For some people, parenting is a dream. For others, an obligation. For scientists, parenting is an opportunity. And if that opportunity requires them to wear a mask and do scientific tickling, that only makes it better.
Clarence Leuba was a psychologist and professor. In 1933 he had two advantages over the other psychologists around him - a newborn son, and a patient wife. Leuba used these advantages to explore whether or not the tickle response is learned or innate. He made his wife swear that there would be no tickling in the house. What's more, the wife was to maintain an unsmiling silence during activities that resembled tickling. No child of Leuba's was going to learn what response was generally given when being tickled!
When the kid was to be tickled for science, Leuba decided to control for any and all variables. He put on a big cardboard mask, to make sure the child couldn't see him smile. And then, behind the mask, he maintained an unsmiling expression - just in case the baby turned out to have superpowers, I guess. He tickled and tickled, but got no response.
One terrible day, his his wife confessed that she had held the kid (called R L Male in the notes), under its arms and bounced it on her knee while laughing. It's tough to say whether that was a tickle, but Leuba felt honor-bound to officially note it. By seven months, the baby was laughing when tickled. Arguably, it took it that long for it to get used to the creepy mask that daddy put on from time to time.
Leuba tried the same thing when his daughter was born. She also started laughing when tickled at around seven months. Leuba concluded that laughing while tickled was an innate response, and wrote a paper about it entitled, "Tickling and Laughter: Two Genetic Studies." His children are still both deathly afraid of masks.