"Just hearing a good hearty laugh sometimes completely changes a person's frame of mind. Laughter is said to be infectious. A good demonstration of this took place on a TV program a year or so ago. It was a comedy show in which studio contestants were paid so much for every second they could keep from laughing while a panel of professional comedians told jokes, made funny faces, and tried everything else they could think of to break the contestants down. One poker-faced contestant steadily withstood the efforts of several hard-working comedians, but broke down immediately when the fourth started his routine. All the comedian did was laugh out loud. Within seconds, the contestant was laughing heartily in spite of himself."
A Help-Your-Self Booklet, 1960