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Why is everyone suddenly talking about tanning their testicles?
I read the scientific literature on UV light and testosterone, so you don’t have to
(Note: To avoid double-emailing everyone, I’m no longer emailing out the audio versions of these articles separately - just posting them. The audio version of this article is at this link).
In mid-April the Fox News host Tucker Carlson announced a new series of documentaries, including the usual predictable topics: crime, immigration, and er, cattle mutilation by extraterrestials (okay, maybe the last one is more unexpected). The one that received the most attention online, though, was called “The End of Men”. Testosterone levels, Carlson claimed, are dropping catastrophically in the West – and that means men are becoming less fertile, weaker, less manly. The only way to stave off this “calamity”? Top up your testosterone by tanning your testicles with UV light.
Carlson is hardly the first to bring this up. Look online and you can find stuff like the following:
“Former MLB Player Gabe Kapler Says Men Who Want to Get Stronger Should Tan Their Testicles” (Complex, 2015)
“Testicle Sunning – Boosting Testosterone By Sunbathing Your Balls” (Tantric Academy)
“I Put a Giant Red Light on My Balls to Triple My Testosterone Levels” (Men’s Health, 2017)
The announcement of the documentaries also had a guest spot from one of Carlson’s pals, and the subject of another of the films, Kid Rock (real name Robert Ritchie, but I don’t think I’m any relation). The testosterone stuff was all a bit much for him. “Dude, stop!” he said, aghast. “Testicle tanning? Come on.”
When I started this Substack, I didn’t think one of my first posts would be “Who’s Right: Tucker Carlson or Kid Rock?”. But it’s still worth digging into this whole issue for two reasons. First, I expect we’ll be hearing a lot more about this in the coming weeks, as Carlson’s full documentary airs. And second, as we’re about to see, there is some important—and potentially worrying—science to discuss.
What we know about fertility and testosterone
Let’s start with something everyone agrees on: the world’s fertility rate has declined. Whether you’re in a low-, middle-, or high-income country, with few exceptions the “total fertility rate”—the number of children per woman—dropped pretty precipitously across the latter part of the 20th Century.
The vast majority of this decline has absolutely nothing to do with testosterone. The main reason for it is economic development: people tend to have fewer children as they and their countries become richer – and that’s incredibly good news. Other things that lower the fertility rate are also related to progress and prosperity: education (particularly of women), urbanisation, the availability of contraception, and so on.
But there also might be negative trends in fecundity – meaning a person’s biological ability to have kids, as opposed to simply whether they have kids or not (the latter being what people call “fertility”). It would be more worrying if fecundity was dropping, because that would take the choice of whether to have kids out of many people’s hands.
This is where things get more contentious. A well-known 2017 meta-analysis of studies on sperm count found that, across a fair chunk of the world (North America, Europe, Australia and New Zealand), men’s sperm counts declined by 50-60% between 1973 and 2011 – and the decline showed no signs of levelling off. Some other scientists have critiqued the meta-analysis: most prominently a paper from 2021 that’s peppered with “humanities”-style language about “interrogating assumptions” and so on, and doesn’t actually seem to disagree with the overall premise – or seems to say there might well be a decline but it might be completely fine. Read it and see if you’re convinced. Meanwhile, another analysis from China in 2021 found very similar results to the meta-analysis.
In any case, to bring us full-circle, if there is a sperm-quality decline, could it—and in turn, the fecundity decline—be partly caused by declines in testosterone levels? There is evidence for that testosterone decline. It’s not just that populations are ageing and testosterone gets lower with age within individual people: studies that look at each successive generation’s testosterone levels tend to find them getting lower and lower. This has been found in data from Israel, Denmark, Finland, and the US (in four separate studies).
Some of these studies find that rising obesity levels might explain part of the generation-on-generation decline in testosterone, but there seems to be a consensus that it’s not all down to this one factor. Other than obesity, the most-discussed factor is environmental pollution: specifically, types of plastics that give off chemicals like phthalates that interfere with the production of hormones like testosterone. I wrote somewhat sceptically about this last year; Tom Chivers’s review of the 2021 book Count Down by the epidemiologist Shanna Swan is also well worth a read (his conclusion was that the book was marginally convincing but also seriously overblown, with its apocalyptic tone and its tendency to blame every medical and societal problem on plastic pollution).
There are a lot of links in this chain – and many of them are made by assumption, or mere plausibility. We’re nowhere near being able to say that X environmental factor (like phthalates) causes testosterone decline, which causes a decrease in sperm quality, which causes lower fecundity, which causes lower fertility. There’s a lot more research needed for that; what we have right now are some pointers and warning signs.
But the idea of a testosterone decline—and the idea that it might cause big problems for people’s lives (and maybe for society more generally)—isn’t some paranoid fantasy – it’s a genuine possibility, taken seriously by reproductive biologists, many of whom look at data like the above and worry about the future. If there was some easy intervention to increase your testosterone, I don’t think it’s ridiculous to at least be interested.
