Spoilers Ahead. So if you haven't seen the film, don't scroll down. 

Cersei
Source: Giphy

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. 

Until now. 

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?

Joker
Source: Cinema Stance.com

Yeah, that is based on an actual mental disorder called the Pseudobulbar Affect, or PBA. 

Arthur Fleck
Source: Balls.ie

This affliction also acts as a prop for director Todd Phillips to give Phoenix the iconic 'Joker laugh'. 

Joker
Source: Giphy

The Pseudobulbar Affect can be described as an emotional disorder where the victim's reaction is completely disconnected from their actual emotional state. This usually manifests itself in form uncontrollable crying and sometimes violent laughter.

Joker
Source: Deseret News

According to Stroke.org, researchers have concluded that the PBA is a result of brain damage and could have links to post-stroke anger. 

Some of Arthur's symptoms fall in line with real-world-PBA. Like the pattern of the episodes he has and the way each of them is an escalation of its predecessor. Once it reaches its peak, it decreases slowly. But then again, some of the symptoms or effects shown in the film are just convenient plot points.

Joker
Source: CNet

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‚Äôs 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.

                    - Stroke

Joker
Source: Gfycat

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. 

Joker
Source: Tumblr

If you watch the movie very carefully, you will notice that these outbursts affect Arthur in many ways, mostly giving him a sense of relief when they are over. Which is not the case with PAB, where no emotional relief follows the episodes. In fact, in the film, his outbursts are credited to brain injury.

Joker
Source: Gozmodo

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.

Joker
Source: Atlantic

The movie oversimplifies the Pseudobulbar Affect for theatrics. Arthur Fleck is always seen laughing while tears are the more common result for someone diagnosed with PBA. That being said, it does stress on the lack of medical facilities for patients struggling with such ailments.