BBM
I know of no research to substantiate your allegation.
"The nation's IQ scores have kept climbing over the past 100 years."
...
"According to his extensive research, IQ test scores in the U.S. increased by an average of three points per decade during the 20th century."
http://theweek.com/article/index/219002/are-americans-smarter-than-ever
In regards to your lead theory, almost all homes at the turn of the century had hazardous lead levels, many if not most of those homes have been destroyed, and the remaining ones should have had the lead levels removed to conform to modern housing codes. Kevin Drum at Mother Jones has done some interesting work on the various research on the effects of lead exposure on crime due to the lowering of impulse control.
Let's see what I can find:
Here is one that seems to apply mainly to Europe:
http://news.yahoo.com/blogs/sidesho...qs-dropped-14-points-over-last-180634194.html
I do read the Daily Mail so it's possible I read about a European study. I didn't remember where I heard or read the statistic, so that's why I said "so I've heard". Thank you for reminding me why it's good to find a link for things. If it's a European study I shouldn't say it's from the US. I find it hard to believe that IQ is up and not down based on what I see around me, but of course people would argue about what is a good IQ test.
Anyway, it's hard to evaluate a study from a news article but that was a possible theory trying to make sense of incomprehensible behavior. You know how they publish headlines saying this is good to eat then not too long after, another study saying it will kill you. You have to look at the actual study and you also have to know who paid for it and who did it to know if there is any truth to it.
Here is info on lead in St. Louis - http://leadsafestlouis.org/
"One child in 20 in the City of St. Louis has a blood lead level at or above the CDC level of concern."
"90 percent of the housing stock in St. Louis City was built before 1978 and may contain lead based paint."
http://leadsafe.aphidesign.com/risk/could-my-home-have-lead/
Side effect of lead - "aggressive and violent behavior"
http://leadsafe.aphidesign.com/risk/why-is-lead-dangerous/
It's not my idea that this might be a factor in high crime rates, I've heard this talked about before. I'm not a scientist so I can't say how much effect it might have. Just something to consider when people see behavior that makes no sense and ask, "why"?