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Many years ago, when I was young and handsome, a friend inveigled me into taking a small role in a film he was making - a proper film, with a real budget and a commercial release, though mercifully it never got much attention.
It was a Cold War spoof called The Last Straw in which the Soviets were plotting to bring the West to its knees by causing the sperm count in Western males to collapse, and I got the Dr Strangelove role.
This friend - let's call him Giles Walker, because that was his name - picked me for the role because at the time I was known for making weighty prognostications on the strategic balance and matters pertaining thereto (you have to make a living). So I played myself, briefing the leaders of the Free World on the appalling strategic consequences if the Soviet plot succeeded.
You cannot even find this film on YouTube now, I'm pleased to report. However, it did give me a head start on considering the appalling consequences of a drastic fall in the sperm count of Western men. This comes in handy at the moment, since that is now actually happening.
In the 15 years between 1989 and 2005, according to a study just published in the journal Human Reproduction, the sperm count of Frenchmen fell by one-third. More than 26,000 men were tested in the study, and the number of millions of spermatozoa per millilitre of their semen was falling by almost 2% a year. If that rate of decline has been maintained since the study ended, the count will be down another 13% by now.
Now, admittedly, counting sperm is tricky. You can fit a hundred million of the little buggers into a teaspoon, they all look alike, and they keep wriggling around. But these results are being taken very seriously because they don't have the usual defects of this sort of study.
Most studies on sperm counts use data from men who donate sperm for artificial insemination centres (who are chosen for their high fertility), and/or from couples who are having trouble conceiving (which may be because of an abnormally low sperm count in the male partner). In neither case is it a genuinely representative sample.
The virtue of the French study is that the country has the Fivnat database, a record of some 440,000 cases of infertility problems at 126 government-funded ''assisted fertilisation centres'' from the 1980s onwards. The researchers chose only the 26,200 cases where the problem had proved to be complete sterility in the female partner - which presumably meant that their male partners were a random sample of the population.
Treatment for infertility is free in France, so there should be no income bias in the data either. For those reasons it is probably the most reliable survey of changing sperm counts over time that has ever been done - and it documents a steep fall in a relatively short time.
The numbers are quite impressive: from 73.6 million sperm per millilitre in 1989 to only 49.9 million per millilitre in 2005. If the rate of decline has stayed the same since 2005, the number now would be around 43 million. Doctors generally regard 15 million as the number below which there will be serious problems with fertility, so there's another 40 years or so before the problem gets really serious. But still ...
There are really three questions here. One, is the same thing happening elsewhere? Two, what's causing it? And three, how much does it matter?
Most other scientific studies in developed countries in the past 20 years have also found falling sperm counts, though none of them matched the French one in scale and precision. There is no comparable research on the trend in developing countries, but it is at least plausible that this may be a global phenomenon.
That mostly depends, of course, on what's causing it. If it's environmental factors, are they the same in rich countries and poor ones? A common theory lays the blame on chemicals in the environment like bisphenol A, found in some plastics, that disrupt endocrine function and change hormonal balances. Another theory blames smoking, drinking alcohol and high-fat diets. These factors vary from one country to another, and more research is clearly needed.
But let us suppose that the trend is continuing, and that sperm counts are also declining in developing countries. Should you lie awake at night worrying that this is a threat to human survival?
Definitely not. If you're really worried about keeping human numbers up, then you should be doing something quite different at night. And afterwards, you might lull yourself to sleep pondering whether it would really be such a bad thing if the birth rate dropped for a while.
If this decline in sperm counts is caused by environmental factors, then it can almost certainly be reversed eventually by doing enough research and then eliminating those factors. In the meantime, however, we are passing through the astounding total of seven billion humans beings, on our way to nine or 10 billion.
That's far too many for this finite planet, and a rapid decline in the birth rate, even to below replacement level, would be a good thing.
Gwynne Dyer is an independent London journalist.