That's not all of it. "Lokeshwar noted that even men with a normal BMI (18.5-24.9) had declining total testosterone levels (P < .05) during the same time frames." lack of hard physical labor in general, or modern diets just being filled with hormones?
Lack of hard physical labor, combined with increasing average caloric intake may contribute to lower average muscle mass & total testosterone while still maintaining a "normal" BMI. I don't have evidence to support this, but this is my guess for falling testosterone within the "normal" BMI range over time (in addition to the effect of environmental pollutants on hormones).
The "real" measure of how fat you are is body-fat percentage, which can vary at a given BMI level. It's not hard to imagine that people have gotten fatter at the same BMI level, as well as people being at the higher end of the "normal" range.
Isn't it a fair question to ask though whether the average BMI of those with a "normal BMI" (18.5-24.9) changed during the time frames of interest? And if it did, couldn't that still be a cause?
It's not the only cause but, from my experience, it's a big one, no pun intended.
I might have made different diet choices as a young man if I knew that body fat aromatizes testosterone into estrogen. This is the primary cause for most cases of gynecomastia, which is why it's rare to see a man with breasts who isn't also overweight to some extent. Obesity is effectively feminizing a large portion of men and lowering their sexual market value. Overall, I think obesity is making people less attracted to each other, which can partially explain the decline in the rate of reproduction.
> which is why it's rare to see a man with breasts who isn't also overweight to some extent.
While (assumedly) true, I feel the need to note that the reverse does not hold generally: 'man boobs' are purely fat in nearly all cases and will go away when someone loses weight.
Gynecomastia is breast tissue and does not go away if you lose weight.
In my opinion, it's the biggest problem facing us. Even this COVID pandemic would have been much less expensive without an obese population, because our hospital capacity may not have been pushed to its limits. CDC says being obese triples the likelihood a person would need hospitalization:
