Can't help wondering to what extent the decline is directly and/or indirectly influenced by both the positive changes (eg increased women's rights and power) and the negative (presumption of masculine "toxicity" and fallibility) in socio-culture over the period surveyed.
Or the negatives (toxic femininity)
Unsurprisingly, the authors didn’t name “women’s rights” or any other feminism-adjacent culture war issues as a cause of declining testosterone. They did name obesity and diabetes.
In other words, if you’re looking for a boogeyman, blame sedentary lifestyles and ultra processed foods.