"The lead–crime hypothesis is the association between elevated blood lead levels in children and increased rates of crime, delinquency, and recidivism later in life."[1]

Basically, during the 20th Century, lead were used in a lot of products, including gasoline and paint. To shocking surprise, it turned out that this lead slowly ended up in our environment, and built up in our bodies.

The hypothesis is that because virtually the entire population got lead poisoned, we became more affected by "learning disabilities, decreased I.Q., attention deficit hyperactivity disorder, and problems with impulse control", which again leads to more crime.

Lead was eventually phased out (no thanks to the companies behind the products), and this correlated with a drop in US crime-rates in the early 1990s.

Since then there's been lots of discussion on why the crime rate dropped. The removal of lead from the environment is one (an alternative, but not mutually exclusive, hypothesis is the legalized abortion and crime effect[2]).

[Personally, I've been wondering how this also affects other qualities of the people affected... Would people born before the 1970s have been more intelligent than they are currently? Could this have given us better political leadership, and better secondary and tertiary effects?]

1: https://en.wikipedia.org/wiki/Lead–crime_hypothesis

2: https://en.wikipedia.org/wiki/Legalized_abortion_and_crime_e...

I wonder when someone will see if the rise of home gaming consoles had an effect. A Nintendo effect, if you will.

Why did the most enormous progress happen when this poisoning was widespread? And it wasn't only once but at least twice: In Ancient Rome lead was also widely used. The whole idea of "chronic toxicity" is bunk. It will eventually turn out that lead is essential.

Lead (Pb) the metal.

https://en.m.wikipedia.org/wiki/Lead%E2%80%93crime_hypothesi...

Some good article here - but this is my favourite.

https://www.motherjones.com/environment/2016/02/lead-exposur...

By the same author.

Lead causes people to behave aggressively and erratically. The idiom “mad as a hatter” comes from the fact that hatters were around lead a lot for some reason I forgot.

Edit: Ah, apparently it was mercury, not lead. Cute anecdote anyway.

Hatters used mercury, not lead.

I knew a person who suffered from lead poisoning (long term occupational exposure).

She wasn't aggressive in any way, just not really "here".

Did she ever get better?

Unfortunately not. Or, any chance for that was lost when she had a stroke.

Lead is needed for the brain. The person is fully there (in fact more than before) they only stop uncontrollably reacting to everything, and stand straight, instead of being all wobbly.

I think you're confusing lead with mercury for the hat-making connection.

why do you need either of those for making a hat?

When felt was made from animal fur, it made it easier and less gross the remove the hair from the hide and soften it.

Previously, urine was used.

Yikes. Knowing what we know now, is mercury the worse option?

It depends. Urine is not pleasant, but is usually safe to work with. I’d rather smell than suffer brain damage. :)

shiny when applied to silk.

Mercury and lead poisoning have similar effects.

Poisoning with lead has been linked with sudden rage attacks and homicides. Lead affects the nervous system.

The abstract of the paper describes it.