Start with the numbers, because the numbers are the story. In 2020, researchers pulled serum testosterone samples from 4,045 American men aged 15 to 39, spanning 1999 to 2016, and found that average total testosterone had fallen more than 25 percent — from 605.39 ng/dL down to 451.22. Even among young men with normal body weight, it fell 20 percent, from 664.8 to 529.2. The researchers adjusted for age, race, illness, drinking, drugs, and exercise. The trend didn't budge.