So now, the moment we’ve been waiting for. The testicle-tanning part.
Literally all the evidence on testicle-tanning
This whole idea derives from one study published in 1939(!) in the journal Endocrinology called “Influence of Ultraviolet Irradiation Upon Excretion of Sex Hormones in the Male”. Scientists at Boston State Hospital shone a UV lamp at either the chest or the genital region of men for an increasing number of minutes (up to 20) across several consecutive days (up to 5). There’s a graph that shows spikes in testosterone levels in the participants’ urine after the light was aimed at the chest, followed by a drop back to baseline levels. But then there’s an even bigger spike—an increase of about 200%—after “irradiation of the scrotum and its vicinity”. This looks promising!
But then… look a bit closer. Firstly, the experiment included a grand total of 5 men. 3 of them were exactly 54 years old and were in the depressive phases of “manic-depressive psychosis”. The other two were 28 and 45, and had “psychopathias with depressive features”. Oddly, the graph doesn’t show an average from across the participants – it shows “the typical reaction”. And since they don’t show the graphs for any of the other men—would that have been so difficult?—we just have to take the authors’ word for the fact this was “typical” of the five men.
There also wasn’t a control group, say with a non-UV light (though I realise asking for a control group in an n=5 study is asking quite a lot – there’s barely a group in the first place). Also, do we trust the methods of measuring testosterone they had back in 1939? Put it this way: as recently as 2016 a review paper detailed all the problems with modern measurement of testosterone: how it can change within an individual throughout the day, how it differs by age, how different measurement techniques can produce highly variable results, among other problems. I’m no expert on hormone measurement, but it seems to me that measures from 80 years ago would be a heck of a lot worse than the ones we have today.
Perhaps a fan of that original study might argue that the effects of the UV light are so obvious and so massive they’d dwarf the effects of these biases. But you can’t get around the fact that this is an uncontrolled study in a sample of 5 people. So let’s see what work has been done to follow up on this pioneering testicle-tanning study.
Perhaps the most promising-looking study that comes up was published in a 2021 paper in Cell Reports, called “Skin exposure to UVB light induces a skin-brain-gonad axis and sexual behavior”. Now we’re talking, right? Well…
The paper does a whole bunch of analyses of mice and humans. I’m always baffled by reading papers that don’t report their sample sizes in some obvious, easy-to-find place—should I really have to count the dots on your graphs to work out how much data there was in your study?—and this is one. For the mice the sample size bounces around between (I think) about 3 and about 24. They ran a huge number of tests on all sorts of behaviours and characteristics for mice who had and hadn’t been exposed to UV lamps. They claimed (among other things) that the male mice found the UV-exposed females more attractive; that those females were more receptive to sexual advances; and that the UV-exposed males had higher testosterone levels.
They also had a sample of either 19 or 32 humans (I can’t work out which), who happened to be having UV light treatment for skin conditions. After UV exposure, they filled in questionnaires, reporting a higher level of “passionate love” for their partner, though also lower levels of attraction to their significant other(?!). For males only, they reported a higher level of verbal aggression. In other words, the results were an exercise in random noise-mining: exactly what you’d expect if you take a tiny sample of people and run tons of significance tests without doing any corrections.
Oh, and even if you thought it might provide evidence for the testicle theory, the following line might be a bit of a downer: “patients were given full body (except their genitals, eyes, and head) narrow band UVB exposure [my italics]”.
In one of several additional analyses, they got the data from that Israeli testosterone-decline paper I mentioned above (just for the 21-25 year-olds for some reason; n = 13,000), and found out that testosterone levels tended to be higher in the summer months, peaking in July. They just show a graph, with no statistical test, but I suppose this is consistent with the idea that more sun exposure is linked with higher T levels. It’s also consistent with a 2010 paper that found this same seasonal effect, and also found a correlation between vitamin D levels and testosterone levels in a sample of 2,299 men. This is at one remove from “light exposure causes increases in testosterone”, and it is just a correlational study, but if you think “sun raises vitamin D, vitamin D helps synthesize testosterone”, then you get the general idea.
