THE BLOG
10/25/2011 04:37 pm ET Updated Dec 25, 2011

Do Brothers Stall Their Sisters' Sex Lives?

Fifteen months ago I gave birth to a baby girl. The child is now a seam-popping twenty-five-plus pounds. Babies, they grow so quickly it's creepy -- my thoughts fast-forward through her teething years to the teens -- and I'm terrified. Problem is, my family lives in New York City where children grow up too quickly. The weenies of tweens should stay in their jeans, but all too often they don't.

The age at which a teenage girl starts to become sexually active depends a lot on her social environment -- peers, culture and so on. It especially depends on the family environment, according to a recent study by Australian behavioral ecologists Fritha Milne and Debra Judge. But here's the thing: family environment is not necessarily influential for the expected reasons, such as curfews and chastity pledges and other parentally-imposed restrictions.

The hidden influence is the younger sibling.

Milne and Judge recruited nearly 200 women and 76 men, all living in or around the city of Perth, Australia, and asked them questions about their family lives and sexual development. The results were that girls with younger brothers only (no sisters) lost their virginity an average of more than a year later (at age 18.3) than girls with younger sisters only. Girls with both younger brothers and sisters lost it nearly two years later on average (age 19.3) than girls with no younger siblings. Younger sisters alone had no impact.

The chastity effect only applied to girls with younger brothers. Having a big brother (or sister) didn't make a girl any less likely to hold onto her virginity. Yet another strange pattern emerged. This one involved the girls' physical maturity.

The more older brothers a girl had, the later she got her first period. Girls with only elder brothers got their first visit from "Aunt Flo" up to a year later (at age 13.6) than girls with older sisters or no older siblings (age 12.7). (This is meaningful given that breast cancer and other conditions are related to earlier menstruation.)

Elder brothers delay physiological maturation, while younger brothers delay behavioral maturation.

What's going on?

Trained as behavioral ecologists, Milne and Judge took a look at the big picture. Daughters are often caregivers. Historically -- and in traditional societies -- a woman with daughters as first- or second-born children has a larger family than a mom whose first children were sons. Elder daughters take care of younger siblings, which frees up Mom to keep popping them out. Boys historically required more resources than do girls, which made a big sister's contributions even more important. As a result, these helpful elder daughters experience a delay in starting their own families. In the modern world where women don't usually start their families until their mid-twenties on average, this is no problem. In the past, females with brothers may have had fewer children over their lifetimes.

The bigger mystery is what's actually behind Big- and Little Brother's stalling effect on their sisters' sexuality. This is unknown territory, so Milne and Judge tread lightly here. The safest theory is that the delays are behavioral. Girls with little brothers lose their virginity later because they're too busy taking care of their siblings to have love lives of their own. Perhaps little brothers, who are slower than female siblings to develop and reach puberty, keep their elder sisters in a more childish mindset. Or perhaps the stress of caregiving slows down puberty.

The researchers should also consider a much more surprising yet equally plausible theory: brothers send out chemical cues (pheromones) in their sweat that inhibit their sisters' sexual development. Odd as it sounds, this would explain the perplexing finding that girls with older brothers get their first periods later than their peers. And, it appears, so do girls who grow up with their biological fathers in the household, compared to their peers with absent dads. Several studies, including here and here and a large one at Penn State that involved over nineteen hundred college students, came to this conclusion. (Interestingly, the same study found that girls growing up in homes with males unrelated to them got their periods earlier than average, suggesting that a non-related male may speed up sexual maturity.)

The sweat-stifles-sexuality theory isn't as far-fetched as it sounds. Other animals -- rodents, for instance -- use pheromones to modulate sexual maturity and fertility in a population. Over the years, a girl would inhale chemical cues in fraternal sweat -- think of all those sock and armpit odors. Those chemicals would hit the hypothalamus of her brain where sex hormones are produced, and slow down the works. The result is that puberty strikes a little later. Evolutionarily speaking, this allows girls to stay in the family nest longer without conflict. The risk of incest is reduced.

Knowing all this, should I try for a son so that my daughter will benefit from the younger-brother effect? Truth is, the data applies to populations, not individuals. There are no guarantees; these are just interesting findings that merit more research. Moreover, I'm in over my head right now with my baby girl's teething and feeding challenges. Sure, I'll want preserve her girlhood for longer than a New York minute. But I also need to preserve my sanity.

If you like this blog, click here for previous posts. If you wish, check out my new book, Do Chocolate Lovers Have Sweeter Babies?: The Surprising Science of Pregnancy.