Scientists this week presented data revealing an apparent halving of average male testosterone levels over the past 50 years, sparking warnings of a male reproductive crisis. Prof Hagai Levine, who led the work, told the Guardian: "It is mind-blowing that testosterone has declined by 50%. This is a lot. Wake up people. Wake up."
Global Sperm Count Decline and Political Attention
Levine's team previously documented a drastic decline in global sperm counts, dubbed the "spermageddon" paper. US health secretary Robert F Kennedy Jr called declining sperm counts an "existential crisis." The research has been seized on in the "manosphere" as proof modern society is emasculating men, and has galvanized concerns about endocrine-disrupting chemicals, air pollution, and global heating.
However, the trend is disputed among scientists. Levine's American co-author, Prof Shanna Swan, suggested sperm counts could hit zero by 2045, but others are highly skeptical. Prof Allan Pacey, a professor of andrology at the University of Manchester, said: "There's a tendency to pick the data that supports our viewpoint. For those who think the world is going to hell in a handcart and we're all doomed, testosterone and sperm count decline make sense."
Skepticism and Alternative Findings
In a more recent analysis using consistent measurement techniques, Pacey's group found no evidence of a substantial decline in sperm count, though sperm quality appeared to be deteriorating. Other attempts to replicate the findings have produced mixed results. Pacey added: "Do I think there's an issue with male infertility? Yes. But sperm count decline isn't one that I worry about."
A less contentious starting point is that male reproductive health reflects overall health. The latest testosterone findings are likely partly explained by rising obesity and diabetes rates. Prof Channa Jayasena of Imperial College London said: "There's been a profound shift in overall metabolic health." Excess body fat accelerates testosterone conversion to oestrogen and disrupts brain hormone signalling. One study showed every one-point increase in BMI was associated with a 2% decrease in testosterone. Being overweight can also increase scrotal temperature, impairing sperm production. Diabetes is linked to lower testosterone, sperm DNA damage, and erectile dysfunction.
Obesity as a Key Factor
Jayasena said: "Obesity could easily account for all of the decline" in testosterone levels over 50 years, a trend he finds convincing. He added: "There's a question mark over whether things like pollution and environmental factors could be contributing as well."
Thousands of studies have examined environmental contaminants on male fertility. Microplastics have been found in seminal fluid, and exposure of pregnant rats to PFAS resulted in male offspring with abnormal sperm. An Italian study suggested pollution could lead to smaller penises, while a US study found average erect penis length increased by 24% over 29 years; both speculated endocrine-disrupting chemicals could alter male development.
Mixed Evidence on Environmental Toxins
Some studies, including one linking air pollution to subtle changes in sperm DNA, are regarded as high quality. But as public interest in microplastics intensified, a "race to publish" led to overlooked contamination controls and bold claims on weak evidence. Prof Rod Mitchell, a paediatric endocrinologist at the University of Edinburgh, said: "There are studies demonstrating the presence of microplastics in the testicles and the extrapolation is that this must be really bad. But they could just be sitting there inert and not doing anything."
Mitchell's highly controlled experiments using human foetal testes tissue in mice found no change in testosterone levels or testicular development from plasticisers, phthalates, or BPA. He said: "The animal studies are misleading." Mitchell places himself "somewhere in the middle" on whether environmental factors drive fertility decline.
Uncertainty and Precautionary Principle
Levine acknowledges uncertainty about biological mechanisms but argues for precaution: "You don't need 90% proof. Let's say there is a 1% chance that something we are doing now would make reproduction extremely rare in 100 years' time. Should we do something about it? I think yes."
For individual men, the issue is time- and finance-limited. Prof Christopher Barratt of the University of Dundee said navigating competing claims is "a nightmare." Male infertility is often treated as secondary in IVF clinics, with men waiting months for diagnoses while female partners undergo tests.
Basics of Male Fertility Care
Barratt said: "It sounds incredibly simple – and quite boring – but we need to get the basics right. The man has to have a physical examination, a history taken and a semen analysis." Sperm analysis has not changed since the 1950s. Pacey said: "It can tell you really blunt stuff. No sperm: we've got a problem. Motility problems: you need IVF or ICSI." Low-quality sperm is typically centrifuged to separate healthier cells. "It's blunt force over ignorance," said Pacey.
Risks of Testosterone Replacement Therapy
Social media marketing of male fertility tests and supplements has expanded, along with clinic "add-ons" rarely endorsed by the UK fertility regulator. Jayasena warned against "low T" or "T-maxxing" with testosterone replacement therapy, which can halt sperm production by reducing the body's own hormone production. He said: "It's like a thermostat in a home – if you put a heater in the lounge it will switch off the boiler. But you need very high levels of testosterone in the testes to make sperm. What we really don't need is a self-inflicted problem of indiscriminate testosterone use."
Promising Future Techniques
Decades of research may soon pay off. Promising techniques include microfluidics systems that "race" sperm through mazes to select the fittest cell. Sperm DNA fragmentation testing is improving, and AI could help select healthy sperm from millions. Barratt said: "You've got such a large number of cells, they look different from each other. Sperm and AI are meant for each other."
Silicon Valley investors back startups targeting lab-grown eggs and sperm. Paterna claimed to have grown functional human sperm in a lab from stem cells and created healthy-looking embryos. Barratt said: "I'm very optimistic that the options for men will look different in four or five years' time."
Avoiding Panic
Mitchell said: "I'm not worried that we're going to die out imminently. Some predictions that male sperm counts will be down to zero within 20 to 30 years – I don't buy that. Many populations are in decline anyway, and not just because of potential slight reductions in male fertility. The problem, in terms of our future, might be more down to other issues about the world we're living in."
