Labile affect or Pseudobulbar affect refers to the pathological expression of laughter, crying, or smiling. It is also known as '"Emotional Lability'", '"Pathological Laughter and Crying"', or, historically, '"Emotional Incontinence'". An individual may find themselves laughing uncontrollably at something that is only moderately funny, being unable to stop themselves for several minutes. Episodes may also be mood-incongruent; an individual might laugh uncontrollably when angry or frustrated, for example.

It is most commonly observed after brain injury or degeneration in conditions such as motor neuron disease (MND, ~20% of patients, particularly those with pseudobulbar palsy), stroke, Alzheimer's disease, Amyotrophic lateral sclerosis (15-45% of ALS patients) or multiple sclerosis (MS, up to 50%).

While not as profoundly disabling as the physical symptoms of these diseases, labile affect can have a significant impact on individuals' social functioning and their relationships with others. In a disease such as Motor Neuron Disease, the majority of patients are cognitively normal; however, the appearance of uncontrollable emotions is commonly associated with learning disabilities. This may lead to severe embarrassment and avoidance of social interactions for the patient, which in turn has an impact on their coping mechanisms and their careers.

Treatment is usually pharmacological, using anti-depressants such as fluoxetine, citalopram, or amitriptyline in low to moderate doses. Recent research has identified a new potential treatment for this condition called Neurodex, currently undergoing open-label trials in the United States. A randomised double-blind placebo-controlled trial in MND found that emotionally labile episodes were reduced by about 50%, leading to an improvement in both quality of life and quality of relationships. Research remains ongoing.

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Mood disorders