Perhaps the most telling thing, though, is that the 2021 study only gives one citation for its statement that “the ultraviolet (UV) component of solar radiation increases testosterone levels in men”: that original 1939 paper! Has there really been nothing else—nothing experimental, where we might be able to assess causality—between then and now to test this idea? There isn’t much. Here’s everything I found:
A study from 1998 where UV lights were shone at 24 men and resulted in no measurable hormonal changes;
A study from 2013 where 20 male rats had lasers aimed at their testicles for a few days – there was extremely weak evidence for a testosterone difference compared to non-lasered rats, after a few days of using one specific type of laser (but the whole thing is so poorly reported that it’s hard to tell what’s going on);
Another study from 2013 where 10 rats were kept under constant light and 10 had a natural light and dark cycle. After 70 days the constant-light rats had higher testosterone levels;
A study from 2014 where 60 fish apparently showed some significant testosterone (and other hormone) differences after UV exposure, but again, the paper is so unclear that it’s very challenging to work out what they’re saying;
A study from 2014 where 16 rats had their testicles lasered and showed no differences in testosterone compared to 8 who didn’t (this one looks like it’s in a fake or scam journal);
A study from 2016 where six rams had their testicles lasered and showed no changes in testosterone (and might even have ended up with damage to their semen);
A conference presentation from 2016 reporting that 38 men with sexual arousal disorder had either a UV or a control non-UV light shone at their face for 30 mins each day for 2 weeks – the UV participants showed “an improvement in sexual satisfaction” and higher testosterone levels. Incidentally, although this is a non-peer-reviewed one-pager with very little information, the authors still press-released it. I don’t think it was ever published in a journal;
A study from 2019 where 10 vitamin D-deficient mice who were exposed to sunlight showed higher testosterone than 10 vitamin D-deficient mice who weren’t, but not as high as a further 10 who were just given vitamin D supplements;
A “pilot study” from 2020 where 6 older men did some resistance exercise and then had UV treatment, and showed no differences in their testosterone levels.
These are not good studies. They’re all tiny, for one thing. Many are published in dodgy-looking journals that clearly care very little about clear presentation of results - they rarely report effect sizes, for example, and sometimes miss even more basic details. The majority aren’t in humans. The majority didn’t aim light at the testicles (none of the human ones did). A couple of them make the elementary statistical fallacy of finding a significant improvement in the treatment group, no significant improvement in the control group, and claiming their treatment works. In fact you have to show that the treatment group had a bigger improvement than the control group (in the immortal words of Andrew Gelman and Hal Stern: “the difference between ‘significant’ and ‘not significant’ is not itself statistically significant”).
In other words: another trawl through the scientific literature; another selection of awful studies from which no conclusions can be drawn, and which may as well never have been done. Go science!
So does it work?
This was my genuine, good-faith attempt to find out whether there might be any decent evidence for the testicle-tanning hypothesis. My conclusion? The basic underlying idea—that raising vitamin D, via sunlight to the skin, might lead to higher testosterone—isn’t completely crazy: it sounds like a plausible-enough hypothesis, and vitamin D and testosterone levels might be correlated - though as with any correlation, there are lots of factors that could be confounders (obesity, for instance, might be a cause of lower vitamin D and lower testosterone). There are also a bunch of mixed results from vitamin D supplementation trials which I won’t cover here – maybe I’ll do that in a future post.
The health fad of testicle-tanning, though, is a lot more specific than that. And after looking up the research, the only reason I can find that shining light on the testicles in particular would be especially useful is that one paper we started with: a very unclear, unreplicated, low-quality study of 5 people, published the same year Hitler invaded Poland.
While we’re talking about Hitler… I’d be remiss not to mention the political angle here. Obviously the “Western males are becoming low-T soyboys” thing is right-coded (the solutions to it, including testicle-tanning and other techniques that come under the heading of—sigh—“bromeopathy” are the exact mirror-image of the kind of left-coded alt-med “wellness” woo you see on websites like Gwyneth Paltrow’s Goop).
Some of the responses to Carlson in the media focused on ridicule for the homoerotic tone of “The End of Men” – and, to be fair, the trailer for the documentary did look like a Tom of Finland picture come to life. But others have argued it’s “no joke” – this whole thing apparently plays into an alt- or far-right narrative. The filmmaker Jackson Katz told USA Today:
'Lower levels of testosterone' is a metaphor for what these movements are really concerned about: reclaiming white men's loss of status and cultural centrality.
That may well be the case. But what about the data I discussed right at the start of this article, which really do imply—with limitations of course—that testosterone levels are dropping? Katz’s statement has a whiff of the following, which might’ve been said by any number of know-nothing right-wing pundits over the past 20 years:
‘Climate change’ is a metaphor for what these movements are really concerned about: reclaiming government control over every aspect of our lives.
The political coding on this issue is potentially quite tragic. If it really is the case that there’s a genuine problem with male testosterone and fecundity—and I’m not arguing there definitely is! But it’s clearly a possibility—then having half the population write it off as a phantasm that only “the outgroup” believes sounds like a pretty bad idea. It certainly hasn’t helped with climate change, vaccines, GMO food*, or any number of other scientific issues that have become hopelessly polarised over the years.
To be clear, this polarisation isn’t just the fault of the people attacking Carlson – it’s absolutely his fault too (and he’s the one gleefully promoting nonsense “treatments” to boost your masculinity). I’m not going to end with some earnest plea not to politicise this issue - it’s way too late for that. But just to note that here’s another scientific area where we’ve made it vastly more difficult than it should be to have a sober, rational conversation. Well done us.
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*UPDATE 26 April 2022. Yoel Inbar got in touch to say that GMO food was a poor choice of example here. That’s because, unlike many other controversial scientific issues, support and opposition to GMOs doesn’t clearly split down left/right political lines, at least in the US. See e.g. this study.
Image credits: Getty; Chodick et al. (2020)