If you have an older brother, odds are slightly greater you won’t be having kids, especially if you live in pre-industrial Finland. According to a new study, which examined the birth, marriage, and death records of three generations of pre-industrial Finlanders, found that those with older brothers were 5% less likely to become parents than those with older sisters.
That disparity applied to both men and women, which researchers believe indicates that the disparity has a biological basis rather than simply a cultural one. It’s possible the stress of carrying a boy damages a woman’s womb, slightly affecting the next child’s health. Those effects could be “spread over a variety of physiological systems,” they said. A previous study, for example, shows that children with older brothers tend to be shorter.