Is pollution really causing penises to shrink and sperm counts to plummet?

We should be sceptical of claims that most couples could eventually be forced to use assisted reproduction.

In the past month, we’ve learned that the human race is going extinct. No, it’s not because of pandemic disease, or even North Korean ballistic missiles. It’s because common-but-dangerous chemicals in the environment, particularly certain kinds of plastics, are wreaking havoc with fertility, lowering sperm counts and causing untold damage to both male and female reproductive systems.

According to the epidemiologist Shanna Swan – who is promoting her new book on this topic – within 25 years, most couples could end up having to use assisted reproduction, and she suggests that chemicals are to blame.

Swan’s claims have received broad (and almost completely uncritical) attention, for instance being featured repeatedly in the New York Times, the Times, the Guardian, and in a viral Sky News tweet that read: “Human penises are shrinking because of pollution, warns scientist”.

My immediate response when I see a claim like this is to be sceptical: at first blush, it seems to be somewhere on the same spectrum as the conspiracy theorist Alex Jones’s infamous suggestion that “they” are “putting chemicals in the water that turn the friggin’ frogs gay” (and this is no idle comparison: Swan specifically mentions the effects on frogs in her book).

But Swan isn’t some anonymous crank from the depths of the internet – she’s a professor at the prestigious Icahn School of Medicine at Mount Sinai in New York City. That doesn’t necessarily rule out the “crank” explanation, of course, but it means it is worthwhile hearing her theory out. So let’s do so.

The first question: is the claimed phenomenon actually happening? So far, so good: there does seem to be a broad consensus that sperm counts are declining, and have been for some time (at least in the West, though there’s evidence from elsewhere too).

There’s much less evidence for Swan’s claims about penis-shrinkage, though; I couldn’t find any studies showing a long-term generation-on-generation decline. Nor did I find anything reliable showing that Swan’s other favoured measure, the “anogenital distance” (which is just what it sounds like – the distance between the anus and the genitals, which is apparently an indicator of birth defects) is shortening over time.

But for the sake of argument, let’s grant that everything Swan claims is happening. What about the mechanism? Is it really the case that exposure to plastics in the environment is related to all these reproductive problems? It does seem to be for lab animals: in studies of rats and mice, for example, exposure to various kinds of phthalates – chemicals added to plastics to make them more, er, plastic and flexible – seem to be linked to the sorts of differences in sperm quality and in various reproductive features that Swan mentions. The idea is that phthalates are endocrine disruptors – they have chemical properties that fool animals’ bodies into producing the wrong amounts of hormones, interfering with their development.

So what about in humans? Here’s where things get murkier. I don’t claim in any way to have done a proper scientific review of the evidence here, but when I looked up some of the studies that Swan cites, I wasn’t particularly impressed.

Take one 2016 study. Swan and her colleagues measured five different kinds of phthalates from urine, and tested their correlations with penis width and two different measures of anogenital distance across three different time points in 168 infant boys. That’s a lot of correlations (45, in fact – and if you look at the paper’s appendix there are even more). They found two or three that were “statistically significant” – that is, unlikely to have been as strong in a world where phthalates had no relation to the genital measures.

But if you run this many statistical tests, you’re bound to find a few that produce “significant” results – purely by chance. Swan and colleagues didn’t use any statistical techniques to control for those chance associations, so it’s hard to know whether their effects are real, or just statistical noise, artefacts of digging around in a dataset with a lot of measures. It’s not a strong piece of evidence that should make us worry about phthalates (other studies often cited by Swan, such as one from Mexico, are small and provide very borderline statistical evidence).

Although there’s no shortage of studies discussing the link between endocrine disruptors and the reproductive system, there really doesn’t seem to be the knockdown evidence from human research that would be required to substantiate Swan’s certainty.

But as I mentioned above, the evidence for a drop in sperm count, at least, does seem to be clear. This is obviously something we need to better understand. Plastic pollution is by no means the only possibility – the list of candidate explanations (which aren’t mutually-exclusive) is long. It includes diet and obesity, sleep, smoking, and our more sedentary lifestyles.

Swan does say that these other explanations might play a part, but still – perhaps in a bid to reframe the debate – hypes up the influence of toxic plastics far beyond the data. There’s definitely a kernel of science here, but for now we don’t need neurotically to avoid Teflon and plastic shower curtains, as Swan and her fans have suggested. The answer, as so often, is to push for scientists to go beyond low-quality research and give this topic the proper analysis it deserves.