Generally speaking, shoppers prefer bonuses (such as a buy one, get one offer) to discounts (50% off), even if the net price is the same. Among other reasons, "the perception is that they're getting something free, and people just love" that, Arul Mishra, an assistant professor of marketing at the David Eccles School of Business at the University of Utah, tells the Health Blog.

But the dynamic that makes the BOGO special or extra quantity so appealing may not work so well with so-called vice foods, according to a study Mishra...