Of course, such a giant, sweeping trend doesn't have just one simple cause, but studies have shown that it might have a few shithouse-crazy ones. So if you're feeling safer these days, science says it could be thanks to things like ...

You might not have noticed this in the middle of all of the bad news that floods the daily headlines, but crime in the U.S. is at its lowest point in pretty much ... well, ever. It's been steadily falling since the early '90s. And nobody knows why.

6 Getting the Lead Out of the Environment

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Lead poisoning is one of those things our ancestors dealt with so thoroughly that we have trouble today realizing what a problem it was. But when our grandparents and great-grandparents were young, lead was in every-damn-thing from gasoline to paint to baby formula (OK, so maybe not that last one). And since lead exposure causes all sorts of fun things, like schizophrenia and low IQ, leaded-up babies were more likely to drop out of school and end up with prison records and regrettable tattoos.



"I shanked a kid for his Lunchables the other day."

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How deeply did lead's toxicity affect society? Well, economist Rick Nevin believes it might just be a major underlying cause of the recent global crime decline.

Nevin studied the criminal histories of nine countries and found that in each case he could link significant crime drops with that country's campaign to eliminate childhood lead poisoning. In his own words, "65 to 90 percent or more of the substantial variation in violent crime in all these countries was explained by lead." You could say that the timing was just coincidence, but each country Nevin studied conducted those campaigns at different points in time, and in every case, 20 years after lead poisoning rates fell, crime started to fall. Another study in 1990 showed that U.S. counties with high lead levels had four times the murder rate of counties with low lead exposure.

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Shotgun victims are also very likely to have high lead exposure.

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Now, as with every entry on this list, we'll never know for sure how true this was -- you could just as easily say that the type of country or era that is advanced enough to successfully deal with environmental problems is also capable of fighting crime. Still, it makes us wonder what stuff we're all being exposed to right now that will turn out to have been making us crazy. We're imagining a future earth in chaos, roving bands of savages driven mad by a decade of iPhone fumes.