Thank you for asking such an interesting question in relation to laughter. Several thoughts came to my mind.
First, nervous laughter is a psychological response to anxiety and tension. Our own body makes us start laughing to relieve the tension, even if we do not really want to and realise that the social situation is a serious one. It is the brain’s way of diffusing tension, or a defensive coping mechanism when a person is faced with something traumatic or distressing. People sometimes laugh when something is sad as they try to deflect going deeper into their emotions. This kind of reaction can be seen as a form of avoidant behaviour. It will thus be useful to talk with a mental health professional to see if more adaptive ways can be used to help regulate emotions and to come to terms with events, situations and memories that traumatise or distress us. Laughing when we are expected to be angry can be confusing to people around us as they will find our behaviour awkward and hard to interpret.
Second, persons with neurodevelopmental delays such as attention deficit hyperactivity disorder, obsessive-compulsive disorder, and autism spectrum disorder will often struggle with appropriate emotional reactions to sad or horrific events. There is also a very small percentage of our society who would fall into the category of a sociopath, these people seem to get pleasure out of others' pain, and they lack empathy or concern for others.
Third, we know there are several brain pathways that contribute to laughter – each for different components of it. For example, brain regions usually involved in decision-making and controlling our behaviour have to be inhibited to facilitate spontaneous and unbridled laughter. Laughter also relies on emotional circuitry connecting areas responsible for experiencing emotion with those required for expressing emotion. Pathological laughter arises from a disconnection between the various pathways in the brain. These conditions include injury to the brain, Alzheimer’s disease, Parkinson’s disease, multiple sclerosis and stroke. An increasingly twisted sense of humour and laughing at inappropriate times could be an early indication of dementia.
