Sex Ratios

Unbalanced sex ratios lead to interesting behavior. It is the one reason I'm happy to be teaching at UW-La Crosse, rather than UW-Platteville.

From the Economist:

So many more women on the beach than men

AS ONE observes the economical use of fabric to contain flesh as it wiggles from one posto to the next on Rio de Janeiro's Ipanema beach, thoughts turn immediately to demography. Like other countries where the population is ageing, Brazil has more women than men. But in Rio the imbalance is particularly marked: for every 100 females in the city there are only 86.4 males, according to IBGE, the national statistics agency. That is far lower than the 95 males per 100 females that is the average for Brazil's big cities. What is going on? And does this explain the size of the bikinis?

The answer lies partly in three forces that have shaped Brazil over the past few decades. First, like the rest of the country, Rio has undergone an extraordinary transition in its birthrate. Although the Catholic church and the government discouraged contraception for decades, Brazilian women decided to have fewer babies. Many opted for sterilisation. One study suggests up to 40% of women aged 15-49 have been sterilised. “You find some families where three generations of women have been sterilised after childbirth,” says José de Carvalho, a demographer at the Federal University of Minas Gerais.

As a result the fertility rate dropped from 6.2 live births per woman in 1960 to around two today, while people are living longer. In the past ten years, life expectancy has risen from 68.9 years to 72.4 years. An older population means more women in relation to men, because women tend to live longer.

Second, during the past 50 years millions of women have moved away from rural areas and towards cities, where they often find jobs in domestic service. This has further skewed the sex ratio in cities compared with the countryside. Copacabana, just down the beach from Ipanema, is one of the most imbalanced places in the whole of the country, thanks to a heavy concentration of the (disproportionately female) over-65s and their maids. The third factor is violence. Rio's murder rate, at 40 per 100,000 people, is shockingly high, and most of the victims are young men.

Evolutionary biologists would draw a straight line between the declining number of potential mates, the need for female cariocas to go to greater lengths to win them, and those small bikinis. But that seems far-fetched, given that much of the imbalance is down to simple ageing. Rio's grannies, who are the most affected, are—by and large—a demure lot. The problem of the bikini is going to require some further study.