Scientists try to tease our causation from correlation, relying on what the data explains best, controlling for factors when they can, and coming up with the best explanation possible...
But you're right, it's not perfect, and there is no singular reason for crime. There are millions.
Parental involvement, education, community involvement, gun control, incarceration rates, drug control, mental health services, income inequality, access to birth control, etc.
However, despite the best efforts of Jim Manzi (expert statistician and conservative, who found that the above could explain up to 20 percent of the following, but failed to replicate those results based on further evidence) no combination of the known crime correlates could explain this:
Steven Levitt (of Freakonomics fame) pleased many liberals, including me, when he published data that showed a probable link between abortion and the drop in crime, which wasn't perfect but explained the phenomenon better than anything else at the time.
That's what I believed up until last month. Call me a flip flopper, but my beliefs change based on evidence.
Cities, states, and countries all over the world have seen a similar rise and decline. The similarity they all share? Not abortion services, not single motherhood, not black youths - just the rise and fall of leaded gasoline, which can explain up to 90% of the crime wave over the late 20th century.
The evidence goes further than the national, state, and city level - follow-up studies have found it on the individual level. Childhood blood lead levels are consistently associated with arrest rates for violent crimes.
All that leaded smog didn't completely go away, it's still in our soil - especially around inner-cities where traffic was worst. It's still in old houses, it's especially in old windows.
If you're not convinced, just wait. Western Europe used leaded gasoline longer, so we can expect crime to drop there in the next 20 years, with a drastic drop over the next 10. In Russia, the drop should happen a little later, since the Soviets used leaded gasoline longer. We should see the beginnings of a crime decrease in Latin America (they need it) around 2018, since they used leaded gasoline until 1995.
If crime decreases don't happen in the countries above, we can label the theory false or flawed.
Regardless, this is all much more interesting to me than Ted Nugent's opinions or txtebow's racial stereotypes.