Reduced-fat peanut butter spreads can be as little as 60 percent peanut. (By law, products labeled "peanut butter" in the U.S. must be at least 90 percent peanut.) Nutritionally void ingredients like corn syrup solids, sugar, and unhealthy oils make up the difference.
Pick a full-fat option labeled "natural," or look for a layer of oil at the top of the jar. Check out the ingredients, too, which should list only two or three items, with peanuts first.