If you’re ticklish, you may wonder why you start laughing or squirming when other people tickle you but not when you try to tickle yourself. This question fascinated Socrates and Charles Darwin and continues to intrigue modern day neuroscientists who research the mysteries of touch.  

Scientifically speaking, there are two types of tickling:

• Knismesis comes from the ancient Greek work for “itching.”  This slower and lighter form of stimulation (think of touching the skin with a feather) Knismesis plays a protective role by making us itchy  and wanting to scratch the skin that’s been touched. This response helps keep us aware of potential dangers, such as quickly getting rid of skin- crawling insects and parasites.
• Gargalesis is a faster and stronger form of tactile stimulation. This term means “laughter” in ancient Greek. True to its roots, it may produce uncontrolled laughing and movements in one person and no response in another.

Gargalesis is more puzzling than knismesis and can’t be explained as just a protective response. Even though your armpits and belly area are very ticklish and contain important vessels and nerves that require protection, it is hard to make sense of why the soles of the feet are ticklish to some people while the face is not.

While different hypotheses try to explain the role of gargalesis, the one that receives most attention suggests that social animals such as primates and rodents tickle their young to stimulate social bonding. 

Researchers who tickled rats’ bellies found that the animals vocalized (“talked”) more and had higher oxytocin levels. These positive interactions made rats enjoy the company of humans they interacted with.

The role of oxytocin—a hormone that mainly plays a role in reproduction and childbirth—in social bonding is well established, which helps us to understand why people with certain neurological conditions respond to gargalesis differently.

• Children with traits consistent with autism are less likely to bond with their parents after being tickled compared to children without characteristics associated with neurodiversity.
• Some people with schizophrenia struggle to anticipate the movements and process that makes most of us not ticklish when we try to tickle ourselves. Due to this difficulty, they may produce strong responses when tickling themselves.
• Your own response to tickling can vary at different times too. For example, you may not be as ticklish when you’re anxious.

The next time someone makes you burst into laughter with a tickle—or you discover that you can’t tickle yourself—you’ll know your brain is doing exactly what it was designed to do: figuring out whether a touch is expected, unexpected or worth paying attention