I heard that first-born males are more likely in younger couples who have sex frequently, because male sperm are more likely to reach the egg just after ovulation. Is there any truth to this, and if so, how is sex ratio maintained?
In humans, as in most sexual species, the ratio of males to females at birth is very close to 1:1, with a very slight bias toward males of about 1.05:1. Statistics collected on sex ratios at birth in the United States from 1940 to 2002 suggest that this ratio is remarkably stable in terms of factors such as age of mother and birth order (1). Age of mother has a very small effect on the male-to-female ratio, which trends very slightly downward from 1.05:1 in mothers under age 15 to 1.039:1 in mothers over age 45. A very slight downward trend is also found in the order of birth, from 1.057:1 in firstborns to 1.031:1 in children born 8th or after. It is not fully understood why there are slightly more males at birth, and very slightly more born to younger mothers and as firstborns, although many studies and speculations have been made. Nevertheless, this initial male bias is very small.
Recently, the availability of sex-selective abortion and pre-implantation sex-selection techniques has enabled the distortion of sex ratios, particularly in cultures where sons are preferred (2). However, in the absence of manipulation, evolutionary theory predicts that a 1:1 sex ratio is an evolutionarily stable strategy. Imagine if more males than females are born. It follows that females will have better mating prospects and can expect to produce more offspring. Therefore, parents who are genetically disposed to produce females will have more grandchildren. In turn, the genes for producing more females are spread, and female births become more common, that is, until the 1:1 sex ratio is attained and females no longer have better mating prospects. The same reasoning applies if there are more females than males. Thus, in most sexual species, the two sexes are usually produced in approximately equal numbers (3).
1. Tj, M., and Be, H. 2005. National Vital Statistics Reports, 53(20).2. Hesketh, T., and Xing, Z.W. 2006. Abnormal sex ratios in human populations: causes and consequences. Proceedings of the National Academy of Sciences of the United States of America, 103:13271–13275.
3. Hamilton, W. D. 1967. Extraordinary sex ratios. Science, 156:477–488.