“I think you can tell it’s for a lay audience,” says Michael Carroll, holding up his book, Your Nuts, its cover illustrated with two walnuts in a string bag. The book has arrived in tandem with worldwide concerns about falling birth rates. “Spermaggedon” is a chunky piece of this puzzle, part of a wider debate about whether and why we’re heading for species-icide.
Are people deciding not to have kids because of climate anxiety and economic precarity? Or is this an unchosen fertility cliff edge, created by forever chemicals and our lifestyles?
This is not a drill, says Carroll, a biologist and reproductive scientist at Manchester Metropolitan University. “There’s compelling evidence to support a historical decline in sperm count and sperm concentration over the last number of decades.” However, it’s just not that simple.First, it’s not a uniform picture and in some populations sperm counts are going up. The Danish Young Men Study, for instance, showed no drop between 1996 and 2022.
In Ireland, a study of 15,000 men between 2008 and 2023 showed a 23% increase in sperm concentration.Second, it’s not a direct read-across from sperm count or concentration to fertility – those statistics don’t tell you anything about functionality. “If you deposit 300m sperm at the cervix, only a couple of dozen might actually reach the egg,” Carroll explains, and we still don’t know really basic things, like “how the sperm find an egg in the female reproductive tract. We can extrapolate from animal experiments and conduct in vitro experiments, but biologically, we don’t really know.”No single issue causes infertility.
Photograph: AleksandarNakic/Getty ImagesThird, “reproductive biology is a very complicated, multi-stepped, timed process requiring two individuals,” and even while we all understand that, any focus on a single issue, whether it’s sperm count or maternal age or generational anxiety, drives us away from an appreciation of that complexity and towards narratives so simplistic as to be basically superstitious. Back in the day, that woo-hoo was all “selfish women are leaving it too late”; now, thanks in part to an overheated manosphere, testicles are centre stage, freighted with notions of masculine destiny being threatened by an effeminate modernity.
These aren’t ideal conditions for understanding what is going on.Carroll grew up in Ireland, the youngest of 12; his unimaginably large family seems salient to his career choices. He brushes off any attempt to psychotherapise him, though, saying this is just how Ireland was in the 60s and 70s (he was born in 1974). “It was very much a theocracy then, and the Catholic church wasn’t in favour of contraception.”He did his PhD in Newcastle, looking at how sperm fertilise eggs in marine animals.
Then, after a postdoc at the Howard Hughes Medical Institute in Dallas, Texas, looking at stem cells, he trained as a clinical embryologist, working in IVF, before returning to academia to teach reproductive science and reproductive biology, while researching the impact of lifestyle and environmental exposure.Although Carroll, who is married, has three children from a previous relationship and a stepdaughter, he is familiar with the pain of infertility, from his experience working in IVF.“It’s a great job, being an embryologist. You’re working at the very beginnings of life, patients really put their trust into you to handle their gametes, the sperm and eggs, and to culture their embryos and get them to a stage where you can give them a family.
It comes with great joy. But you also have to be prepared sometimes to break devastating news.”Carroll’s book on the role of sperm in declining birth rates.
Illustration: Fiach PressSome types of azoospermia – where no sperm is produced in the ejaculate – have no cure or treatment. It can be caused by anabolic steroid use, which is sometimes reparable when you stop taking them, but sometimes not. “Don’t take health advice from social media!” Carroll says, and writes, often.The global fertility picture, though, can’t just be blamed on TikTok.
The total fertility rate (TFR) is the average number of children a woman will have in her lifetime. In 1963, it was 5.3; by 2024, it was 2.2.
Most European nations are well below replacement rate (UK, 1.6; Spain, 1.1) and the only reason the world population isn’t falling is that people are living longer and infant mortality rates have dropped. Carroll keeps the economics front of mind: “some estimates put the total cost of raising a child at $280,000 [£209,000].
I don’t know anybody now who could afford to have 12 children.” Plus there are other social factors, including climate change anxiety. Older maternal age is plainly a factor, though paternal age should be considered more than it is.
From 40 to 45 is considered advanced paternal age, and “older men tend to have higher levels of DNA damage or mutations, so there’s a higher prevalence of neuropsychiatric disorders in their offspring – bipolar will be one, schizophrenia will be another”.But what researchers are starting to call the “disappointment gap” – between the size of family people want and the one they have – is often driven by brute biological realities for both men and women, and where sperm is concerned, individual choices don’t cover it. In 2005, the epidemiologist Christopher Wild coined the expression “exposome”: the combination of environmental and lifestyle factors an individual would be exposed to throughout their life.
Pollution, endocrine-disrupting chemicals, plastics, sunshine, processed foods, “everything matters,” says Carroll. “The best way to examine a drop in any biological marker would be prospective surveillance studies, where you follow populations over time.” The Danish data came from military recruits; the Irish sperm research, which was a retrospective study, took place in a fertility clinic.Carroll wants to broach the isolation of infertility. Photograph: Florence Law/The GuardianIf longitudinal research yields a frustratingly mixed picture, the understanding of sperm itself is maddeningly partial.
We can measure numbers, but that doesn’t tell us anything about function.For sheer talismanic potency, though, sperm and testosterone are in a class of their own, drawing in social constructs of masculinity and meaning, which collide with these unexplained or only partially understood declines to produce narratives that are unscientific and often work directly against the science. In tandem with that, unwanted childlessness is rising sharply; infertility in broad terms is about 30% down to the woman, 30% down to the man and 30% unexplained.
And for men, conventionally the silent partner in this discourse, a diagnosis of low fertility or infertility engenders “feelings of shame, demasculinisation, questioning their own identity and self-worth,” says Carroll. Whether as an embryologist or academic, he just wants people, with or without testicles, to understand what’s going on.Seeing sperm as magic, and its absence a curse, isn’t a new thing.
