Perhaps the oldest theory of humor, dating back to Plato and other ancient Greek philosophers, suggested that people find humor in, and laugh at, earlier versions of themselves and the misfortunes of others because of feeling superior. In the 18th century, The Theory of Release, evolved; that is, laughter allowed people to let off steam or release pent-up nervous energy. The Arousal Theory then focused on the cognitive aspects of laughter. While stress increases arousal, laughter helps to reduce this arousal. As a result, the situation doesn’t seem as negative. All in all, this theory proposes that laughter can help reduce negative emotions and unpleasant feelings. Today, many studies that examine the relationship between humor and aging focus on one prominent humor theory called “The Incongruity-Resolution Theory.” (Gerontology, Vol. 59, Issue 5, August, 2013). According to the theory, humor emerges from two incompatible parts that at first do not seem to make sense, but, when the incongruity is resolved, the humor is appreciated. This joke exemplifies the theory:

O’Reilly was on trial for armed robbery.
The jury came out and announced, ‘Not guilty.’
‘Wonderful’, said O’Reilly, ‘does that mean I can keep the money?’

So, humor has remained one of the most important human activities, one that is enjoyed daily by people in every culture and at every age. Resulting laughing is universal: no matter how old one is, what language one speaks or what one’s physical or mental experiences. It’s also instinctual. Infants laugh almost from birth; it’s not a learned behavior. Humans are hardwired for laughter. Research suggests that elderly people enjoy humor more than younger people, but that they have increasing difficulties in understanding it.
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Science has some answers to the mystery of human laughter for seniors. Laughter decreases stress hormones and increases immune cells and infection-fighting antibodies, thus improving one’s resistance to disease. Laughter triggers the release of endorphins, the body’s natural feel-good chemicals. Endorphins promote an overall sense of well-being and can even temporarily relieve pain.
Laughter clearly also serves a social function. It is a way for us to signal to another person that we wish to connect with them. We’re also thirty times more likely to laugh in a group.