Read the text below and answer questions 14-26.

The Science of Human Laughter

A. Human beings love to laugh. It's such an obvious fact that it's easy to overlook. Laughter, like music and language, is a fundamental human trait. Common sense tells us that laughter is associated with happiness. However, there is also a body of scientific evidence proving that laughter is good for us. Studies show that laughter strengthens relationships in both personal and professional life. It has also been established that laughter improves cardiovascular function, boosts the immune system and releases beneficial hormones into the bloodstream. However, according to psychologist Dr Peter Shrimpton, humans might all laugh, but they often don't remember doing it. 'All the studies show that we laugh more frequently than we realise,' says Dr Shrimpton. 'Perhaps because it is such a basic part of human nature, we tend not to notice when we are laughing.'

B. Infants typically give their first laugh around three to four months of age, long before they can talk. But according to biologists, this isn't because they find something amusing; it is rather a form of non-verbal communication. They laugh to form a closer connection to the people they are with, and adults are little different. 'There is a widespread belief outside the scientific community that we laugh because something is humorous,' says sociologist Jocelyn Barnes. 'While this is true, just as commonly the real purpose of laughter is to promote bonding with other individuals or groups.' This may be partly because it is almost impossible to imitate laughter; even trained actors struggle to mimic a laugh convincingly. So if someone is laughing, the chances are they are being genuine. There's even difference between a real and a fake smile. In the 19th century, the French neurologist Guillaume Duchenne found that a genuine smile activates the zygomaticus major and orbicularis muscles, and this in turn causes line to develop called 'crow's feet' at the outside corners of the eyes. No crow's feet appear if the smile is put on.

C. There is certainly nothing new about joking and laughter. Attempts to be humorous have been found from ancient Egypt, dating from 2600 BC. And a long and detailed joke book called The Laughter Lover, which was written in ancient Rome, still exists today. While of considerable historical value, it may not be all that amusing any more. A professor of classics, Heinrich Ahrends, has studied many such ancient sources and concluded that tastes in jokes have evolved markedly with the passing of the centuries and that the jokes of our forebears would not get much of a laugh today – and vice versa, no doubt. Nonetheless, studies show that almost everyone can find amusement in some form or other. There is a rare neurological disorder named aphonogelia that prevents some people from laughing out loud. However, they may still be amused or entertained, but just express it in different ways.

D. Much more common is contagious laughter: laughter that spreads uncontrollably between people, sometimes referred to as 'getting the giggles'. Many people will have experienced this themselves, particularly as children, though it also occurs in adults. On one infamous occasion, a group of BBC cricket commentators got the giggles while broadcasting live on radio. And in January 1962 in Tanzania contagious laughter spread through a group of students. Ninety-five pupils were affected and one girl laughed continuously for 16 days. Eventually the situation became so bad that the authorities at the school felt obliged to close it temporarily. In general, however, it is possible for most people to suppress laughter in circumstances where it would be inappropriate. Scientists believe this is possible because in the brain's cerebral cortex there appears to be a laughter switch over which humans have some conscious control.

E. What is becoming clear to scientists is that laughter is highly complex. It appears, for example, that laughter has the power to override other emotions, at least temporarily. Neurologist Nikki Sokolov is studying the network of brain circuits and neurotransmitters that regulate laughter and other emotions. She hopes her work may provide further insights to explain the processes involved when laughter occurs simultaneously with other, seemingly contradict emotions, such as crying, for example. Another aspect of humour's complexity is that it is so subjective. What makes one person laugh will be met with stony silence by another. Writer David Mackenzie recognized this from the reactions his own jokes received. Intrigued, Mackenzie conducted an international online survey to establish exactly what makes people and what doesn't, and was surprised by the diverse and often contradictory variety of topics and scenarios that were listed in each category. Understanding humour is still as much an art as science, according to theatre critic Jake Gottlieb. 'Stand-up comedians are a remarkable type', says Gottlieb. 'Making jokes for a living is a serious business. You need to be a psychologist and social commentator, be empathetic, self-aware, observant, stubborn and have great timing. Not many of us are so multi-talented. ' Perhaps not, but we can still enjoy the instinctive humour of our family and friends, and perhaps sometimes but a ticket for a show.