Less young men too - birthrate way down. When I was in 1st grade or so (late 60s early 70s), it was the highest population ever for that cohort. Young men commit a disproportionate rate of violent crime. Less young men, less violent crime.
IOW, there were many, many changes during the period under discussion. Latching onto one of them as “the cause” of the decline is not real science.
Are you saying that I’m violent? ARE YOU?
Let’s step outside, bub, and settle this thing RIGHT NOW!
/sarc