This article cites research that goes way beyond the usual sort of correlation. How much stronger would the evidence need to get?
And in case hackernews has become so infected with the kind of reddit thinking that can only see in black and white - yes I think reducing lead in gasoline is a very good thing.
In this case, lead exposure and crime rate are correlated, but maybe lead doesn't cause crime at all: maybe something else causes crime that also happens to correlate with lead exposure. Who knows, maybe a certain pesticide was used at the same time that lead gasoline came into vogue, and that's really the true cause of the rise in crime rates.
In research like this, when you can't do a manually controlled experiment, you have to control for hidden variables by some other means. And that's precisely what the investigators in this article did: they varied the input data to try to "shake out" other variables that might be behind the correlation. They looked at different time scales, different geographies, and different demographics, in an attempt to control for hidden variables that might be related to any one of those things. Every time you vary the input data and keep finding a correlation, your evidence of a causative relationship goes up.
The gold standard, of course, would be to expose two random, double-blind sets of infants to lead and to a control substance and see what happens. But since that would be unethical (as we have reason to think lead is bad for you), we're stuck with animal studies or retrospective studies. Personally, I find the evidence in this article impressive, but it would take quite a lot of looking into their specific methods to come to any real conclusion.
But in the case of lead, the causal chain from lead exposure to worse behaviour is fairly well understood.
The whole thing sound like classic fitting the preconceived conclusion. But there's also something like this:
>>>> Not only does lead promote apoptosis, or cell death, in the brain, but the element is also chemically similar to calcium.
How lead is chemically similar to calcium? They are on opposite ends of the periodic table and have very different chemical properties. That, of course, does not prevent lead from messing with ion channels, but saying lead is chemically similar to calcium is wrong.
Then you go on to explain how lead is like calcium in its bioavailability, so in relation to the operations of a human body, lead is chemically similar to calcium (only dangerous).
http://brain.oxfordjournals.org/content/126/1/5.full
For example, lead’s ability to pass through the blood–brain barrier (BBB) is due in large part to its ability to substitute for calcium ions (Ca2+)
How do you show that something is not caused by a hidden Z? All one can hope for is a preponderance of evidence. If the following passage from the article is true, then the preponderance here is stronger than a generic correlation-is-not-causation objection can just dismiss:
"In fact, use of leaded gasoline varied widely among states, and this gave Reyes the opening she needed. If childhood lead exposure really did produce criminal behavior in adults, you'd expect that in states where consumption of leaded gasoline declined slowly, crime would decline slowly too. Conversely, in states where it declined quickly, crime would decline quickly. And that's exactly what she found."
Thus your hidden Z would have to move in sync not only with X and Y, but also with their widely varying rates in different states. That's a high bar – especially if there are legal and institutional explanations for why leaded gas was phased out at different rates in those places.
For example, if you're doing an experiment, you should report everything that you think might make it invalid — not only what you think is right about it; other causes that could possibly explain your results; and things you thought of that you've eliminated by some other experiment, and how they worked — to make sure the other fellow can tell they have been eliminated.
I did not see such explanation done in this case - it looks like everything described talks about what matches the hypothesis, and no effort is done to look for alternative explanations and refute them.
>>>> Thus your hidden Z would have to move in sync not only with X and Y, but also with their widely varying rates in different states.
Use of gasoline is highly correlated with use of cars, which again is highly correlated with income, population density, business activity, etc. Any of which can be also associated with crime. I.e. if both effects are caused, say, by population density (I do not say it is, I just take it as an example), then gasoline use would raise and crime would raise when population density raises (say, because some economic factors attract people, like it happens now in North Dakota due to shale gas boom) and both would fall when some factors cause population to move away - as it happening in Detroit, for example. There might be also other factors.
>>> In fact, use of leaded gasoline varied widely among states, and this gave Reyes the opening she needed. If childhood lead exposure really did produce criminal behavior in adults, you'd expect that in states where consumption of leaded gasoline declined slowly, crime would decline slowly too. Conversely, in states where it declined quickly, crime would decline quickly. And that's exactly what she found.
And this is exactly what she would find if both were strongly related to some third cause Z, but completely causally independent from each other. The fact that the MJ author takes it as evidence confirming the causation means they do not understand how causation needs to be proven. It is not evidence for causation, since it equally applies to both when causation exists and when it does not.
Yes, this means causation is hard to prove. It is.
Does that mean that causation is proved? Of course not. But it does mean that the evidence here is stronger than garden-variety correlation.
By doing a controled study.
Of course, in this case, we're better not knowing than paying that kind of cost.
How could the entire population ever be better off 'not knowing'? If there was a button to exchange a few babies or animals lives for the possible outcomes presented at the end of the article, I would do it in a heartbeat.