Sophie Scott is the group leader of the speech communication neuroscience group at the Institute of Cognitive Neuroscience, University College London. She recently gave a TED talk on laughter and frequently contributes to the Guardian. Check out her article entitled Why Do People Laugh? You asked Google– and here's the answer.
Scott states, "It’s now becoming clear that though laughter is an essential social sign of affection and affiliation, it may be even more important than that. Research into couples has shown that people who deal with unpleasant, stressful situations with positive emotions like laughter not only feel immediately better, they are also happier in their relationships and stay together for longer. Laughter is a phenomenally useful way for people to regulate their emotions together – and feel better together. In this context, jokes and humour may form incredibly useful reasons to laugh together."
The following image is from her study The Social Life of Laughter.
According to the study: Upper panel shows oscillograms for speech and laughter, middle panel shows spectrograms for laughter and speech, lower panel shows chest expansion dynamics for metabolic breathing, laughing and speaking. The time scale on the x axis is the same for each (breathing, laughing and speaking). Both speaking and laughing are distinctly different from metabolic breathing, in terms of chest wall movements. Laughter is characterized by very rapid contractions of the intercostal muscles, resulting in large exhalations followed by individual bursts of laughter: the spectral modulation of laughter by supra-larangeal structures is minimal. Speech shows a fine pattern of intercostal muscle movements, which are used to maintain constant sub-glottal pressure at the larynx and to provide pitch and rhythm to the speech. Unlike laughter, speech also shows considerable spectral complexity reflecting movements of the supra-laryngeal articulators.