Thatcher effect

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 British former Prime Minister Margaret Thatcher on whose photograph the effect was first and most famously demonstrated. The effect was originally created by Psychology Professor Peter Thompson in 1980.

Overview

The effect is illustrated by two originally identical photos, which are inverted. The second picture is obviously altered so that the eyes and mouth are vertically flipped, though the changes are not immediately obvious until the image is viewed in normal orientation.

This 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 hypothesised 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.