I don't think the crack epidemic is very relevant since it was primarily a US only thing. I do agree that all the hypotheses have problems.
Young people (say, 14-25) are just as murderous as ever.
I'd be very interested in seeing a citation for that. I was under the impression that probability to murder has gone down among all age brackets. Certainly in the late 1980s there were warnings that the temporary boom in young people born from the baby boomers were going to cause an increase in crime(see e.g.here), and yet crime rates kept going down in the 1990s.
There is by the way one very strong piece of evidence for the lead hypothesis: individual states had different amounts of lead and we can look at the varying crime levels in those states. That data shows that lead levels account for (with about a 20 year delay) a large part of the change in crime levels. See here. There's one serious confounding factor: more progressive states were more likely to clamp down on lead levels earlier, but it doesn't seem to be that serious a complicating factor in this context.
Yes, improved policing is underspecified, and that's my fault: I don't think highly of the hypothesis and so I didn't bother expanding into what proponents mean. They mean using detailed analysis to locate crime hotspots and better forensics (so one is more likely to catch repeat offenders). Most proponents of this seem to combine it with the high incarceration rate it seems.
Did you click on my link? Surely if you did, you would recognize the source and not need me to provide a precise citation.
Another serious confounder in that study is that the author is an economist.
I was under the impression that probability to murder has gone down among all age brackets.
Oh, come on. Everyone takes as their baseline the crack epidemic.
An article by Nyan Sandwich on More Right.
This is the section that I am particularly interested in discussing, building better models of this has clear consequences for futurism as well as ambitious effective altruism: