Does losing a parent make children more sexually precocious? Some scientists think so, based on evolutionary arguments. But one of the largest studies of its kind has found that the effect depends on how the parent was lost.

Evolutionary psychologists predict that children who lose a parent will develop faster. The loss is a signal that the road ahead is difficult, so an optimal reproductive strategy should get triggered: Have as many kids as early as you can, because you might die early like your parents did. The strategy is a type of “predictive-adaptive response,” examples of which are seen elsewhere in the animal kingdom. For example, some mammal babies develop thicker fur if their mothers have spent less time in the sun.

If there really is a predictive-adaptive response related to parental loss and sexual activity in humans, it's hard to prove. Dozens of studies have found a positive correlation between losing a parent to divorce or abandonment—the vast majority being the father—and daughters starting menstruation earlier, having sexual encounters earlier, and ultimately giving birth to more children. (The sexual development and activity of girls is easier to track than boys because of menstruation and childbirth.) But it is difficult to separate out the environmental factors such as violence, toxins, or malnutrition—all of which children are likely to share with their parents. What’s more, there may be genetic factors that make parents more likely to abandon their family, which their children also carry, perhaps having effects on their sexual behavior.

To get around these confounding factors, Peeter Hõrak and Markus Valge, a biologist and psychologist, respectively, at the University of Tartu in Estonia, turned to a natural experiment. A biologist named Juhan Aul started collecting meticulous data about the development of his fellow Estonians in the 1930s. He ultimately tracked the development of some 50,000 people. After World War II broke out, many of the girls in the study lost their fathers to the fighting. Aul passed away in 1994, leaving his treasure trove of data for science.

"There is still a lot to discover," Hõrak says. "Since the names and birth dates of these persons were recorded in the original study, [we were] able to identify most of the subjects." It was then just a matter of looking them up in Estonia's population registry, which records the number of children. Hõrak and Valge narrowed it down to 1678 girls, 12% of whose fathers died during the war. If there really is a predictive-adaptive response, then the bereaved girls should go into puberty earlier and have more children over the course of their life, compared with Estonian girls of the same age who did not lose their fathers.

The results were a resounding defeat for the theory. Girls whose fathers were killed did not go into puberty earlier, nor did they have their first child any earlier, Hõrak and Valge report online today in Biology Letters. Indeed, contrary to the prediction, the bereaved girls had slightly fewer children over the course of their life—1.43 on average compared with 1.73. And they were 7% more likely to remain childless.

The outcome is probably not due to malnutrition. Hõrak points out that the bereaved girls grew just as quickly to the same height and weight on average as the rest. "We can thus reject the possibility that girls with dead fathers were short of bodily resources for speeding up their maturation."

So if losing a parent through divorce really does cause girls to become sexually precocious, as dozens of studies have suggested, why is death different? One possibility is that there is simply no evolutionary effect at play. Hõrak notes that losing a father through death, more so than divorce, has an economic impact on the entire family, and having children is expensive. Perhaps the bereaved women were simply making sensible life choices.

But researchers who favor evolutionary explanations aren't giving up yet. According to an idea proposed in 1982, divorce and death might signal different reproductive strategies in Western culture: In divorce, the child may get the signal that parental bonding and investment in family is less important, whereas in death, the grieving widow may signal the opposite. If there is a predictive-adaptive response, says Bruce Ellis, an evolutionary developmental psychologist at the University of Arizona, Tucson, then "father absence due to separation or divorce tends to accelerate reproductive strategies while father absence due to death tends to have the opposite effect, resulting in later onset of sex and reproduction," he says. The Estonian study "nicely converges with this original theory."