I've seen people (including myself) that laugh for no apparent reason in really serious situations, such as in an argument or when receiving bad news. Although the other party is clearly very upset, it seems they have the worst possible reaction: they start laughing. It's probably not because they find it funny, so what does trigger it?
This could be what the Psychology Today article "Why We Laugh", (Lickerman, 2011) refers to as 'nervous laughter', suggesting that this response is both for reassurance (as suggested by Tyler Langan's comment) and also a means to build resilience in the face of potential trauma, specifically (from the article):
This may explain why some psychologists classify humor as one of the "mature" defense mechanisms we invoke to guard ourselves against overwhelming anxiety (as compared to the "psychotic," "immature," and "neurotic" defense mechanisms). Being able to laugh at traumatic events in our own lives doesn't cause us to ignore them, but instead seems to prepare us to endure them.
Further, it is suggested here, that the nervous laughter is a means to protect our dignity and sense of control.
Laughter relieves stress and lightens the mood.
- Dugan, D. O. (1989). Laughter and tears: Best medicine for stress. Nursing Forum, 24, 18–26. doi:10.1111/j.1744-6198.1989.tb00814.x
- Toda, M., Kusakabe, S., Nagasawa, S., Kitamura, K., & Morimoto, K. (2007). Effect of laughter on salivary endocrinological stress marker chromogranin A. Biomedical Research, 28, 115-118. doi:10.2220/biomedres.28.115
- White, S., & Winzelberg, A. (1992). Laughter and stress. Humor, 5, 343-355. doi:10.1515/humr.19126.96.36.1993
I have long suspected that laughter has a dual-purpose as a defense mechanism, when the "serious" situation is an embodied assailant.
I think it laughter is counter to the nature of an assault, which creates an element of confusing that the laugh-er (prey) retains control over. This concept of retaining control follows suite with the long-standing ideas that laughter relieves stress and increases coping in the laugh-er.
I think this underscores why laughter from attacked-characters in stories is a cross-over signal for their indomitable nature.
You can take this a level further into the psychology of such situations: when the prey introduces a confusing component to the 'theater of war', they can attempt to infer many things about their assailant via the response given to this introduced-confusion.
- When it comes to serious situations, one may react with feeling the events as surreal. One may have trouble believing that what is happening is actually real.
- What happens next is that one may be confused and dissociate with the event:
In philosophy, "the Absurd" refers to the conflict between (1) the human tendency to seek inherent value and meaning in life and (2) the human inability to find any. In this context absurd does not mean "logically impossible", but rather "humanly impossible". The universe and the human mind do not each separately cause the Absurd, but rather, the Absurd arises by the contradictory nature of the two existing simultaneously https://en.wikipedia.org/wiki/Absurdism
- Now the incongruity theory for humour say we may respond with laughing:
The incongruity theory states that humor is perceived at the moment of realization of incongruity between a concept involved in a certain situation and the real objects thought to be in some relation to the concept. https://en.wikipedia.org/wiki/Theories_of_humor
This in addition to what others have answered, that laughter may be a form of defense mechanism. One starts to laugh as a way to reduce emotional stress.
First of all we have the relief theory that postulates that laughter serves a homeostatic mechanism by which psychological tension is reduced. According to this theory humor relieves tension caused by fears. You have probably observed how you tend to become more creative and make funny remarks when confronted with situations that are packed with tension and discomfort, such as right before you are about to give a public presentation, or right before an exam. We also use dark humor to distance ourselves from horrifying images and stories of war and human atrocities. http://www.popularsocialscience.com/2012/10/25/why-we-laugh-at-things-that-are-not-funny/
There is a theory that laughter like this expresses a reaction to absurdity.
When a situation is serious but cannot be assimilated because of dissociation or shock, it is in the same state as an absurd statement -- you are trying to take it seriously, or find meaning in it, but you just can't. You keep giving up, but then trying again.
Since the situation is real in this case, the impulse toward laughter can be much stronger than real humorous laughter evokes. In my experience it is accompanied by a specific feeling of dissociation, like living in a Lewis Carroll novel.
People who use forms of therapy that involve planned confrontation can evoke this kind of irresistible laughter by challenging a core belief so directly that the client detaches in the same way they might in an emergency.