Compare the two faces above – no big difference? Now press “Rotate” or use the slider. Also select Merkel as alternative to Thatcher.
Decoding of facial expression works best in that orientation where faces are seen most of the time, namely upright. This is now well known with artificial neural networks: they only work in the ranges where they’ve been trained.
Thompson P (1980) Margaret Thatcher: a new illusion. Perception 9:483–484 [PDF]
Occurs also in mokeys: Adachi I, Chou DP, Hampton R (2009)
“Smiling Angela Merkel” photo attribution: Spencer Irvine ©2013
Wikipedia “Thatcher Effect”