Do we learn to laugh when tickled or is it an innate response? That is the question psychologist Professor Clarence Leuba set himself to examine using his own children, no less, as experimental subjects.
In 1933 he decided that he would not laugh in the presence of his first child while tickling him (Leuba, 1941). Everyday life in the Leuba household, therefore, was devoid of tickling except for one special experimental period. During this period he would cover his own face with a mask while tickling his son so his facial expression was hidden.