alt Hacker News> testosterone levels have dropped without a solid explanation
There is a solid explanation.
First, before the adoption of mass spec, studies used a less accurate method of measuring testosterone that overstated testosterone levels.
Also, the studies showing the population level decline in testosterone generally controlled for obesity (which naturally lowers testosterone) using BMI. But BMI is a very crude measure.
When studies control with better methods like BMI + waist circumference, and only compare samples using the mass spec measurement method, the unexplained population level decline goes away. After fixing the measurement method, what remains of the decline can be explained by BMI + waist circumference. In other words, modern men are more prone to obesity and metabolic syndrome, which naturally reduces testosterone. Case closed.
That study appears to be US only, while global studies have shown the effect.
For that matter, some _animal_ studies have shown declining testosterone as well. That doesn't seem to be well-studied, but if it holds up it would make me lean toward it being something environmental (e.g. microplastic pollution)