You probably have heard of the data about men's average testosterone levels over the past few generations. Every year that passes the average level goes down about 1%; the average 40 year old man in 2017 had about 30% lower levels than the average 40 year old in 1987. And this descent maintains its rate, it doesn't seem to be slowing down.

Testosterone levels affect growth, behavior, cognitive skills and intellectual capacity, propensity to develop depression and other afflictions, bone density, muscle development, genital development, fertility... it's incredibly important, for so many health and psychological issues, to present a healthy level of testosterone.

But you can't just change that. Your lifestyle choices only affect its production to a small degree; it's mostly determined by genetics and the first phases of growth (below age 14 or so). Which takes me to the point:

Testosterone levels can't be increased too much through external factors: most of it is determined at birth, and another big chunk of the equation is already determined by age 14 approximately. And the free fall appears to continue: 1% lower average levels every year. At this rate, if you take a man from 2040 and take him back to 1987 his testosterone levels would be closer to a woman's than a man's. And that has, is already having, incredibly heavy consequences.

No group of scientists has determined what is causing this. Obesity, the lack of smokers, lack of physical exercise... all external factors were studied and they turned out to only account for a small part of the change. They just don't know what in the world is happening.