Thatcher Effect (Upside-Down Face Illusion)

Grotesque one way up, normal the other

Take a photo of a face, cut out the eyes and the mouth, turn each of them upside down, and paste them back in place. The right way up, the result is grotesque: an unsettling leer you notice in a fraction of a second.

Now turn the whole photo upside down. The altered eyes and mouth are still there, but the grotesqueness mostly vanishes. The face looks like a normal face upside down, and you have to search for what's wrong, or turn it back, to be sure anything is. The demo above lets you do exactly that. This gap between monstrous and barely noticeable, produced by nothing more than rotating the picture, is the Thatcher effect.

Faces are read as a whole

Recognising a face isn't a matter of checking the eyes, nose and mouth one by one. For upright faces, the brain reads the arrangement of the features as a single pattern, in particular the distances and angles between them. Researchers call this holistic or configural processing. Faces also have dedicated brain regions; the best known, the fusiform face area, responds much more strongly to faces than to other objects (Kanwisher, McDermott & Chun, 1997).

This configural reading is fast and automatic, but it's tuned to a lifetime of seeing faces the right way up. Turn a face over and it largely stops working. You can still tell it's a face, and work out whose with some effort, but the brain falls back on checking features piece by piece. An upside-down eye is still recognisably an eye, so piece-by-piece checking doesn't flag it as wrong; only the upright, whole-face reading would have noticed that it doesn't fit the rest.

Face recognition in general suffers when a picture is inverted, far more than recognising other things. Robert Yin showed in 1969 that turning photos upside down made faces much harder to recognise than houses and other familiar objects (Yin, 1969). The Thatcher effect is more specific: one particular kind of distortion, which is glaring in an upright face, becomes almost invisible upside down.

Why it's named after Thatcher

The psychologist Peter Thompson, then at the University of York, published the effect in 1980 in a one-page paper using a doctored photo of Margaret Thatcher, who was Prime Minister at the time (Thompson, 1980). Using such a familiar face made the demonstration memorable, and the name stuck. It has been reproduced with countless other faces since and is a staple of teaching about face perception. The face on this page is AI-generated rather than a real person, so the effect can be shown without altering anyone's photograph.

Like change blindness and inattentional blindness, the Thatcher effect shows how much less the brain checks than it seems to. Here the check is for something as important as the layout of a face, and turning the picture over is enough to switch it off.