A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Are male fertility rates shrinking?

January 28, 2005

Dear Cecil:

While flipping around cable recently, I came across a show discussing human fertility. Among its claims was that male fertility rates (sperm counts) have declined by 50 percent in the past 30 years and continue to decline. They suggested one possible reason for this is that as much as 80 percent of hormones in female contraceptives get flushed into toilets and end up in our drinking water. Is this true? Will fertile males one day be an endangered species? Should I start saving up my sperm to auction off to the highest bidder?

Cecil replies:

What you do with your sperm is not something the rest of us need to know, Louis. Fact is, people in the third world are reproducing just fine — it's the developed world that may have reason to worry. Birthrates in the industrialized countries have plunged to the point that the population of some nations (though not the U.S.) will start falling over the next few decades — Russia's already is. Nobody knows if that's just because people don't want a lot of kids or if more insidious forces are at work, and if the latter what they are. Female hormones in the environment is one candidate, but lots of others have been suggested. For example, you might want to be careful with that laptop — according to one study, it could be cooking your cojones.

Concern about declining male fertility first caught the attention of the scientific community in 1992, when a Danish team published a study ominously entitled "Evidence for Decreasing Quality of Semen During Past 50 Years." No, it wasn't the Bush family, silly. Niels Skakkebaek and his associates reviewed 61 papers published between 1938 and 1991 containing sperm-quality data for a total of 15,000 men. Results: Semen volume and sperm counts were way down — from an average of 3.4 milliliters with 113 million sperm per milliliter in 1940 to 2.75 and 66 million in 1990. At the same time, testicular cancer and other genitourinary abnormalities were on the rise. Coincidence? Maybe, but the researchers noted that Danish men had five times more testicular cancer than Finnish men and 47 percent fewer sperm.

Critics were quick to fault the Danish study for alleged methodological shortcomings. Subsequent research has shed little additional light — some papers support the Danish conclusions, others say they're baloney. A few studies have raised eyebrows, notably one by Sheynkin et al (2004) reporting that men who perched a computer in their lap for an hour increased their scrotal temperature 2.6 to 2.8 degrees Celsius, possibly playing havoc with their sperm-making capability. While that may be so, the reported drop in sperm quality began long before the arrival of laptops.

In short, we don't really know what's going on with male fertility. What we do know is that the population of many industrial countries will soon level off or drop. According to the UN, only four developed countries — Albania, Iceland, New Zealand, and the U.S. — reported a fertility rate of two children per woman or higher in the 1990s. (Replacement level, or the number needed to maintain the current population, is around 2.1.) In 2000, 64 countries accounting for 44 percent of world population had fertility rates at or below replacement. The population of Japan is expected to peak in 2006 and decline 14 percent by 2050; Italy's population is projected to drop 22 percent. In eastern Europe, Bulgaria, Estonia, Georgia, Latvia, Russia, and Ukraine will have 30 to 50 percent fewer people by 2050. Population increase in the U.S., meanwhile, will be largely driven by immigration and the high birthrate among Hispanic women.

For those who recall the "population bomb" panic of the 1970s, most of this probably sounds like good news. However, while we used to think that declining birthrates were strictly a matter of conscious choice, some scientists now wonder whether one reason we don't have more children is that we can't. A host of biological factors can contribute to reduced fertility, including the trend toward starting families later in life (older women have a tougher time conceiving); more sexually transmitted diseases; increased obesity, smoking, and alcohol consumption (in some quarters, anyway); and greater exposure to chemicals.

That last one's the scariest. A few alarmists contend we're committing slow suicide with environmental poisons. One book, Our Stolen Future (Colborn, Dumanoski, and Myers, 1996), argues that industrially-derived "hormone disrupters" — particularly chemicals that behave like estrogen and interfere with fetal development — threaten our ability to reproduce. (Fears about contraceptive residue in water are an extreme form of this view.) Skeptics note that male infants are naturally exposed to high levels of their mothers' hormones in the womb without harm. Still, while the falling birth rate may be an entirely benign phenomenon, this is one area where you want to know for sure.

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