The study, led by University of Warwick psychology researcher Sophie Nightingale, relied on an online test. The researchers started with 10 real (in other words, not manipulated) pictures from Google Images. Six of them were subjected to five different forms of data manipulation each—some producing physically possible images, some staying within the bounds of reason—to create an additional 30. These manipulations included air brushing, the introduction or subtraction of people or objects, changes in lighting, and changes in landscape geography. Participants, 707 in all, were each shown 10 random images—always including all five manipulation types and all five original images, but never a repeat of the same type of manipulation or base image—and asked to determine their authenticity. If you're curious, you can take the test yourself here.