All sons know that we get more flak than daughters. Does “You’ve taken years off my life” or “Why can’t you be more like your sister?” sound familiar?

Unfortunately, a new study, published this week in Science, shows that these refrains may hold an uncomfortable truth. In preindustrial times, sons really did have adverse effects on mothers. And worse news still for male children is that daughters helped to offset the negative effects of their male siblings.

Scientists studying Finnish church records have discovered that there was a biological cost to having sons — each son took an average of 34 weeks off a mother’s life. The effect is cumulative: more sons = shorter life span.

A group of researchers led by Samuli Helle of the University of Turku in Finland used the church records to explore questions about the evolution of human populations. Animal and plant studies can be difficult enough, but work on human evolutionary biology is usually complicated by modern innovations and medicine.

In preindustrial societies, however, the distorting effect of such factors is weaker. Until the early 20th century, Finland was preindustrial; its mortality rates were “raw,” the product of natural selection, unaffected by things like hot running water, central heating and advanced health care.

What’s more, clergymen from Finland’s Lutheran church meticulously recorded census data, births and baptisms, and marriage and death/burial registers for each Finnish parish from 1640 to 1870. The records even documented the same data for the aboriginal people living in Finland, the Sami. The Sami data set is unique and perfect for an evolutionary study.

“These records are amazing,” said co-author Virpi Lummaa, of the University of Cambridge, in an e-mail interview. “Thanks to the greedy kings of Sweden, who wanted records of everyone capable of paying tax, there are detailed historical records of everyone in Scandinavia for this old period.”

“The Sami people we studied differed significantly from 18th-century Finns,” said Helle in a separate interview, “and they cannot be regarded as typical Finns of that period. For example, the Sami had their own distinct genetic history and are quite distantly related to Finns and other Europeans.”

That’s because the Sami were around before Scandinavia was colonized, before borders were drawn. They inhabit areas from central Norway to Sweden, through the northern part of Finland (6,500 of them live there today) and into the Kola Peninsula of Russia. “In contrast to the Finns, who mainly practiced agriculture,” said Helle, “Sami were mainly nomadic reindeer hunters, and also practiced hunting and fishing. They lived in small villages, and some of them followed the seasonal migration of reindeer herds.”

Scientists already know that sons exact a higher price on mothers, physiologically, than do daughters. Male embryos grow faster, and when they are born they are heavier than females. But the long-term effects of having sons remained unknown until Helle and her colleagues went through the Sami data.

The Finnish scientists found that the life span of mothers was not related to the total number of children they had. Instead, maternal life expectancy decreased with each son born and raised to maturity. The number of daughters born did not have a statistically significant effect, but the number of daughters raised to adulthood had a positive effect on the mother’s life span that outweighed the negative effect of having sons.

“It is predicted that boys would be more costly than girls,” said Helle, “but we did not suppose that girls would be so beneficial for their mother’s longevity, since the production of girls also imposes costs on the mother.”

Girls, the scientists say, help their mothers in everyday tasks — and this prolongs the mothers’ life spans. But why do sons have such a negative effect?

“Pregnancies with male fetuses have elevated maternal testosterone levels,” said Helle. Testosterone is known to reduce the effectiveness of the immune system, so “if testosterone has a negative effect on the mother’s immune defense ability, the production of many boys may have a detrimental effect in the long run.”

But the question all mothers — and their sons — will want to know: Is the price of bearing and raising a son as expensive now, in modern societies?

“Probably not,” said Helle, “because living conditions have much improved since the 18th century, and things like food scarcity probably don’t limit life span in today’s industrial societies. But I don’t know if anyone has yet looked at this in modern humans, so it might be too early to say anything for sure . . .”

It seems, then, that it’s safe to have sons. Anyone who wants to have one might like to follow guidelines from traditional Finnish folklore. Couples who wished to have a son would put a hammer or an ax under the bed. To further boost the chance of a son, fur gloves and hats were worn during the sex act. Men unable to grow a good beard, by the way, were said to father only female offspring.

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