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Thatcher effect
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Thatcher effect

The Thatcher effect or Thatcher illusion is a phenomenon where it becomes 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 Prime Minister Margaret Thatcher on whose photograph the effect has been most famously demonstrated.

Demonstration of the Thatcher Effect

The illustration above depicts two identical photos, although the picture on the left has been inverted. As can be seen, the picture on the right has obviously (and quite grotesquely) been altered so the eyes and mouth have been vertically flipped, whereas the identical changes are less immediately obvious in the picture on the left.

This is thought to be due to specific psychological processes involved in face perception which are tuned especially to upright faces. Faces seem unique despite the fact that they are statistically 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. When a face is upside down, the configural processing cannot take place, and so minor differences are more difficult to detect.

This effect is not present in people who have some forms of prosopagnosia, a disorder where face processing is impaired, usually acquired after brain injury or illness. This suggests that their specific brain injury may damage the process that analyses face structures.

See also