Laughter is a primitive reflex common to most animals not just humans

Humans don't have a monopoly on laughter, says Silvia Cardoso. A behavioural biologist at the State University of Campinas, Brazil, she says it's a primitive reflex common to most animals: even rats laugh. She tells Sophie Petit-Zeman that too little laughter could have serious consequences for our mental, physical and social well-being.
Laughter a universal phenomenon, and one of the most common things we do. We laugh many times a day, for many different reasons, but rarely think about it, and seldom consciously control it. We know so little about the different kinds and functions of laughter, and my interest really starts there. Why do we do it? What can laughter teach us about our positive emotions and social behaviour? There's so much we don't know about how the brain contributes to emotion and I think we can get at understanding this by studying laughter.
Only 10 or 20 per cent of laughing is a response to humor. Most of the time it's a message we send to other people--communicating joyful disposition, a willingness to bond and so on. It occupies a special place in social interaction and is a fascinating feature of our biology, with motor, emotional and cognitive components. Scientists study all kinds of emotions and behaviour, but few focus on this most basic ingredient. Laughter gives us a clue that we have powerful systems in our brain which respond to pleasure, happiness and joy. It's also involved in events such as release of fear.
My professional focus has always been on emotional behaviour. I spent many years investigating the neural basis of fear in rats, and came to laughter via that route. When I was working with rats, I noticed that when they were alone, in an exposed environment, they were scared and quite uncomfortable. Back in a cage with others, they seemed much happier. It looked as if they played with one another--real rough-and-tumble--and I wondered whether they were also laughing. The neurobiologist Jaak Panksepp had shown that juvenile rats make short vocalisations, pitched too high for humans to hear, during rough-and-tumble play. He thinks these are similar to laughter. This made me wonder about the roots of laughter.