Attention, shoppers: If the cart you selected has a handle greased with Vaseline, you may be an unwitting participant in an undercover experiment.

Ditto if you find an envelope stuffed with cash hanging out of a mailbox.

More than 600 people unknowingly took part in a series of "field experiments" in Groningen in the Netherlands designed to test the "broken window" theory, which posits that bad behavior begets bad behavior. That is: if someone sees, say, graffiti scrawled on a building, he or she will be tempted to do the same or commit some other illegal or mischievous act.

In fact, sociologists often cite the theory as a possible reason that petty crime in New York City dropped in the 1990s after the city scrubbed buildings, trains, buses, walls...clean of graffiti.

In an attempt to test the theory, psychologist Kees Keizer and his colleagues at the University of Groningen report online in Science that they conducted six experiments.

In their paper, "The Spreading of Disorder," they explain that they scattered shopping carts around a parking lot to provide a chaotic backdrop for one experiment.  They then smeared their handles with petroleum jelly to discourage customers from messing with the setup. (They say the deterrent worked "without exception".)

The researchers discovered that people in the shopping cart–laden lots were more likely to throw fliers placed on their windshields on the ground than those in a cart-free lot (58 percent compared to 30 percent).

In another test, they left envelopes visibly containing cash sticking out of public mailboxes and found that folks were more likely to make off with the money when the mailbox was covered in graffiti. In four other experiments, the researchers consistently found that graffiti, litter, fireworks (apparently a no-no in the Netherlands in the weeks leading up to the New Year), and other violations led people to litter, steal and do other naughty things.

(Photo from iStockphoto/Egor Mopanko[Egorych])