Men who thought giving their moms flowers for Mother's Day was enough to show their appreciation might have another think coming. According to a report published in the current issue of the journal Science, having boys actually shortened the life span of mothers in preindustrial times. Raising daughters, in contrast, had a positive effect on a woman's longevity.

Samuli Helle of the University of Turku in Finland and his colleagues analyzed church records from 1640 to 1870 to investigate how total family size and the number of children produced affected the longevity of 375 nomadic Sami women in northern Scandivania. In order to focus on long-term effects, the researchers studied only postmenopausal women who lived to be more than 50 years of age. They found that neither the total number of children born nor the number of offspring raised to adulthood affected how long a mother would live. Bearing a son, however, significantly shortened a woman's life by an average of 34 weeks compared with giving birth to a daughter. What is more, the team determined that raising daughters to adulthood had enough of a positive effect to overcome the sons' detrimental influence.

To explain how sons shorten their mothers' lives, the scientists propose that giving birth to heavier babies was more taxing for women in preindustrial times. Moreover, carrying a boy is associated with higher levels of testosterone (which can suppress the immune system) for mothers-to-be. Grown daughters were more likely to stay within the family to help their aging mothers with everyday tasks. The findings, though intriguing, probably don't translate to modern mothers, because these effects are no doubt dampened by modern medicine and the reduced numbers of children that today's mothers bear. But it's something to keep in mind next year while shopping for Mother's Day.