The short answer

We don't know for sure why people cry when they laugh, but this phenomenon is likely caused by your tear ducts getting stimulated when you use laughing face muscles, your body sensing an opportunity to reduce stress, and/or your brain getting confused as to what emotion you are feeling.

The long answer

Laughing so hard that you cry is a fascinating phenomenon, but unfortunately there is not too much research on this topic. However, I'll share some potential theories behind why we sometimes cry when we laugh hard.

Theory #1: Your tear ducts are getting stimulated by you laughing hard.

When you laugh really hard, the scrunching up of your face muscles puts pressure on your tear ducts. This could lead your tear ducts to trigger what are known as "reflex tears," which are produced as a reflex to a stimulus. Reflex tears happen when, for example, sand gets in your eyes and your body starts crying to flush out the sand.

Theory #2: Your body is trying to soothe you and reduce stress.

Both laughing and crying have been found to decrease the levels of cortisol and other stress hormones in our bodies. It's possible that, as your brain is reducing these stress levels via laughter, crying follows naturally as a way to further soothe you.

Theory #3: Your brain is confused about what emotion you are feeling.

Your limbic system is controls many functions relating to emotions, including the responses of both laughing and crying. Because the neural pathways that trigger laughing and crying are so closely related, it's possible that a really strong laugh confuses your brain about what emotion you are feeling and triggers crying.

Sources

Baker, B. (2016, June 22). Infrequently Asked Questions: Why do we cry when we laugh? PhillyVoice. Retrieved January 14, 2023, from https://www.phillyvoice.com/infrequently-asked-questions-why-do-we-cry-when-we-laugh/

Columbia University. (n.d.). What causes laughter? Go Ask Alice! Retrieved January 14, 2023, from https://goaskalice.columbia.edu/answered-questions/what-causes-laughter/

Gračanin, A., Bylsma, L. M., & Vingerhoets, A. J. J. M. (2014, May 28). Is crying a self-soothing behavior? U.S. National Library of Medicine. Retrieved January 14, 2023, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4035568/

Yim, J. E. (2016, July). Therapeutic Benefits of Laughter in Mental Health: A Theoretical Review. U.S. National Library of Medicine. Retrieved January 14, 2023, from https://pubmed.ncbi.nlm.nih.gov/27439375/