Spoilers Ahead. So if you haven't seen the film, don't scroll down.
Joaquin Phoenix's Arthur Fleck/ Joker has a very distinct habit of bursting into a loud and obnoxious laugh at some very wrong moments.
All previous Jokers have also had their own distinct laughs. Be it Hamil's maniacal cartoonish laugh or Ledger's creepy baritone laugh, they have all been personal to the actors who play the character. The creators have never given a reason for their laughs.
Remember the scene, where Arthur is making faces at a kid but when his mom asks him to stop, he gets stressed and bursts out laughing?
Yeah, that is based on an actual mental disorder called the Pseudobulbar Affect, or PBA.
This affliction also acts as a prop for director Todd Phillips to give Phoenix the iconic 'Joker laugh'.
According to Stroke.org, researchers have concluded that the PBA is a result of brain damage and could have links to post-stroke anger.
In reality, PBA and stress have a direct correlation to each other as the laughter and tears have no real connection to a person's actual mood at the time.
In PBA, there’s a disconnect between the frontal lobe (which controls emotions), the cerebellum and brain stem (where reflexes are mediated). The effects are uncontrollable and can occur without an emotional trigger. Those with PBA have involuntary bouts of crying, laughter or anger.
Furthermore, tears are more common than laughter which often leads to the misdiagnosis of PBA as depression. However, it is possible that one person can suffer from both at the same time, as Arthur Fleck does in the film.
See, while director Todd Philips has made Joker to be a film about mental illness, he has done very little to actually dive into the subject. As a result of which, these ailments just act as plot points to help the audience keep make sense of Arthur's descent into madness and villainy.