A healthy relationship

FOR hormone-addled teenagers, finding a date can often seem to be a matter of life and death. As it turns out, that may not be so far from the truth. In a paper in the August issue of Demography, a team of researchers led by Nicholas Christakis of Harvard University reports that men who reach sexual maturity in an environment with few available women are at risk of dying sooner than their luckier confrères. The team points out that this finding may have important implications for public health in countries such as India and China, where sex ratios are skewed against women.

The idea that a dearth of available women hurts male longevity has been around for some time. There are several reasons why such a hypothesis makes sense. It is now well established that marriage has a beneficial effect on health and survival. Since women are traditionally the caregivers, these benefits accrue especially to men. If there are fewer potential mates around, men may delay marriage or forgo it entirely, losing out on these nuptial niceties. In addition, with more men and fewer single women, the intense competition for a mate is likely to be stressful. Such early-life stress is known to have effects on health that can last for years.

As reasonable as it all sounds, the hypothesis that a skewed sex ratio leads to shorter male lifespan has never been confirmed in humans. To put it to the test, Dr Christakis and his team made use of two unusual sets of demographic data. The first, known as the Wisconsin Longitudinal Study, consists of a third of all those who graduated from high school in the state of Wisconsin in 1957—about 10,000 people. The male-to-female ratio in each person's graduating class is known, and provides an indicator of the ratio during the sexually formative years of the study's participants. The second set of data consists of 7½m white men who were enrolled in America's Medicare programme in 1993. The researchers found the year and state in which each participant's Social Security number was issued, which typically happened between his 15th and 25th birthdays. The sex ratio of his contemporaries was then calculated from state-level census data.

In the Wisconsin sample, Dr Christakis looked at those who had died before their 65th birthday. For the women, there was no significant relationship between their school's sex ratio and their age of death. For the men, however, a significant relationship did emerge. A percentage-point increase in the male-to-female ratio of a man's graduation class led to a percentage-point increase in his likelihood of dying before the age of 65. The Social Security data, moreover, suggest that a lack of women during men's teenage years still haunts their health decades later.

The average white American male who was 65 in 1993 could expect to live another 15 years. Dr Christakis found, however, that those who had come of age around the most available women had a life-expectancy three months longer than that of the least favoured. Three months may not seem a huge difference, but according to Dr Christakis it is comparable to the benefit an elderly person can expect from exercising or losing some surplus weight.

In an American context, these results are, perhaps, no more than an interesting curiosity: at the age of 15, boys outnumber girls by about 4% and the ratio shrinks towards equality thereafter. In China, however, it is estimated that there are now 20% more men of marriageable age than women—the result of selective abortion and infanticide consequent upon the country's “one-child” policy. That bodes ill for the future health of China's menfolk.