What might explain these findings? We know that the age of menses is at least partly genetic. But higher body fat is also believed to hasten puberty—and girls raised by single mothers, often struggling financially, may have less healthy diets. Moreover, in many animals, exposure to the sex pheromones of a genetically related older male (such as a father) suppresses a young female's pubertal development—probably due to an evolutionary adaptation to prevent inbreeding. Conversely, the presence of an unrelated male tends to speed up maturation. A study of more than 1,100 girls suggested this might also be true in humans. Those with stepfathers in the household matured faster than those without. And the younger the girl when her biological father left home, the earlier she began to menstruate. In terms of how much earlier, the actual effect was fairly small: Girls without fathers at home began to menstruate four to five months before those who lived with both parents. But considering that early pubertal development is associated with a host of negative outcomes—teenage sexual activity and pregnancy, substance abuse, depression, anxiety, poorer performance in school—"small effects," says University of London professor Jay Belsky, PhD, a leading child development specialist, "can have big implications."