For men’s testicles, it is the best of times, it is the worst of times. The population of the earth has swollen to 8 billion, and is forecast to hit 9.7 billion by 2050, which suggests male powers of procreation are in bullish good health.

But disturbing global research has shown that men’s mean sperm concentrations tanked by 51.6% between 1973 and 2018, from 104 million sperm per millilitre of semen to 49 million. One team of scientists can review two decades of studies and see no “significant” changes in sperm counts. Another team can analyse data from every inhabited continent on earth and identify an alarmingly accelerated decline, from 1.16% per year to 2.64% per year since 2000.

Some experts think any declines may just represent natural fluctuations, which grab headlines and feed dark cultural fears of a decline in masculinity. Other experts insist the infertility issue – which is primarily linked to environmental chemicals and unhealthy modern lifestyles – has sinister parallels with the climate crisis: wake up, look at the data, and take action before it is too late.

Sperm count controversy

Semen is a sticky business. But there is one thing the scientific community does agree upon: it is scandalous that male infertility – which could become an “existential threat” to the human race – remains so poorly understood.

“It’s frustrating that after 20 years, we’re still debating whether or not this is true or not,” says Allan Pacey, a Professor of Andrology at The University of Sheffield Medical School. “And that says a lot about how well or how important male reproductive health figures in our funding streams.” Professor Richard Sharpe, of the MRC Centre For Reproductive Health at The University of Edinburgh, says men have been “left by the roadside,” in part because they are often “reluctant” to talk about infertility, but also due to “chronic” underinvestment in male-focused research.

Male infertility can be caused by myriad factors, such as low sperm counts, poor sperm motility (swimming skills), genetic defects, damage to the vas deferens (the tube which carries sperm out of the testes), low testosterone, erectile dysfunction and more (read our guide, “10 simple ways men can boost their fertility“).

Analysing sperm concentrations is therefore a crude measure of overall fertility, but it’s a strong marker. The World Health Organisation says sperm concentration is related to time to pregnancy and pregnancy rates, making it a useful predictor of conception.

Still, as men generally have 15 million to 300 million sperm swimming around a single millilitre of semen, it can be tricky to count all those little tadpoles. The best way is to use a haemocytometer – a counting-chamber slide engraved with a neat little grid. Using a microscope, scientists can tot up the sperm in each square, and then calculate the concentration by using clever adjustments and equations. Below 15 million is deemed low – and will make natural conception challenging. Two hundred million is not necessarily better for conception than 40 million. But men with readings below 40 million will generally have more difficulties. Some experts call this number the “tipping point” towards fertility issues.

Fertility in freefall

In recent decades, many studies have identified terrifying drops in sperm counts, while other papers have questioned the results.

But the big shockwave came in 2017 when epidemiologist Dr Hagai Levine, of the Hebrew University of Jerusalem, Dr Shanna Swan, an epidemiologist at the Icahn School of Medicine at Mount Sinai, New York, and their colleagues published a huge meta-analysis of studies based on semen samples taken between 1973 and 2011.

The study revealed that the average sperm count had nosedived from 99 million per millilitre to 47 million – nudging close to that tipping point of 40 million. Dr Swan wrote an accompanying book, Count Down, which suggested sperm counts could, in theory, reach zero by 2045. As with climate change, male infertility is shifting gradually and invisibly. This is the denial phase. Next comes panic. Then disaster.

But not everyone was convinced. Sceptics suggested that changes in sperm-count technology, methodology and controls over the years make such grand claims impossible. And men’s sperm counts vary hugely with age, health and location, so reliability is a big problem. For example, sperm dislike higher temperatures, so levels can drop by 30% during the summer. But after a few weeks of sexual abstinence, sperm levels can soar ten-fold. French researchers concluded that, although a decline is “credible,” sperm health varies even between local areas of a single country, so we can’t – yet – make planet-wide conclusions. Any declines may just be down to local environmental or lifestyle trends.

Another team, led by experts at Harvard University, published a paper which suggested that sperm levels may vary naturally “within a wide optimum” over time or within a population, so it would be helpful to explore the nuances of “local social, political and environmental context” instead. This seems wise. They also highlighted how high levels of migration make it hard to analyse a ‘stable’ population over time; how the data used by Dr Levine was limited to Western populations; and why not all those older studies are reliable.

But some of their other arguments are less convincing. They stress that there is no clear evidence that higher sperm counts (at least above that key 40 million marker) improve fertility. True. But with the data suggesting average levels may have been slashed in half to 49 million, that’s a bit like saying rising sea levels are not around our necks yet, so don’t panic about global warming.

They also fretted about how this issue is being co-opted by alt-right men’s groups who rage against a perceived decline in Western masculinity. Worrying what extreme keyboard warriors might think seems strangely peripheral to the core issue of whether humanity is heading to oblivion. And besides, Dr Levine and Dr Swan have since pointed out that if this is all down to ‘natural’ variation, why aren’t more studies showing increases too?

A terminal decline?

Around the world, several experts in male reproduction concluded that if you keep finding the same trends, whatever the obvious limitations, something is wrong. In 2018, experts at the International Symposium on Spermatology felt compelled to make a public statement demanding more research into these “alarming” male fertility issues.

