Coincidence? I think not
Online posting: February 26, 1997
Published in print: April 15, 1997 CMAJ 1997;156:1115
Re: Declining sex ratios in Canada, by Dr. Bruce B. Allan and associates, CMAJ 1997;156:37-41 [full text / résumé]
The following letter is reprinted from The Actuary, the newsletter of the Society of Actuaries, March 1997, with permission. - Ed.
I recently bought my teenage son a T-shirt at a souvenir shop. It read, "Hours in the day -- 24; Beers in a case -- 24: Coincidence?"
It turns out that many occurrences in our daily lives that at first appear to be coincidental turn out not to be. Let me add to the list.
Although you may not have noticed it, the ratio of male births to female births is dropping. Whereas the ratio used to be 105 boys to every 100 girls, there has been a measurable decline over the last 20 years (a loss of 2.2 male births for every 100 live births from 1970 to 1990). The experts are looking for an explanation for why this ratio, which used to be extremely stable, has changed. They cite older age of parents, environmental pollution, fertility drugs and so on. But maybe the explanation is more subtle and yet more profound.
We know that, in nature, the ecosystem maintains a remarkable stability. It is well documented that when certain species become overpopulated the size of their litters decline, and when their population is sparse, the size of their litters increases. This change can be quite remarkable, two to three times the number of live births in one cycle -- whatever is needed to maintain the species.
For the human race, the ratio of 105 male births to every 100 female births was remarkable. Given the higher mortality rate among males, this ratio created almost equal numbers of males and females at the ages of reproduction.
However, things have changed. Survival in general has improved, especially at younger ages. The difference between the mortality rates among males and females, which until 1970 had always favoured females, has actually narrowed. Much of this can be explained by smoking habits. Deaths from accidental causes are down, especially, again, among young males.
Thus, were the natural world to continue to produce 105 boys for every 100 girls, we would have an oversupply of males in the reproductive ages, rather than an equilibrium. What was needed to compensate was a decrease in the ratio of male to female live births. And guess what? That's exactly what has happened!
Coincidence? I do not know. However, I do know that not everything in demography has an easy actuarial explanation, which is what makes the discipline so fascinating.
Robert L. Brown, FSA, FCIA, ACAS
Professor of Actuarial Science
University of Waterloo
Waterloo, Ont.
rbrown@sfu.ca
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