If you’ve ever thought, while chasing after your son, “This kid will be the death of me,” well…you may just be right! This according to Virpi Lummaa, a Finnish evolutionary biologist who searches through centuries-old birth, marriage and death records looking for patterns. What she’s learned from studying the premodern Sami people of Finland:

Among this group, she found that 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.

Boys may also be costly for their younger siblings. Those born after a son tend to be physically slighter, have smaller families, and are more likely to die from infectious diseases. These results held even if the older brother died in childbirth.

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.

In these cases, Dr. Lummaa believes that testosterone exposure in the womb is to blame.

Another odd finding: While the presence of a grandmother in the family home is associated with the survival and later reproduction of her grandchildren, grandpa provides no such service:

“If anything there’s a negative effect,” she concludes. This could be because of the cultural tradition of catering to men, particularly old men. “Maybe if you had an old grandpa, he was eating your food,” she speculates.

Of course, these findings are based on people who lived before modern medicine, and it’s rare in the developed world to find a grandfather wrestling a chicken nugget from a toddler. Interesting, nonetheless.

Gotta go, the boys are tearing the house apart…

Posted by MommaSteph.