Hundreds of millions of years of evolution have given us one great reproductive insight: Incest is icky.

Inbreeding is the king of evolutionary no-no’s. Related parents are more likely to have similar genetic mutations, so children of incest are at a high risk for birth defects and disease.

A natural aversion to incest is not the only mechanism that may have evolved to prevent inbreeding. A new paper published in the American Journal of Human Biology concludes that having a male in a household can affect the timing of menarche, when a young woman first gets her period.
The Pennsylvania State University team who performed the study attributes the effect to pheromones that evolved to thwart inbreeding. Yes: In a sense, the constant smell of dad is enough to stall puberty.

The researchers asked nearly 2,000 college-aged women when they began menstruating as well as questions about the makeup of their families. They found that when no father was present in a household, girls started to menstruate significantly earlier, on average, than girls raised with a father.

“We are not saying that if daughters mature early with the biological father present, inbreeding will inevitably result,” said lead author Robert Matchock, a psychologist at Pennsylvania State University, via e-mail. “Rather, over millions of years of primate evolution, females who did delay sexual maturation might be at a slight advantage because any offspring that they produced would have a reduced chance of having deleterious recessive mutations.”

The power of the pheromone to stymie inbreeding extends to a control of sexual desire, Matchock said.

Some studies have shown that daughters are averse to the smell of their fathers.

“Olfactory cues are known to also operate, largely unconsciously, in guiding human mate choice and in sexual arousal, in male-male competitive encounters, and in mother-child identification,” said Glenn Weisfeld, a human ethologist at Wayne State University.

But Weisfeld said that the authors do not conclusively demonstrate that these changes in timing of menarche are due specifically to pheromones. Merely showing a statistical relationship between father absence and age of first menstruation is not enough, he said.

“There would have to be some measure of response to pheromones, such as demonstrating a specific aversion or attraction to them, or detection of them,” he said. “The reasoning here is circular.”

Matchock says the behavior of other species supports his pheromone thesis. Past studies have shown that young, female prairie dogs are less likely to go into heat if their fathers live with their family groups. Similarly, female tamarins exhibit more sexual behavior when they are removed from their natal group.

Matchock said his team’s results for half-brothers and step-brothers—their presence associated with early menarche—were consistent with the “male effect” observed in animals: Exposure to genetically unrelated males triggers earlier sexual maturation in females.

In addition, the authors observed that the number of sisters, especially older sisters, correlated with delayed menarche. Mammals frequently experience delayed sexual maturation when same-sex competitors are present because the chances of mating are reduced. For instance, according to Matchock, grouped female mice often experience delayed sexual maturation, and this mechanism is clearly pheromone-based.

“The take-home message is that we are mammals,” Matchock said. “The underlying biological mechanisms that mediate physiology and behavior in other mammals probably apply to humans as well.”

Originally published September 26, 2006


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