Psychologist Clarence Leuba banned tickling in his household when his son was born.
Except during certain experimental periods.
He wanted to to see if laughing when being tickled is instinctive or something learned.
So during these experimental periods he would put on a cardboard mask so his son wouldn’t see his face and tickle him. The baby laughed anyway.
Leuba, however, felt the experiment was ruined because his wife later confessed that the boy saw her laughing when she bounced him on the knee.