Here's what a neuroscientist knows about people who aren't 100 percent truthful. By Emma Haak Photo: Carlos Fierro/Getty Images If you're trying to figure out if your friend was truly sick–or just fibbing–when she canceled dinner plans at the last minute, see if she answers your questions with questions. Responding that way is a tell, says Daniel Levitin, PhD, a professor of psychology and behavioral neuroscience at McGill University and the author of the new book, A Field Guide to Lies.