The study states the majority of the effect is for fairly predictable causes like age and obesity, but I found this interesting:
> After controlling for confounders—including year of study, age, race, BMI, comorbidity status, alcohol and smoking use, and level of physical activity—total testosterone was lower among men in the later (2011-2016) versus earlier (1999-2000) cycles (P < 0.001)
Excluding all those confounders, there's not much we're left with. Environmental toxins?
> After controlling for confounders—including year of study, age, race, BMI, comorbidity status, alcohol and smoking use, and level of physical activity—total testosterone was lower among men in the later (2011-2016) versus earlier (1999-2000) cycles (P < 0.001)
Excluding all those confounders, there's not much we're left with. Environmental toxins?