This article was published by The Conversation on 4th March 2021. Find it here.
A doomsday scenario of an end to human sperm production is back in the news recently, now with the added threat of shrinking penises, thanks to articles like this opinion piece published on The Guardian by environmental activist Erin Brockovich, inspired by Prof Shanna Swan’s new book Countdown.
Such extraordinary claims require extraordinary evidence. Is this science or sensation?
Correlation does not equal causation
Prof Shanna Swan is an epidemiologist who studies environmental influences on human development. Epidemiologists find associations between disease and potential contributing factors, like lung cancer and smoking, but their work does not identify the causes of disease — just because two things are associated does not mean one is causing, or caused by, the other.
The Guardian article leads by referring to “hormone-disrupting chemicals that are decimating fertility” but causation is far from established. It’s reasonable to expect that endocrine disrupting chemicals like BPA and PFAS can affect reproduction in males and females, given available evidence, but there is not irrefutable proof.
Selective reporting
In 2017, Prof Swan published an exhaustive study showing an apparent drop in men’s sperm counts between 1973 and 2011, of 59.3%. What’s not often mentioned is the decline in sperm count was observed only in groups of men from North America, Europe, Australia, and New Zealand, but not in groups of men from other parts of the world.
When Prof Swan and her colleagues combine the data from all countries, they see a decline over the years because the studies of ‘Western’ men outweigh those of other countries. Prof Swan and her colleagues worked hard to avoid bias when conducting their study, but selection bias, publication bias and other limitations of the original work used as the basis for their study could be influencing the results of the larger study.
There are many studies from different parts of the world that show declining sperm counts but the reasons for the apparent decline are unknown. Blaming chemicals in the environment overlooks other important modifiable causes such as chronic disease, diet, and obesity, which individuals can act upon to improve their fertility.
Erroneous extrapolation
The Guardian article isn’t the only one to make the statement that Prof Swan’s research “suggests sperm counts could reach zero by 2045”. The 2017 study boils down to a straight line drawn between sperm counts of one group of men in 1973 and another group in 2011.
Just because a straight line can be drawn through data does not justify extrapolation of a line beyond its earliest and latest data points. We know that sperm counts of men in the early 1940s were around 113 million/ml, not the 145 million/ml you get from extrapolating backwards from Prof Swan’s research. When Prof Swan told Axios, “If you look at the curve on sperm count and project it forward” she was encouraging unjustifiable and unscientific interpretation of her data. When Ms Brockovich writes, “That would mean no babies. No reproduction. No more humans”, that is hyperbole. That’s just not science.
Whether or not declining sperm counts equate to reductions in fertility, in terms of babies being born, is not proven. Sperm counts would have to fall to one-third of their current level, to reach 15 million/ml, which is believed to be the cut-off for male fertility but there is no sign of that happening.
Relax, your penis is not shrinking
Body-shaming claims of shrinking penises are obvious clickbait but only a single study, of 383 young men from the Veneto region of Northeast Italy, links men’s penis size to the types of chemicals blamed by Prof Swan and Ms Brockovich for declining sperm counts.
Within Veneto are geographic zones that vary in their contamination by PFAS.
A group of 212 men who live in areas with maximal or intermediate PFAS exposure and have high levels of these chemicals in their bodies, had an average penis length (8.6 cm) about 10% lower than the average (9.7 cm) of a group of 171 men from an area without exposure.
There are a few features of this study that impact on the reliability of the observations and their generalisability to other populations:
- Men were grouped according to where they lived, not where they were born. Since genital measurements are determined before birth, the environment during their mothers’ pregnancies is more relevant to penis size than where the men lived at the time of the study. Some men will likely have relocated from their place of birth but how many, and in which directions, is not known.
- The exposures to PFAS of men living in the contaminated regions of Veneto are extreme, because of decades of industrial pollution. How the potential effect of such large exposures relates to smaller and more common exposures, like from plastic food wrap, is not known.
- The study is missing details about its subjects and the conditions under which measurements were made. It is usual to exclude people with conditions that might affect study outcomes but it’s not clear if this happened in the study of Veronese men. Variables that influence penile measurements (like temperature, posture, and the position of the penis) are not mentioned.
Even if Veronese men with high PFAS levels in their bodies10 do have shorter penises, the same study shows their sperm counts are normal, which fundamentally contradicts Prof Swan’s premise.
From a semantic perspective, for penises to be “shrinking” they must be getting shorter over time, on either an individual or population scale. We cannot find any reports of men’s penises shortening as a consequence of environmental pollution. Available data (here and here) certainly do not suggest a decline in penis size over the past few decades.
While environmental pollution is a pressing concern, the evidence suggests the catastrophic collapse of human reproduction and an accompanying mass shrinkage event is more sensationalism than science.
A commentary on ‘Plummeting sperm counts, shrinking penises: toxic chemicals threaten humanity’ (18 March 2021) by Associate Professor Tim Moss, Healthy Male Health Content Manager