Sons are tough on their mothers. Whether it is heavier birth weights, amplified testosterone levels or simple, hair-raising high jinks, boys seem to take an extra toll on the women who gave birth to them. And by poring over Finnish church records from two centuries ago, Virpi Lummaa of the University of Sheffield in England can prove it: sons reduce a mother’s life span by an average of 34 weeks.

The 33-year-old Finnish evolutionary biologist, aided by genealogists, has scoured centuries-old tomes (and decades-old microfiche) for birth, marriage and death records—and clues about the influence of evolution on human reproduction. Historians, economists and even sociologists have long used such tactics to explore their fields, but Lummaa is among the first biologists to enlist Homo sapiens as an animal whose population can be followed over time.

After all, humans are relatively easy to track and offer the signal advantage of occasionally keeping detailed records. “I always wanted to work on primates,” Lummaa says. “But if I wanted to collect a similar data set on wild chimps, I would be struggling. I’ve decided to study another primate in the end.” Of late, one of her subjects has been premodern mothers among the Sami people of Finland, who are famous for their reindeer herding.

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.

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. Researchers have seen such hormonal influence in other animals, including lab rats and cows. When a cow has mixed-sex twins, the female is occasionally born sterile because of testosterone influence.

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. “Biological differences between male and female are not determined [simply] by the chromosomes one inherits at birth,” says anthropologist Christopher Kuzawa of Northwestern University. The sibling effects “impact reproductive success and thus have evolutionary significance.”

The results are somewhat puzzling, comments Kenneth Weiss, a biological anthropologist and geneticist at Pennsylvania State University. He notes that “if twinning is genetic, then there should be a slight selection bias against it, so that twinning would be kept rare. But some animals twin routinely.” Given the seeming conflict, he says, “there are dangers in overinterpreting ‘fitness’ effects, even if the observation is correct.”

That danger is especially acute when trying to apply the results to the present day. Access to effective birth control, an abundance of food, and low child mortality rates would all obscure the evolutionary influences seen in the preindustrial data. “It’s almost a shock when you realize that 100 to 150 years ago, 40 percent of babies died before they reached adulthood,” even when adulthood was defined as age 15, Lummaa notes.

But most of the world’s population still lives under similar conditions to those faced by preindustrial Finns. “Who gets the most kids and who puts forward the most genes are still going to be the people who make up the next generation,” Lummaa says. “There is no reason why the principles of evolution wouldn’t apply.” And she hopes to test her Finnish findings against more modern demographic information, such as the ongoing collection of health records for families in the Gambia, on Africa’s west coast.

Lummaa and her colleagues have also begun to parse the Finnish records to understand grandparents and the evolutionary conundrums they pose. Her group’s previous research has shown that grandmothers provide direct aid in ensuring the survival and reproduction of their grandchildren. The same records revealed, however, no such benefit from fathers and grandfathers. Whereas having a father around did seem to aid children in getting married earlier, a living papa did not increase the number of grandchildren.

“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.

Or, possibly, longevity in men is simply a by-product of selection for longevity in women. And it could be that because men can reproduce throughout their lives, they are less vested in anyone other than their own children; Lummaa is examining whether men continue to procreate into old age, although a monogamous culture such as Finland’s argues against it. “Men past 50 had a chance of finding someone who wanted them,” unlike a woman past childbearing years, Lummaa says. “What, if any, benefits do men get from reaching old age?”

Lummaa and her colleagues are also using the data to explore issues of class, showing that the rich in olden times produced more heirs than the poor. Populations in the richest parts of the world seem to have reversed that long-standing trend. 

“Perhaps under the current circumstances, we are investing in quality over quantity,” Lummaa speculates. “The satisfactory answers are still kind of missing.”

But the deleterious effects of males—and the benefits of grandmothers—stand out clearly. This does not bode well for Lummaa, who gave birth to a son, Eelis, in March. “I can certainly see that it’s taking a lot of energy, and I’m sure it’s aging me,” she chuckles. “How on earth these women managed to give birth every year is truly amazing.”

And certainly there will be no shortage of sons, despite their costs. “If you produce a really, really good son, he can produce a lot of offspring,” Lummaa notes—the best outcome from an evolutionary standpoint. “You might lose more by producing a son, but you might win more as well.”