A doomsday scenario of an
end to human sperm production has been back in the news recently, now with
the added threat of shrinking penises.

Professor Shanna Swan, a US
epidemiologist who studies environmental influences on human development,
recently published a new book called Countdown.

In it, she suggests sperm
counts could reach zero by 2045, largely owing to the impact of a range of
environmental pollutants used in manufacturing everyday products: phthalates
and bisphosphenol A (BPA) from plastics, and per- and poly-fluoroalkyl
substances (PFAS) used, for example, in waterproofing. Under this scenario, she
says, most couples wanting to conceive would need to rely on assisted
reproductive technologies.

She has also warned these
chemicals are shrinking penis size.

Such extraordinary claims require
extraordinary evidence. I would argue the evidence is not strong enough.

Correlation
doesn’t equal causation

Epidemiologists find
associations between disease and potential contributing factors, like lung
cancer and smoking. But their work can’t identify the causes of disease — just
because two things are associated doesn’t mean one is causing, or caused by,
the other.

An article written by
environmental activist Erin Brockovich in The Guardian in March leads
by referring to “hormone-disrupting chemicals that are decimating fertility”.
But causation is far from established.

It’s reasonable to expect
chemicals that affect hormone function in our bodies, like BPA and
PFAS, could affect reproduction in males and females, given available evidence.
But we don’t have irrefutable proof.

Selective
reporting

In 2017, Swan and several
colleagues published an exhaustive review study showing an apparent
drop in men’s sperm counts of 59.3% between 1973 and 2011. This research
informs the arguments Swan makes in Countdown and those we’ve seen in the
media.

What’s not often mentioned
is the fact the researchers only observed a decline in sperm count in groups of
men from North America, Europe, Australia, and New Zealand, but not in groups
of men from South America, Asia or Africa.

When Swan and her colleagues
combined the data from all countries, they saw a decline because the studies of
“Western” men outweigh those of men elsewhere (in the number of studies and
participants).

The problem with
extrapolation

Swan’s 2017 study boils down
to a straight descending line drawn between sperm counts of groups of men
studied at different times between 1973 and 2011.

Just because a straight line
can be drawn through the data, this doesn’t justify extrapolation of that line
beyond its earliest and latest data points. It’s unscientific to assume trends
in data exist outside the range of observations.

We know sperm counts of men
in the early 1940s were around 113 million sperm per ml of semen, not the
roughly 140 million/ml you get from extrapolating backwards from Swan’s
research. Concluding sperm counts will reach zero in 2045, based on extrapolating
forward from the available data, is just as likely to be incorrect.

When Swan told news
website Axios “If you look at the curve on sperm count and project it
forward” she was encouraging unjustifiable and unscientific interpretation of
her data — even though she acknowledged it was “risky” to extrapolate in this
way. Unfortunately this caution is too often unmentioned.

For example, Brockovich
writes: “That would mean no babies. No reproduction. No more humans.” That’s
hyperbole. It’s just not science. 

Relax, your
penis isn’t shrinking

Claims of shrinking penises
are obvious click bait. But only a single study, of 383 young men from the
Veneto region in north-eastern Italy, links men’s penis size to the types of
chemicals Swan attributes to declining sperm counts.

Within Veneto there
are geographic zones with varied levels of PFAS contamination. A
group of 212 men who live in areas with high or intermediate PFAS exposure and
have high levels of these chemicals in their bodies, had an average penis
length of 8.6cm, about 10% lower than the average of a group of 171 men from an
area without exposure (9.7cm).

But a few features of this
study affect the reliability of the observations and whether we can generalise
them to other populations.

1.     Men were grouped according to where they lived, not
where they were born. Since genital size is 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 where they may have moved
to and from, we don’t know

2.     The levels of PFAS exposure for 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 to pollutants, like from plastic food
wrap, we don’t know

3.     The study is missing details about its subjects and
the conditions under which measurements were made. It’s usual to
exclude people with conditions that might affect study outcomes, such as
congenital abnormalities, but it’s not clear whether this happened in the
study. Variables that influence penile measurements (such as room
temperature, posture, and whether the penis is held straight or hanging) are
not mentioned.

And from a semantic
perspective, for penises to be “shrinking” they must be getting shorter over
time, on either an individual or population basis. I cannot find any reports of
men’s penises shortening as a consequence of environmental
pollution. Available data don’t 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 accompanying penis shrinkage is thankfully a
pretty unlikely prospect.