In 1889, Charles-Édouard Brown-Séquard, a professor of experimental medicine, mashed up the testicles of dogs and guinea pigs, extracted the juice, injected himself with it and reported “a radical change … I had regained at least all the strength I possessed a good many years ago” (he was 72 at the time). This unleashed an industry of quacks, flogging “Brown-Séquard Elixir” or, more commonly in the US, the “Elixir of life”.
To be fair to Brown-Séquard, the literature at the time on what testicles may or may not be capable of was very undeveloped, and he was like the Withnail of medical research, gamely experimenting on himself throughout his life, once covering himself in varnish to discover the properties of skin (he was found unconscious and had to be cleaned with alcohol), another time giving himself cholera by eating the vomit of a patient, to see if he could cure it with opium (he was found unconscious again and had to be resuscitated).The Withnail of medical research … Charles-Édouard Brown-Séquard. Photograph: HUM Images/Universal Images Group/Getty ImagesWhat concerns Carroll is the revival of that quackery, this time attached to testosterone. “What’s being promoted, especially by podcasters and social media influencers, is that testosterone is a quick hack to make you feel better and more invigorated, it’s a new elixir.” There is an ambient association of testosterone with “masculinity, aggression and competition, but behaviour is complex and it is too simple to put it down to the level of a hormone released from an endocrine gland”.
A lack of vigour is more likely to be down to poor sleep or diet, and testosterone replacement therapy won’t put a dent in that, but it will along the way do the exact opposite of what you took it for. “It’s a very sensitive pathway of signals, the brain telling the testicles to produce testosterone. It’s called the hypothalamic-pituitary-gonadal axis.
Some testosterone will be released to do its thing around the body; more is produced in the testicles. And that’s important for spermatogenesis” – or sperm production and development.
If you take supplementary testosterone, he says, “it’s telling the brain to shut down your factory downstairs because we have enough testosterone now, thank you. It’s a negative feedback loop, spermatogenesis is affected, and then there’s poor sperm or no sperm output because of that”.It’s so pronounced with bodybuilders, who use testosterone replacement therapy to build muscle, that they get testicular atrophy, where their testicles shrink from underuse.
It even has a name: “The Mossman-Pacey paradox,” Carroll writes, “whereby men enhance their visual cues of reproductive fitness while reducing their actual ability to reproduce.” Finasteride, a medicine that arrests and sometimes reverses male-pattern baldness by blocking the hormone DHT, has the top-line impact of boosting testosterone (because testosterone converts into DHT so, unconverted, it stays in the bloodstream). But libido, erectile function and mood track the androgen environment in general – so blocking DHT, like taking synthetic testosterone, is another signal to the brain to shut down the factory.Paternal age should be considered more than it currently is in the global fertility picture.
Photograph: Posed by models; AleksandarNakic/Getty ImagesAlmost more from a geopolitical than a spermatozoic point of view, it’s sobering to see testosterone make it into US defence policy, with Pete Hegseth announcing over the summer the “High-T Department of War”, mandating that all servicemen over 30 have their testosterone tested, to maximise mental and physical readiness, strength and warrior ethos. It’s not even the stupidest thing to come out of the Department of War recently, but it’s a stark illustration of manosphere misinformation working its way up to policy, flooding the knowledge environment.And the problem with that is, it causes a lot of heartache. “A lot of men think that if they’re producing semen and they can get an erection, their bits are working,” Carroll says. “They don’t realise what’s going on within the testes, what can affect the quality of the sperm, what can reduce their capacity to reproduce.
They don’t even start to look for the information until they’re trying to start a family and there’s a problem.”Smoking is strongly associated with lower semen volume, lower sperm count, and increased DNA damage. Obesity has been linked to reduced testosterone levels and lower sperm count, but not all studies concur on this, and most obese men are fertile (Carroll is careful to stress this in his book, possibly to ward off anyone thinking they can rely on BMI as a contraceptive).
The most solid correlation is between obesity and infertility rather than low sperm count, which also has the most obvious mechanical causation – fat cells produce an enzyme, aromatase, which can convert testosterone to oestrogen. This can directly impair the function of testicular cells.
Human bodies are amazing, though sometimes in annoying ways.Manosphere misinformation working its way up to policy … Hegseth addressing the troops. Photograph: Chip Somodevilla/Getty ImagesMore broadly, it’d be good for everyone to “know how it works”, Carroll says. “Plenty of lads go to the gym and they’ll know a lot about muscle physiology and contraction and nutrition, and they’ll be able to use that knowledge to make informed decisions to get the physique or fitness level they want.
It should be the same with reproductive health. If you know your testicles need to be two to four degrees lower than body temperature, and prolonged high temperatures can damage sperm production; if you know the risks of STIs, which can cause infertility not just in males but in females as well; if you know that taking Finasteride or TRT can have an impact; this will all help you make informed decisions and ask the right questions.”Fundamentally, Carroll wrote Your Nuts to broach the isolation of infertility.
World Health Organization estimates that around 17.5% of the global adult population will experience infertility at some stage. People (like me) rail against the asymmetry of the discourse, that women’s health and choices are endlessly analysed and policed while men and their nuts slip under the radar, but it’s a painful and alienating experience, being the one that nobody talks about. “We’re in an infodemic era,” says Carroll, “and a lot of that information is not correct.
I just want to tell people what we know, and more importantly, what we don’t know.” Your Nuts: The Science of How They Work and What it Means for Your Fertility by Michael Carroll is published by FiachPress (£22.99)