Analysis of the birth records of the Sami people of Finland by Virpi Lummaa has yielded fascinating information about the detrimental effects that bearing sons have on their mothers. Scientific American reports on Lummaa’s findings:
those who bore sons had shorter life spans than those who gave birth to daughters. This discrepancy has to do with birth weight—male babies are typically larger—as well as testosterone. “Testosterone can compromise your immune system; it can affect your health,” Lummaa says, and the mothers of sons proved especially susceptible to endemic infectious disease, such as tuberculosis. “Boys are a little bit more costly” to raise than girls as well, because they drain more physical resources from their mothers, she adds, as has been seen in other mammals, such as the red deer. Sons also are not as likely as daughters to stick around to help their mothers out later in life.
Unfortunately, the havoc that sons cause is not limited in impact to the mother:
More recently, Lummaa and her colleagues have been studying how sons are not just tough on their mothers but also hard on their siblings. Those born after a son were physically slighter, had smaller families and generally had a greater chance of dying from an infectious disease. The effects held up whether the elder brother died in childhood or not, suggesting that the negative outcome is not a result of some direct sibling interaction, such as competition for food, regular beatings or the practice of primogeniture, in which the eldest brother inherits everything. “Big brothers are bad for you,” Lummaa explains. “If the fifth-born was a male, then the sixth-born is doing worse.”
This phenomenon is particularly evident in twins where one is male and the other is female. Of 754 twins born between 1734 and 1888 in five towns in rural Finland, girls from mixed-gender pairs proved 25 percent less likely to have children, had at least two fewer children, and were about 15 percent less likely to marry than those born with a sister. This brotherly influence remained the same regardless of social class or other cultural factors and even endured if the male twin died within three months of birth, leaving the female twin to be reared as an only child.
The reason that the female half suffers, Lummaa speculates, is because of testosterone exposure in the womb. . . . Whatever the cause, there is no question of the outcome: mothers of opposite-sex twins end up with 19 percent fewer grandchildren than moms of same-sex twins, meaning evolution would seem to favor the latter.