Stepfathers: A Mysterious Cause of Early Menarche

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The age of menarche, or the onset of menstruation, has become earlier over time, shifting from 13.3 years for women born before 1920 to 12.4 years for women born in the 1980s. One cause of earlier menarche is the greater availability of food, which allows girls to accumulate the body fat needed to begin the reproductive process. However, researchers have identified other, more mysterious causes that may help to explain why some girls menstruate much earlier than others. One of these little-known causes, termed the male effect has been documented in both humans and non-human animals.

What is the male effect?

The male effect is typically used to describe a phenomenon among non-human animals—including baboons, mice, sheep, deer, and cattle—in which the introduction of an unrelated adult male causes pre-pubertal females to menstruate and reach puberty earlier. Although researchers rarely use the term with humans, a similar effect is found for girls who have stepfathers or live with unrelated father figures. In a study of children of twins conducted by Mendle and colleagues (2006), girls with stepfathers or who lived with unrelated adult men reached menarche earlier. Another study that tested both biological father absence and stepfather presence found that the latter was a better predictor of early menarche among girls who were separated from biological fathers. Specifically, Ellis and Garber (2003) found that the girl’s age when the unrelated man moved in was more important than her age when her biological father left in predicting her age of menarche. Furthermore, girls whose mothers reported high stress with the unrelated male partner not only menstruated earlier than those with low stress but also spent on average three more years with the unrelated father figure, indicating much higher exposure to him.

Pezibear / Pixabay

Instability in the household might trigger girls’ bodies to focus on short-term reproduction rather than long-term well-being.

Source: Pezibear / Pixabay

Why does the male effect occur?

Researchers do not know exactly why the male effect occurs. However, several different mechanisms have been proposed. The first set of explanations centers around stress. The introduction of an unrelated adult man to the living space can be stressful, which can result in weight gain and body fat, which in turn, causes early menarche. Additionally, household instability caused by the event might trigger girls’ bodies to focus on short-term reproduction rather than long-term well-being.

Second, the modeling of paternal investment, or investment by the father, may play a role. As a result of the stepfather (or unrelated father figure), girls may come to view paternal investment as relatively unimportant for reproduction, and thus engage in sexual risk-taking (a consequence of early menarche) and short-term relationships versus long-term relationships with men as a reproductive strategy.

The final explanation is simpler yet rather off-putting. The presence of an unrelated adult male might induce girls’ bodies to prepare to mate with the men. Interestingly, research on non-human animals suggests that pheromones of the unrelated adult male may cause early menstruation. Additionally, pheromones of the biological father may inhibit early menarche. Researchers have used similar explanations with humans, pointing out that pheromones from men’s axillary sweat glands affect women’s menstrual cycles in experimental studies. However, studies on human pheromones are relatively scarce and poorly conducted, and the presence of human pheromones is still debated.

Altogether, the male effect is mysterious and somewhat disconcerting. However, given the many negative consequences of early menarche for girls, such as sexual risk-taking and an increased risk of both mental and physical illnesses, future research should examine the strength of this effect and ways to reduce it.

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