There's a very credible argument that decreasing lead exposure over the last few decades (banning leaded gasoline) has driven a large portion of the decrease in violent crime.

Decreasing testosterone could also be a contributor, but it's unlikely to be as significant a factor from what I can tell.

See e.g. https://en.wikipedia.org/wiki/Lead%E2%80%93crime_hypothesis or https://www.motherjones.com/kevin-drum/2018/02/an-updated-le....