Compare the two faces on the right – no big difference? Now press “Rotate” or use the slider. Also select Merkel as alternative to Thatcher.
Decoding of facial expression worka best in the orientation where faces are seen most of the time, namely upright.
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”