Menarche, a girl’s first period. The event that marks the beginning of a girl’s womanhood, sexuality and fertility. Right? Well, not quite.

Before getting into the evolutionary arguments about the adaptiveness of the preference for adolescent girls I want to first clear up some misconceptions about menarche and it’s significance to the human mating system and the issue of the fall in the age of menarche since the nineteenth century.

Old enough to bleed, old enough to breed?

It’s a common belief that a girl’s first period marks the beginning of her fertility and signals her readiness to start reproducing. This isn’t quite true. Girls don’t become able to conceive until their ovaries start releasing eggs (ovulating) and this doesn’t typically start until about 3 years after menarche. There have also been rare cases in which a girl has got pregnant before her first period. Since menarche represents the end of a girl’s first menstrual cycle if an egg was released in that first cycle it’s possible for her to conceive before having had her first menstrual bleeding.

The natural age of consent?

Another misconception is that menarche represents a kind of natural age of consent and marks a girl’s physical readiness to have sex. Although many primitive foraging societies do have taboos against having sex with a girl before she starts menstruating these rules are really just as much social inventions as the age of consent in our societies rather than a part of the natural human mating system. Because primitive societies don’t have a number system they can’t have a precise age of consent like we do. Instead menarche is often used as a marker of a girl’s sexual maturity and taken to define the age it becomes acceptable to have sex with her. Taboos against sex with pre-menarche girls aren’t universal either. In the Ache tribe, which were a very primitive people when they were contacted in the 1960s, all girls would lose their virginity before menarche. By the the time they started menstruating at about 14 most girls would have already had sex with several adult men in the tribe. Some tribes don’t pay much attention to menarche and some even believe that having sex with a girl is what causes her periods to start. It’s common for a girl to bleed a bit the first time she has intercourse and it’s easy to see that menstruation could be believed to be some kind of continuation of that first bleeding.

These taboos are often violated too just like the age of consent laws in our societies. Every society has taboos and ideals about how people should and shouldn’t behave but the way that the people really behave is often a bit different. In 1930s England, for example, it was shameful for a woman to have sex before wedlock. To get an idea of how often this taboo was violated studies have been done looking at the difference between the ages that women at the time got married and when they had their first baby. In about half the cases the difference between the two was less than nine months. At a time it was taboo and extremely shameful for a woman to have sex before she got married most of them were doing it anyway and half the brides were walking up the aisle pregnant. Similarly, in the Yanomamo tribe it’s taboo for a girl to have sex before she has menarche and gets officially married which typically occurs at about 13-14 but the anthropologist Napoleon Chagnon found that about 2/3 of Yanomamo girls become sexually active before then at about 12. This figure is probably an underestimation too since a certain amount of sex goes on in secret. It’s my opinion that about 12 is really the age that girls have evolved to become sexually active whether they’ve started menstruating or not.

Something else that needs to be appreciated is that primitive foragers without governments to control them are usually pretty savage and it’s kind of naive to believe the men are always so opposed to having sex with girls before menarche. The men in these societies beat their wives and burn them with firewood, kidnap teenage girls from other tribes and gang-rape them for several days before forcing them into a marriage, raid their neighbours and commit mass-murder, and sometimes engage in cannibalism, but when it comes to consummating their marriages with their child brides all of a sudden we’re supposed to believe that they’re such good noble men who always wait until after menarche? Some Native American peoples would kidnap kids from other tribes, tie them to trees and burn them alive. It was fine for men to do that but having consensual sex with a girl shortly before menarche was strictly forbidden? Let’s be realistic. In societies this savage I don’t think they’re going to be too bothered by someone having sex with a girl before she starts menstruating. What’s so bad about doing that anyway? If she’s come up to that age and she’s horny and ready to start having sex then so what if she hasn’t had her first period, what does it matter? They may have some ideal that men shouldn’t have sex with their wives until menarche but that’s all it really is, an ideal not some “sacred law of the tribe”.

If the onset of menstruation was when mating was supposed to begin then we’d expect girls’ libidos to suddenly kick in at menarche and possibly for girls to physiologically advertise that they’ve started menstruating. But we see no such things. One month before menarche, one month after a girl looks and behaves the same. You can’t tell just by looking if a girl has started menstruating, it’s not accompanied by any changes in behaviour and it’s not physiologically advertised at all. Likewise, the onset of ovulation (and consequently the onset of fertility) isn’t advertised or accompanied by any changes in behaviour either. Because the human mating system involves long-term relationships in which copulation takes place regularly it’s not necessary for females to advertise, or the males to know, the precise age at which they become fertile. What matters instead is a female’s total future fertility and the amount of offspring a man can expect to get from her over the long-term.

