With overpopulation the way it is, might just be a literal safe guard built into our DNA.

Modern life is neutering us

@jordan That's not how DNA works. And there's no overpopulation in the West (or, arguably, anywhere). This is more likely due to a foreign substance introduced into our precious bodily fluids.

" there is no overpopulation in the West (or, arguably, anywhere)",
Really?
Tell that the the commuter on the Northern line...
Why are we devastating so much land to feed and house our population if it's not increasing?

Even a level of well below 15 million does not imply infertility either.
However if there is a decline then there will be a reason, whether its in the way the participants are chosen, remembering there will be both social and religious reasons of why the samples might not be representative of the population.
Later studies might be more representative, so maybe the real norm is a lower?
There is of course the worrying probability that pollution or life styles is the cause. But low sperm count can often be improved dramatically in months by simply improving diet (fruit etc.) and other life style changes, including, and surprisingly, less rigorous exercise!
So don't panic (just) yet!

@El Bonko
Actual how genes are expressed can be altered by changes in environment and not just DNA - its like a short term change to how the 'DNA' is used, which from a survival perspective makes sense.
If our bodies picked up something that hinted at overpopulation, then its not impossible its a response. For example a lot of estrogen in the environment might be indicative of overpopulation, so the mechanism kicks in to slowly reduce population rather than a crash.
Above just a possible mechanism - not saying it is!
As for overpopulation, its a matter of definition, but a population that can't healthily sustain itself or with a population dense enough to allow disease to run through it without a 'fire' break is an overpopulation!
London easily makes that definition, as do many other cities!

Another change that needs looking in to is the number of young men losing their hair prematurely. Among my 65-70 year old friends about half of us still have a fairly full head of hair but among our sons most are going bald in their 20s and 30s. Worse yet, the trend seems to be accelerating with the younger ones showing markedly thinning hair right out of college. This was rare 50 years ago but is quite common now. What other effects are we going to see as time goes on. Another interesting thing is diet. Many of the people I know that grew up eating bacon, greasy sausage, real butter, and eggs are living into their late 80s to early 90s. Those of us that are 20 years younger and grew up in the fast food era aren't fairing so well. About a third of my high school class mates were already dead at our 50th reunion and another third in badly failing health. The average lifespan will soon be dropping at a rapid rate. Even with medical advances, science and the proliferation of chemicals is killing us.

We're still arguing that the west is over populated huh? Look at the birth rates compared to the replacement fertility rates. In short you need about 2.1 kids per man/woman just to maintain the population. The west is way below that as is Japan and a bunch of other places.. There's a ton of really smart and educated comments in New Atlas articles - the ones crying about over pop are not those. Spend just a few minutes doing some research - population numbers are not the problem. I just read an article a few weeks ago that tried to make the case that Venezuela's population was the reason for all their shortages and crime. It'd be funny if it wasn't so sad..

This trend precisely matches the "chickafication" of the West and Japan. The brain has a powerful effect on the body.

One word: Chemtrails...
Seriously, our western diet, prescription and "other" drug consumption, built environment, and stress are all factors we should study.