The Joker laughs due to a condition called Pseudobulbar Affect (PBA), which causes uncontrollable and inappropriate episodes of laughter or crying. In the 2019 film "Joker," Arthur Fleck (the Joker) has this condition, which is depicted as laughter triggered by stress or emotional turmoil, even in moments where laughter is socially inappropriate. This laughter serves as an expression of his inner pain and mental distress rather than genuine humor. The condition is often linked to neurological issues such as brain injuries, which the film reveals as part of Arthur's backstory. The laughter reflects a disconnect between his actual emotions and outward displays, contributing to his character's complexity and tragic nature.

Joaquin Phoenix, who portrayed the Joker in the film, suggested that the laughter might not solely be due to a medical condition but could also be an expression of Arthur Fleck's suppressed true nature emerging, making the laughter symbolic as well as symptomatic.