From an evolutionary perspective, making babies with a closely related family member is a cardinal sin.

A new study suggests that like many animals, humans have evolved built-in mechanisms to help keep this from happening.

The study finds that the absence of a father, the presence of half- and step-brothers and living in an urban environment are all associated with the earlier onset of a girl's first period, known as menarche.

Meanwhile, the presence of sisters in the household while growing up has the opposite effect.

The researchers speculate that the findings are part of an evolutionary strategy to prevent inbreeding and that it is regulated by chemical signals, called pheromones, that influence our behavior without our knowing it.

Why inbreeding is bad

Menarche can occur anytime between age 8 and 16, but usually takes place around age 12. It signals the looming onset of fertility.

Inbreeding is breeding between close relatives. If practiced repeatedly within a population, it creates a situation in which the genes among individuals are overly similar — that is, the genetic diversity of the population is reduced.

This is bad because the onset of a sudden threat, such as a deadly virus, can exploit a common weakness and wipe out an entire population.

Inbred individuals are also more likely to have physical and health defects, as well as lower levels of fertility.

The study, which is one of the largest and most comprehensive of its kind, will be detailed in an upcoming issue of the American Journal of Human Biology.

Unexpected findings

Pennsylvania State University researchers Robert Matchock and Elizabeth Susman analyzed questionnaires collected from nearly 2,000 female university students.

The women were asked about their families and the environments they grew up in as well as the age at which they experienced menarche.

The participants were around 20 years of age on average when the study was conducted and the majority of them, about 87 percent, were Caucasian.

Out of the group, 326 of the women reported growing up in families without fathers for most of their lives, 587 had at least one older sister and 200 grew up in urban environments.

Contrary to what was predicted, having a stepfather in the house was not associated with a delay of menarche.

One possible explanation, which comes from animal studies, is that female sexual maturation speeds up in the presence of any unrelated male who is fertile.

Also surprising was the fact that the presence of half-brothers did not delay sexual maturation.

If the goal is to prevent inbreeding, having sex with a half-sibling who shares 50 percent of your genes might also seem like a bad idea.

However, Matchock speculates that half-brothers are "genetically different enough so as not to activate some of these anti-inbreeding strategies."

Other research has also shown that people tend to pick mating partners who are like themselves but not too similar, Matchock said.

City life and older sisters

The finding that urban girls had earlier ages of menarche than girls who grew up in rural areas is consistent with past research, but the cause for the discrepancy is still unknown.

One possibility is that city life provides more opportunities for social interactions outside the home and thus more opportunities for sex with members outside the immediate family.

Contrary to previous studies, neither the total number of siblings nor the numbers of brothers were associated with later onset of menarche.

The presence of sisters, and older sisters in particular, did delay menarche, however.

"It may be a signal that indicates that the population is very high [or] it may have something to do with the menstrual synchronization which we see in sisters living together," Matchock told LiveScience.

The marked effect of older sisters is not surprising, the researchers say, since older sisters would presumably be more dominant and more active when it comes to exuding pheromones.

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