Then in 2022 Dr Levine and Dr Swan dropped another bombshell, in the form of an updated global study, which this time included data from 53 countries, in every inhabited continent on earth, including previously neglected regions like Asia and Africa. To aid consistency, they used only haemocytometer studies, and excluded men with known infertility risk factors. This was the attention-grabbing study which revealed those headline-making stats, including the 51.6% drop in sperm counts, and the accelerated decline since the year 2000.

Who to believe? Dr Channa Jayasena, a specialist in reproductive endocrinology at Imperial College London, believes the problem is real. “There appears to be a trend for sperm counts lowering – and we can’t explain that away by chance,” he insists. “There’s variability of course – different men and different populations – but they checked all five continents, using the totality of evidence that we have. And I think it fits with the narrative that if you look worldwide, we are overall less healthy than we were.”

Professor Pacey remains open-minded but unconvinced. “There might be (a crisis),” he says. “But I don’t think that paper is strong enough to draw that conclusion.” He says other studies show strangely different results. But his central argument – outlined in a 2013 paper – is that advances in sperm-counting technology, manufacturing processes, training and quality controls mean modern tests do, generally, pick up lower readings, owing to improved accuracy. And inconsistencies in analysis remain “frightening” – even today. “When we send the same sperm sample out to 200 labs, the variation between those labs is huge,” he says. “And if we can’t get it to within a reasonable level of margin of error in 2023, how on earth can we say what we were doing 30 years ago was robust?”

Dr Jayasena is less concerned about these shifts in counting technology and methodology, as, in basic terms, “the ability to look under a light microscope since the 1970s hasn’t changed.” And Dr Levine has also countered that the accelerated sperm count decline since the year 2000 suggests inferior 1970s counting methods isn’t the problem here.

Here and now

The debate continues – and neither side is wrong. But Professor Sharpe believes that the sperm count conundrum is a distraction. “Personally, I’m reasonably convinced by the data that there has been a fall [in sperm count],” he says. “But we can’t ever go back and check [how tests were done in the 1970s] so we can’t prove it. So the focus should be on what we know about men of reproductive age today.

“That is the sort of research that we were involved in with colleagues in Denmark and Finland and other places in Europe. And what that shows is that across northern Europe, at least, about one in six of every young man of reproductive age has a sperm count that is low enough that we know it will negatively impact fertility. It doesn’t mean they’re infertile. It means that they will take longer to impregnate their partner.”

Dr Jayasena offers a different prevalence here in the UK, but he agrees that this is a growing problem. “We know that 10% of couples have difficulty getting pregnant within a year of trying. And we know that about half of the cases are accounted by problems with semen quality in the male. So we’re talking about at least one in 20 men – but that figure may be rising.”

So what could be triggering this apparent rise in male infertility? Dr Jayasena, Professor Pacey and Professor Sharpe all agree that unhealthy modern lifestyles are a likely factor. Many infertile men are simply unlucky, owing to genetic issues or physical trauma, but infertility is now clearly linked to obesity, sedentary lifestyles, ill health, smoking, excessive alcohol consumption and drugs such as cocaine or steroids. Any rise in male infertility would sit grimly and predictably alongside the known rise in obesity, diabetes and heart disease.

Chemical consequences

There is also growing evidence that environmental chemicals may be at fault. Dr Jayasena was involved in a major review which looked at how chemicals found in plastics, food packaging, pesticides and cosmetics may disrupt hormone function and impair sperm health. This process begins in utero, when the foetus is developing, but continues due to exposure throughout adulthood.

The search for answers continues. But what most men want to know is: how can I protect my own fertility right now? Here things get much simpler. Dr Sharpe says regular exercise and maintaining a healthy weight are crucial, while Dr Jayasena suggests avoiding long, hot baths, or draining endurance exercise. Professor Pacey says: “When I used to see men in clinics, it was the obvious things: throttle back on drinking, don’t smoke, wear loose pants, and fruit and veg are better than burgers and chips.” Reducing your use of household cleaners and plastic food packaging or plastic bottles would also be a sensible move.

The other key message is to wise up now. Owing to shifts in home ownership and cultural norms, couples are waiting longer to have babies. So belatedly discovering you have a low sperm count can be problematic. “Most couples are not putting their fertility to the test until the female partner is in her 30s, and often late 30s, and at that time her fertility is reduced by 30-60%,” warns Professor Sharpe. “So if she has a partner who has a low sperm count, it’s going to take them longer to achieve a pregnancy. And time isn’t on their side.”

Prevention over cure

If a man has been struggling to impregnate his partner for over a year, the NHS can arrange tests. But home testing kits are available from pharmacists like Boots and Superdrug, and private clinics also provide analysis. Such methods are imperfect, but they can kick-start the quest for self-knowledge.

Treatments, such as in vitro fertilisation (IVF) and hormone therapy, may help some infertile men. But most treatments have to be paid for privately, and they offer no guarantee of success. So for the average man, identifying problems early, and protecting your fertility with lifestyle changes, remains the best proactive approach.

In the future, Dr Jayasena would like to see dedicated men’s health clinics, with regular testing and improved education. Dr Levine has said that this kind of surveillance would enrich scientific research and give men vital feedback on their fertility. But for now the best thing a man can do is to help himself. Optimising your general health is a first step towards respecting your reproductive health. And nobody else is going to do it for you.