The inventor of Doritos envisioned this snack in 1964 as a marketing powerhouse that could deliver endless varieties of new flavors. But none of the formulations would surpass Nacho Cheese, whose irresistible taste sent Doritos into the processed food hall of fame, and more recently into a partnership with Taco Bell. I visited Steven A. Witherly, a food scientist who wrote an insider’s guide, “Why Humans Like Junk Food,” and we raided his lab to taste and experiment our way through the psychobiology of what makes Nacho Cheese Doritos so alluring.