How Daylight saving time fights crime

When in 2007 Congress decided to extend the period daylight saving time, known as DST for short, by four weeks each year — three weeks in spring and one in fall — MIT researchers saw an opportunity:

“This [increase] produced a useful natural experiment for our paper which helped us isolate the effect of daylight from other seasonal factors that might affect crime,” authors Jennifer Doleac and Nicholas Sanders

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It probably doesn’t come as a surprise to anyone that criminals thrive in the dark, and now we know by exactly how much: when DST comes into effect in spring, researchers identified a 7% decrease from average in daily number of reported robberies, and a whopping 27% decrease during the now-brighter evening hour. They also found strong evidence of a decrease in the incidence of rape in the evening hours.

And, according to the paper, thos decrease during evening hours isn’t accompanied by an increase in the morning.

But wait…DST shifts an hour of darkness from sundown to sunrise so, how can there be a decrease in overall daily crime? It’s dark for just as long each day, only at different hours, it doesn’t make sense!

It turns out it’s laughably simple — criminals just aren’t morning people.

“Most street crime occurs in the evening around common commuting hours of 5 to 8 PM,” the authors write, “and more ambient light during typical high-crime hours makes it easier for victims and passers-by to see potential threats and later identify wrongdoers.”

And as every crime carries a social cost — pain, suffering and lingering trauma, but also including monetary losses suffered by the victim, medical costs and lost earnings for the victim, police funding, legal services and incarceration costs for the government — the team wanted to see how many pennies we can save if we extend the period when DST is in effect.

Previous studies have estimated the total social cost of a single robbery at somewhere around $42,000, and that of rape at $240,000.  When you add it all up, a three-week increase in DST in the spring of 2007 saved the country some $246 million, the authors estimate.

“Assuming a linear effect in other months, the implied social savings from a permanent, year-long change in ambient light would be almost 20 times higher,” they conclude — several billion dollars annually.

They do caution, however, that this is just an assumption and that more research would be needed to determine whether the drop in crime from enacting permanent DST would hold true year-round.

Their work only adds weight behind the push to permanently extend DST year-round, a tiny change that has the potential to save hundreds of lives just from alone, and and . On the flipside, it could negatively affect certain crops and the farmers that grow them, and there are concerns about leaving schoolkids waiting for buses in the dark hours of the morning (though maybe we could change that too).

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