The only way to get around these obstacles would be to
feed one group of people doses of lead and compare them to
another in a controlled setting, or isolate two groups of
people, give one group the right to birth control and
abortions, deny the other group those rights, and keep
them from finding out about each other. Things that cannot
be done for ethical reasons, and things that prevent
social psychology from rising to the level of science.

And yet in another comment, you say that cosmology, which has the same problem with not being able to vary the inputs and observe the effects on the outputs, is a real science. Why? Because it offers falsifiable hypotheses; you can make a prediction based on certain data, then test that prediction against other data.

The exact same is true of this topic. As I pointed out in my comment above, in portions that you conveniently ignored, there are strong predictions that this research makes, which can be tested. The original paper on this topic was written in 1999, and there is already 15 years worth of additional data since then; the next 10 to 15 years will provide even more data, which could falsify or could be consistent with this hypothesis.

This is why social psychology cannot be a science, and it
explains why the field has the terrible reputation it has
and deserves.

I don't know why you bring up social psychology, as this study is not social psychology. It's more environmental sociology. Social psychology is the study of how people's behavior is related to their beliefs and attitudes about other people. This study is about how chronic, population wide lead poisoning relates to criminal behavior.

And while I share your skepticism about social psychology, the mere presence of corrupting influences such as the ones you mention, the desire for snappy, headline-ready results, does not mean that an entire field is devoid of value or not science. There are other fields, such as pharmacology, which likewise have troubling corrupting influences, and yet can also be real science that produce real value.

Anyhow, we've diverged pretty far from the original topic, as the original topic is not even within the field of social psychology.

you haven't the slightest idea what makes science science.

you have no idea what you're talking about. Not a clue

you do not know what it is. How do I know this? You
defended social psychology for over four hundred words
without even mentioning it.

Please refrain from the ad-hominem attacks. They bring the level of the discussion down considerably.

I do not mention social psychology because this research is not social psychology.

This research is founded on well known, rigorous research on the effects of lead poisoning.[1] This is not some surprising, out-of-left-field result which came out of the blue from some small study on a handful of college undergrads, which is the problem with many psychological and social psychology findings; this is based on building on many years of research and meta-analysis of epidemiological research. The original paper on this topic[2] was written in 1999, follow up papers from other authors have been published in 2007[3].

There have been further studies confirming this effect; this 2008 paper in PLOS Medicine[4] measured prenatal blood lead levels in pregnant mothers, childhood lead levels, and later correlated that with arrest rates. They too found a strong correlation. Other studies on the same cohort have shown a strong correlation between blood lead levels and reduced brain mass[5].

Here's a paper that compares several of the hypotheses that you have brought up[6]. Now, I take this paper with a large grain of salt, because it clearly comes from a biased source, the Heartland Institute, which is clearly biased towards libertarian findings; and the point of the paper is to argue against drug prohibition, or excessive enforcement of it (I am sympathetic to that viewpoint, but view their research on the topic with a heavy dose of skepticism). However, they compared several of the proposed determinants of violent crime rates, such as incarceration rates, access to legal abortion, environmental lead, and so on, and the only one of them (outside of the one that they're obviously promoting, drug prohibition) which held up with a consistent correlation was environmental lead.

[1]: https://en.wikipedia.org/wiki/Lead_poisoning

[2]: http://www.ricknevin.com/uploads/Nevin_2000_Env_Res_Author_M...

[3]: http://www.nber.org/papers/w13097.pdf?new_window=1

[4]: http://www.plosmedicine.org/article/info%3Adoi%2F10.1371%2Fj...

[5]: http://www.plosmedicine.org/article/info%3Adoi%2F10.1371%2Fj...

[6]: http://news.heartland.org/sites/default/files/c11845.pdf