Take a look at these 2 graphs
In the first graph, you see what is purported to be a casual relationship. Namely, as lead levels increased from 1941 to 1969, crime increased 23 years later (another interesting note: most violent crimes are committed by those aged 16-19, so this graph is off by roughly 4-7 years - http://www.fbi.gov/about-us/cjis/ucr/ad ... t93-01.pdf). However, note that the red line denoting crimes ends in 2005. But, since the red line so closely follows the blue, and if this is a valid theory, then it should be predictive, right? In other words, if we know the lead level, we should be able to predict the number of violent crimes for 2006-2010. Roughly, they should be about:
YEAR - # Crimes
2006 - 475
2007 - 400
2008 - 300
2009 - 200
2010 - 100
Contrast that with the last graph. It shows that crime rates for 2006-2010 are actually:
YEAR - # CRIMES
2006 - 475
2007 - 475
2008 - 450
2009 - 430
2010 - 400
Today, with lead levels almost non-existent, our violent crime levels should mirror that, if lead is to blame. However, if the abortion theory is right, we would not expect crime to go away, because all babies born into "bad" circumstances aren't aborted, just some. So, we would expect crime to level off at some point . . . which is what it has done.
In other words, for a theory to be accurate, it has to be predictive. The lead theory may account for some portion, but not enough to drive the trend. Otherwise, it could predict the trend. On the other hand, the abortion theory has done exactly what you would expect a valid theory to do - predict a trend.
