How nature figures out when we get our periods

Research has offered us some fascinating morsels into how nature determines when we get our periods. If you don’t grow up with your father in the household, but have half or step brothers, you are likely to get your period earlier. Conversely if you have sisters in the household growing up, you are likely to get your period later.

Researchers think that this is an evolutionary strategy to help prevent inbreeding, with the whole system regulated by pheromones – these little molecules can influence our behaviour without any conscious input from us. These pheromones are sniffed up our noses all the time, providing us with messages from other people.

When periods normally appear

A period arrives at some stage between the ages of eight and 16, usually around age 12. What has been discovered is that it depends on who you live with as to when you are more likely to get your period.

Skinny girls get their periods later than other girls, because you have to have at least 17 per cent body fat for the hormones to be produced. This makes a difference, even if you have non-blood or half-blood brothers in the house growing up.

Menarche (pronounced men-arc) is the medical name for your first-ever period.

Why inbreeding is bad (and why we find it funny)

Inbreeding is when two close relatives make a baby, with certain populations (usually islands or other isolated areas) being the butt of many incest and two- or three-headed relative jokes. This incest is typically down to a lack of sexual options, so these populations are bound to turn to each other in times of need. (Right?)

When this inbreeding is repeated over and over, the genes become too similar and diversity is heavily reduced. When our genes are too similar, any threat that wipes out some of us (like a flu) can wipe out all of us in one fell swoop. Ultimately we develop low levels of durability as a species in these areas.

Part of the beauty of our normal mating rituals is being attracted to the scent of someone with a very different immune system to ourselves. This ensures that the baby we make will have, theoretically, the best of both worlds. The offspring of inbreeding, however, often have physical, mental, and fertility challenges, because the same (possibly dud) genes are just swirling around, getting passed on and on and on.

Humans find inbreeding funny because we like to make fun of people, especially those whose parents couldn’t keep their hands off each other despite being siblings. Anyone who has ever grown up with a sibling knows all too well just how much you loathe them growing up. The idea of having sex with them, for fun or even out of desperation, seems ludacris.

It is a phenomenon, however, that in some cases family members who didn’t grow up with each other (say a biological parent and a child, or half or full siblings) may sexualise and romanticise their relationship with one another. They can find that their feelings towards one another haven’t had the chance to sour by living together for many years as part of a family.

The study results – unexpected findings

The study, conducted at Pennsylvania State University by Robert Matchcock and Elizabeth Susman, collected questionnaires from 2,000 female university students. The questions were about their families, growing up, and the age they got their first period.

  • The women were all about age 20, with 87 per cent Caucasian
  • 326 women grew up without fathers present for most of their lives
  • 587 had at least one older sister
  • 200 grew up in an urban environment

Having a stepfather in the house was not associated with a delay in the first period, contrary to what was expected by the researchers. The researchers theorised that this was due to – as many animals display – female sexual maturation speeding up in the presence of a biologically unrelated fertile man (like a stepfather).

Having half-brothers was also not a cause of a delay in a girl’s first period, which was also a surprise. The researchers thought that if the goal was to prevent inbreeding, a half-brother who shares half the genes was too close, but turns out not so.

Some other research has found that we tend to pick someone who is like us, but not too like us. This could make a half-brother, in a pinch, a ‘good enough’ mate.

Girls in the city got their periods sooner than girls from the country, but there was no reason supposed for this, besides more opportunity to come in contact with males outside of the family.

Total number of siblings, not numbers of brothers or sisters, was associated with later first periods, however having other sisters – older sisters in particular – did delay a girl’s first period.



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Jessica Lloyd - Vulvovaginal Specialist Naturopathic Practitioner, BHSc(N)

Jessica is a degree-qualified naturopath (BHSc) specialising in vulvovaginal health and disease, based in Melbourne, Australia.

Jessica is the owner and lead naturopath of My Vagina, and is a member of the:

  • International Society for the Study of Vulvovaginal Disease (ISSVD)
  • International Society for the Study of Women's Sexual Health (ISSWSH)
  • National Vulvodynia Association (NVA) Australia
  • New Zealand Vulvovaginal Society (ANZVS)
  • Australian Traditional Medicine Society (ATMS)
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