Definition 1 of 2
They claim that it is an ancient aspect of yoga and has many health benefits.
While it is commonly accepted that happiness most definitely leads to a more fulfilled and enriched life, there seems to be no evidence of such benefits coming from forced laughter or ridiculous and scary cult like behavior in public spaces.
Daughter - Mummy i'm scared! (Clutches onto mummy for dear life).
Mummy - I'm sorry beta you had to see this.
Definition 2 of 2
But those who find it most beneficial and constructive might be those who still have a lot of juice left in them. Again, might be.
It is a quick shortcut de-stressor. Not just in public parks, it is also available on conference calls on telephones in some places. Happens also in some offices in Japan and USA, everyday usually before the daily grind begins.
The technique has no apparent basis in Yoga or other sanatana dharma practices.
Patanjali's Yogasutras and the Geeta several times talk about techniques of overcoming a negative feeling by bringing on an opposite feeling effortfully to overcome yogantarayas (hurdles), a plain displacement technique used in modern psychology. The secular Sankhyas have endorsed such techniques, being a major contributors to Yogasutra development.
Such meditation techniques involve displacing a feelings of anger with muda(pleasantness), of hatred with karuna(kindness) etc. so a scattered mind could be brought back into a state of peace.
Buddhism and Confucianism seem like more probable sources of this cultivar of a de-stressor if you insist on a connection to the ancient. Also, Sufism might have helped.
Hindu culture is well-known to frown upon loud laughter, almost as a vulgar indulgence. Hasya being one of the major rasas, hasn't had a great run, not until the times of Tenali Ramakrishna. And we desis desist LOL except in text messages, online or in parties, that too limiting it with people of same sex and age-group.
Laughter clubs are spreading like well, like how laughter spreads: a bit contagiously.
Also, a lot of phenomena that appear intangible to scientific investigation; like happiness, pain, faith, love, intuition, sleep, sexuality, laughter included are extremely subjective.
Isn't subjectivity the only thing that makes life interesting, ever-new and funny-not-so-funny?
However, subjective these stuff may be, no denying that they are all psychosomatic. Nothing you ever feel could ever be felt without the instrument of body being there. I dunno, what evidence Mildly Racist has had for orgasms she gets if at all, other than experience itself. Try. You may get it. You can never get it without trying.
Humans are condemned to be apes by essential nature and learn a lot by mimicking. Apes and humans historically and currently observably resort to simulation techniques to stimulate certain gratifying mental states, masturbation being a major one of such phenomena.
In laughter clubs, as in most human group activities, inhibition level gets lowered in individual participant's mind, peer behaviour encourages partaking in the shared activity, and then there arrives a moment of discovery especially for those with crippling mental contractions that they can really break out of a mould, break out of a shell. Could there be people who haven't laughed loudly in years?
I attended a laughter club session in Bangalore just once more than 10 years ago. It is rewarding. I love to do it again.
I can laugh at myself, and alone, but it's more like drinking alone, elevating but without any excitement of a party.