It's well known that lead is very poisonous and can cause neurological problems, especially in young children, so we should not be exposing people to it regardless of whether it's responsible for long-term crime trends. With that said though, I find a lot of these analyses very dubious. The UK banned unleaded paint in 1992, and petrol in 1998 (as far as I can tell no serious move was made to even reduce the usage of it until the late 80s). Under the "Banning lead causes crime rates to drop 23 years later" hypothesis you'd expect violent crime in the UK to still be rising, or at best for the trend to have reversed just a few years ago. This isn't the case however - in actual fact the violent crime rate began to drop in the early 90s, just like in the US.
http://news.bbc.co.uk/1/hi/uk/7511807.stm
The USA banned leaded fuel in
1996. They had a similar lead-time, therefore it's not a discrepancy. 1996 vs 1998 is pretty close.
But the USA brought in unleaded in 1976, and the UK only brought in unleaded in 1986!
That sounds like a problem but then you can find citations to say that the UK cut allowable lead fuel concentations in half starting in 1976, from 0.84 g/l to 0.4 g/l, then they cut them again to 0.15 g/l in 1986. So the dates
should actually correlate with the USA data. Both countries had 20 years of phasing down lead concentrations in leaded fuels. While the USA brought in unleaded a bit earlier, they were actually coming down from a higher concentration of lead than the UK, it started at ~1.05 g/l in the USA
Similar with paint, once, it was normal for leaded paints to be up to 50% lead, but by the end, it was a small percentage:
http://www.environment.gov.au/protection/publications/factsheet-lead-alert-facts-lead-house-paintThe recommended amount of lead in domestic paint has declined from
50% before 1965, to 1% in 1965. In 1992, it was reduced to 0.25%, and in 1997 it was further reduced to 0.1%. So, when you say "lead in paint" it dropped by a factor of 50 between the end of WWII and 1965. Current "leaded" paint is 500 times less lead than the 1950's lead paint. So the UK 1992 ban on leaded paint was the end of a long series of reductions anyway.