A mirror installed in an El Paso grocery cart is part of an effort to get Americans to change their eating habits, by two social scientists outmaneuvering the processed-food giants on their own turf, using their own tricks: the distracting little nudges and cues that confront a supermarket shopper at every turn. The researchers, like many government agencies and healthy-food advocates these days, are out to increase consumption of fruits and vegetables. But instead of preaching about diabetes or slapping taxes on junk food, they gently prod shoppers — so gently, in fact, that it’s hard to believe the results. …