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These traits (violence and low IQ) are both caused by exposure to lead. This study is only novel and interesting if they controlled for that (at least using age).

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In 3.4a they say the groups show "Absence of differences between groups in potential confounding variables (e.g., number of participants per group, socioeconomic level, educational level, among others)." Intragroup ages are provided in Table 2.

I do not know why you were voted down, I have seen a few articles stating the same. It showed violent crime has decreased and the main correlation was the reduction of lead in the air.

In 1975 lead was eliminated from auto fuels in the US. As lead in the atmosphere decreased so did violent crime. Crime stats were from the FBI. They noticed also the IQ increased as time went on too.

So like you, I think they need a better control group. Maybe based upon year born and lead levels as the child grows to adulthood ?

This is a minor point but leaded fuel for autos wasn't phased out until much later than 1975.

My memory from back then is iffy as I was rather young. But I think you are correct, maybe it was a phase in.

I know I went looking for another vehicle back then due to lead and rumors saying unleaded gas could eat away at the engine gasket.

I ended up moving to a unleaded car in 1983, indicating I could still find leaded gas after 75.

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