How nature figures out when we get our periods

When will I get my period

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

Usually your period arrives at some stage between the ages of eight and 16, usually around age 12. 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, which 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 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-headed relative jokes. This is usually down to a lack of sexual options, so these populations are bound to turn to each other. When this inbreeding is repeated over and over, the genes become too similar and diversity is heavily reduced. When our genes are all too similar, any threat that wipes out some of us can wipe out all of us in one fell swoop.

The offspring of inbreeding often have physical, mental and fertility challenges. We find this funny because we like to make fun of people, especially those who look different to us, and 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, seems ludacris.)

    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 an unrelated fertile man (like a stepfather).

Having half-brothers was also not a cause of a delay in a woman’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 members outside of the family.

Total number of siblings, nor numbers of brothers or sisters, was associated with later first periods, however having other sisters – older sisters in particular – did delay a woman’s first period.