The Thatcher effect or Thatcher illusion is a phenomenon where it becomes more difficult to detect local feature changes in an upside-down face, despite identical changes being obvious in an upright face. It is named after the then British prime minister Margaret Thatcher, on whose photograph the effect was first demonstrated. The effect was originally created in 1980 by Peter Thompson, professor of psychology at the University of York.
The effect is illustrated by two originally identical photos, which are inverted. The second picture is altered so that the eyes and mouth are vertically flipped, though the changes are not immediately obvious until the image is viewed upright.
The Thatcher effect is thought to be due to specific psychological cognitive modules involved in face perception which are tuned especially to upright faces. Faces seem unique despite the fact that they are very similar. It has been hypothesized that we develop specific processes to differentiate between faces that rely as much on the configuration (the structural relationship between individual features on the face) as the details of individual face features, such as the eyes, nose and mouth.
There is evidence that rhesus monkeys as well as chimpanzees experience the Thatcher effect, raising the possibility that some brain mechanisms involved in processing faces may have evolved in a common ancestor more than 30 million years ago.