Declining sperm counts: fertile ground for study but hard results prove slippery

A new study appears to confirm a drastic drop in sperm counts in developed countries over the last 40 years, but critics says the study has fundamental flaws

Human spermatozoa under a microscope.
Have sperm counts really fallen by 50% in the last 40 years?
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In 1992, Danish researcher Elizabeth Carlsen and her colleagues set global alarm bells ringing with a report in the British Medical Journal that sperm counts were plummeting. Was this the beginning of the end of the human race or just a case of bad science? Other studies found no change in sperm counts, or even rising counts. The topic has been a lightning rod for controversy ever since.

Now a new report by an international team led by Hadassah-Hebrew University in Jerusalem claims to have resolved the controversy by sorting through four decades of studies. The alarming conclusion is that the sperm counts of men in Western countries have plummeted by more than 50% in the past 40 years. “This study is an urgent wake-up call,” says lead author Hagai Levine.

Other experts, however, are yet to be convinced the findings are as dire as claimed. “I suspect they are on to something but we don’t need to go running around like Chicken Little,” says Robert McLachlan, the director of Andrology Australia, based at Monash University in Melbourne. “We don’t actually have a decline in male fertility that I’ve seen.”

There’s no doubt people are finding it harder to have babies; but much of that can be attributed to the fact women and men are delaying the age at which they first start trying to procreate. For both sexes, ageing takes a toll on fertility.

Are declining sperm counts a contributing factor? Monitoring the trend in populations over time turns out to be problematic. Generally speaking, men are reticent about offering up their sperm for studies. Researchers have long suspected those who do volunteer have concerns with their fertility. In other words, studies might inadvertently pick out men with lower sperm counts, so the results do not represent the general population.

As the number of fertility clinics has soared in recent decades – 4-7% of babies in Western countries are now conceived in labs – the number of concerned men is also likely to have increased. One finding in support of this bias in studies is that the sperm counts of Swedish military recruits, who are all tested, have shown no change over a decade.

Another concern relates to comparing sperm counts from different clinics over the decades. While there is a standard World Health Organisation protocol, it’s not like the standard that defines a unit of measurement such as the kilogram. “People claim it’s reproducible,” McLachlan says, “but it’s not.”

Levine and colleagues were aware of these concerns. That’s why they trawled through 2,518 published scientific papers from across the globe and weeded away most of them to settle on just 185. Studies were rejected, for instance, if they included men with known fertility concerns due to genital abnormalities or pre-existing disease.

The selected 185 studies involved 42,935 men, and showed that sperm concentration and total sperm counts have dropped more than 50% in four decades in North America, Europe, Australia and New Zealand. There was no similar trend in South America, Africa or Asia.

The team, which included endocrinologists and andrologists, used numerous statistical methods, as well as “sensitivity analysis”, to slice and dice the data to eliminate potential bias. Just two months ago, Levine says, their findings stood up to the scrutiny of colleagues at the International Congress of Andrology in Copenhagen. “I hope this time there is a consensus that there’s a problem,” he says, “and we can move on to find a solution.”

But the intrinsic flaws in these studies means that’s a big ask.

Some of the authors also seem quick to lay the blame at the feet of environmental chemicals. “The fact that the decline is seen in Western countries strongly suggests that chemicals in commerce are playing a causal role in this trend," says Shanna Swan, a co-author based at the Icahn School of Medicine at Mount Sinai, New York.

Environmental pollution, however, is arguably more rampant in less developed countries, so they presumably should have seen the same problem.

Levine says they may have failed to see the problem because there was not as much data from these countries.

While McLachlan, acknowledges there may be a problem, he points to other factors. Paracetamol taken in pregnancy, for instance, has recently been shown to reduce a son’s fertility.

And the fact that infertile men are now able to have children by injecting their sperm into their partner’s egg means that infertility is passing to the next generation.

While the new study will fire up the debate, McLachlan says we’re unlikely to ever get a clear resolution “since you can’t get compulsory tests from all males”.

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Elizabeth Finkel is editor-in-chief of Cosmos.
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