The answer is really simple. Genetics. It's clear for european states.
During WW2, all the real men went off and fought in the war. Most of the european men died. The only men left were the pansies. Thus, the only men left to breed with were girly men.
Same thing happened here around the Vietnam Era. The "Real Man" started losing ground to the "Girly Man" around the same time as we started to lose the vietnam war. Women started breeding with the "Girly Man," and ever since then testosterone levels have been falling sharply.