where there are 9 males for every female (a 9:1 sex ratio).  We’ll start with 10 females and 90 males and assume each female can lay 10 eggs.  If every female breeds and lays 10 eggs, the result is 100 offspring – 10 females and 90 males.  So the average number of offspring in this population is 10 for females (100/10) but only 1.1 for males (100/90).

A normal female will have one daughter who has ten offspring, and 9 sons, who each have 1.1 offspring.  So her genes, programmed for a 9:1 sex ratio, will be passed on to 20 grandchildren.

Now, what if a female has a DNA mutation that leads her to lay 4 male eggs for every female egg (4:1 sex ratio) instead of 9 male eggs for every female egg.  When she lays her 10 eggs, she’ll have two daughters and eight sons.  Each daughter will have 10 offspring, for a total of 20, and each son will have 1.1 offspring, on average, for a total of about 9.  So the genes of the female who has 2 daughters and eight sons will be passed on to about 29 grandchildren in total.  Over time, the genes that lead to a 4:1 sex ratio will increase compared to normal 9:1 genes.

There is a very strong advantage for genes that lead to having daughters.   

In fact, if another female has a DNA mutation that leads to a 1:1 sex ratio, she’ll have five daughters and five sons.  Each daughter will have 10 offspring, and each son 1.1 offspring, for a total of 55 grandchildren with her genes, compared to only 20 for the female who lays a 9:1 sex ratio.

As the genes with a 1:1 sex ratio spread, and the population becomes more evenly balanced, the advantage disappears.  Eventually, if there are 50 females and 50 males, the average number of offspring becomes the same.  There are 50x10=500 offspring.  The average female has 10 offspring and the average male now also has 10 offspring (500/50).  At this point, the advantage of having extra daughters disappears and the sex ratio stabilizes at 1:1.

This analysis works exactly the same if you start with a 1:9 sex ratio, 1 male to 9 females.  In the beginning, the 1 male passes his genes on to an average of 90 offspring, whereas each female has an average of only 10.  So a female that had more male offspring would pass her genes on to far more grandchildren than one who had the 1:9 ratio.  Mutations that led to more males would be favored until, again, the sex ratio is 1:1.

This theory seems to apply fairly universally across the animal kingdom.  There are notable exceptions, which typically involve inbreeding, where it can be advantageous to have more daughters than sons.  But in most cases the sex ratio at breeding age is very close to the predicted 1:1.