From: Scientific American: What Finnish Grandmothers Reveal about Human Evolution.

No animal compares to humans when it comes to studying populations over time. Easy to track and occasionally living in relative isolation, Homo sapiens is the only species that keeps detailed records. That is why biologist Virpi Lummaa of the University of Sheffield in England started in 1998 to comb through Finnish church records from two centuries ago for clues about the influence of evolution on reproduction.

"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."

The 33-year-old Finnish biologist, aided by genealogists, has pored through centuries-old tomes (and microfiche) for birth, marriage and death records, which ended up providing glimpses of evolution at work in humanity’s recent ancestors. Among them: that male twins disrupt the mating potential of their female siblings by prenatally rendering them more masculine; mothers of sons die sooner than those of daughters, because rearing the former takes a greater toll; and grandmothers are important to the survival of grandchildren. "I’m trying to understand human reproductive behavior from an evolutionary perspective," Lummaa says.

Most recently, Lummaa and her colleagues studied the effect of males on their female twins. 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 more than 15 percent less likely to marry than those born with a sister. This impact remained the same regardless of social class and other cultural factors and even if the male twin died within three months of birth, leaving the female twin to be reared as if she was an only child, the researchers reported in Proceedings of the National Academy of Sciences USA. [continue]

Thanks to Sarah for pointing out this story.