We can disprove the “menarche = ready to mate” misconception further with some comparisons to our ape relatives. In chimps, bonobos, orangutans, and gorillas the females don’t only become sexually active after menarche but often just before it too. In chimps the ages of menarche and the onset of mating are both about 10 but these are just averages. A chimp female may become sexually active at 9 and start menstruating at 11, or vice-versa. The ages of menarche and the onset of mating form two largely independent distributions that happen to have roughly the same mean. Menarche doesn’t always precede the onset of mating and I believe the same must have been true for ancestral humans.

Menarche is not abnormally early today

It’s often pointed out that girls in nineteenth century Europe didn’t have their first period until about 16 and that since then the age has steadily fallen to the current value of about 13. The inference many people make from this is that menarche is happening abnormally early today. But this actually gets it backward. It’s not that menarche is abnormally early today but rather back then it was abnormally late due to poor living conditions and diets. Since then living conditions and nutrition have improved and menarche has fallen back to the “natural” age of about 13.

There’s nothing unusual about girls having menarche at about 13 as they do today. We have medical texts from thousands of years ago mentioning that girls were starting their periods about 13 in places such as Ancient India, China, Rome and Egypt. In primitive foraging societies living in fertile habitats like the Amazon jungle menarche is about 13 too. In the Hadza and San tribes, which are often used as models for prehistoric people, girls don’t have menarche until about 16 but these people are living in such deprived conditions in semi-desert habitats and don’t make the best models for prehistoric populations.

menarche amazonians

Ages at menarche and first birth for South American foragers (Kramer 2008). Average age menarche is ~13. Average age first birth is ~18 (meaning first conception is ~17).


During our evolutionary history humans lived in some of the most fertile habitats in the world. Prehistoric Europe, for example, was a fertile land full of big well-fed animals such as giant bulls called Aurochs, giant bears, packs of wolves, lions, several species of elephant and herds of grazing animals. The humans there at the time were robust and healthy living on a diet of plenty meat, root vegetables and other plant materials. Fertile habitats like this are now occupied by states and farmers. Most of the few foraging populations remaining today have been pushed out into marginal habitats where they live in states of deprivation and this is presumably the reason menarche occurs late in these populations.


Hadza country today. Not much to hunt here. Andy Lederer/Flickr.

Prehistoric Europe

Prehistoric Europe, or something like it.

In healthy, properly nourished populations menarche is usually about 13. We see this all around the world whether it’s Europe or the Amazon jungle today, or Ancient India and Greece. The only time we see very high ages of menarche is in populations living in deprived conditions such as poor people in nineteenth century Europe or primitive foragers living in marginal habitats. Considering these facts I estimate that prehistoric girls usually had menarche about 13 and from this we can estimate the onset of ovulation to have been about 16 and first pregnancy would have been soon after at about 17. These ages are obviously just rough averages. Some girls would have had their first baby at 12 and some not until their 20s but most first pregnancies would have probably occurred in the 15-19 bracket.

Anticipated questions and comments

-You said there are ancient medical texts mentioning that girls started their periods about 13 but weren’t they only talking about the healthy upper classes? Maybe girls in the poorer classes had menarche later?

Yes, most of the texts refer to the well-nourished elites. Peasant girls living in poorer conditions usually had menarche later. But this is exactly the point. In healthy, properly nourished populations menarche is usually about 13. The peasants in these ancient societies appear to have been less healthy and well-nourished than prehistoric hunter-gatherers.

-I’ve read that the age of menarche has evolved to be variable. You seem to be saying it’s “supposed” to be about 13. Isn’t it more complex than that?

Yes, I’ve been a bit simplistic. The variability of the age of menarche seems to an adaptation for coping with variations in the food supply. When food is plentiful have menarche about 13, when food is scarcer delay menarche until about 16 and don’t start reproducing until you’re more physically mature. The point I’m making is that prehistoric people usually appear to have been well-nourished so on average menarche was probably closer to 13 than 16.

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