Why do some people cry or laugh when they are sad, hurt, happy or think something is funny? Why do they even cry or laugh at all? Did someone invent these actions or do they come naturally? Do we know if early humans laughed or cried?

Humans are a social species, and as such, it is very important for an individual to express his or her feelings and emotions to fellow individuals. Laughing and crying are expressions of very basic emotions--happiness/joy/contentment and pain/sadness/confusion/anger. Sharing the first strengthens social bonds between individuals; whereas by sharing the last, an individual can achieve help and comfort. Incidentally, solitary, non-social animals probably hide their feelings because there is no reason to do otherwise.

Humans are very strongly oriented towards vision and sound, and most of our communication is based on vision and hearing--using facial expressions and other body movements together with speech and other sounds. Laughing and crying are both types of communication which use these forms of expression. One can only wonder what laughing and crying would be like, if we were like many insects and other species that communicate largely via smells, pheromones and tactile stimuli.

No one invented laughing and crying, no more than someone invented walking or sleeping. Babies laugh and cry all by themselves, without being taught how to do so. These actions simply evolved as the most adequate means for our species to express these basic emotions. Early humans most certainly laughed and cried, as did our ancestors and so do chimpanzees. Laughing and crying are part of our human heritage.

Irene Tjorve , Follebu Norway

If you study how babies develop, you can see a partial answer to how the actions probably evolved in humans. Babies do not have to be taught to cry. As all parents knows, even new born babies use crying to show they are unhappy, long before they are capable of much else.

Babies start to smile some time after four weeks. It is believed that their first smiles are accidental. However, they soon learn that this wins a pleasing reaction from their parents. So they smile deliberately when happy. Laughter develops in a similar way a little later.

However, we all smile and laugh to show pleasure and amusement, even across cultural divides. So there must be an instinctive element as well.

Katrina Campbell , Faringdon Oxfordshire

Humans are much more aware of themselves and others than the rest of the animal world. We can comprehend the past and the future, the pain, suffering and futility of most people's lives compared with the lives we would choose to lead.

Somehow, most people manage to carry on. Laughter and tears are two defence mechanisms that allow us to relieve our pent-up emotions and stop us from going stark raving mad.

F Grisley , Barry Glamorgan

Languages, contrary to popular belief, hinder communication. If you don't know how to express yourself in a way that your fellows understand, you are being hindered by your vocabulary and language--monolinguists have trouble conversing with foreigners. So, to make life easier we replace words with emotions.

Laughing tells everyone you are happy. Wailing with agony is easier than submitting an oral report explaining why something hurts and you wouldn't mind a bit of help. On the other hand, the use of canned laughter seems to imply we need to be taught when and how to laugh.

Daniel Schutze , Chislehurst Kent

I would like to add to the debate from a clinical viewpoint. Laughter is basically a voluntary action, but crying is not. The simple test is to laugh out loud--which can be done easily on demand. But, unless the habit has been cultivated over time, it is impossible to cry tears at will.

Smiling is a voluntary relaxation of the facial muscles and it conveys a message to other people that we are feeling happy and nonaggressive. It relieves us and puts others at ease. We need no more than to smile to convey what our feelings are. However, laughter is a much more complicated affair.

The act of laughing relaxes the face, chest, spine and abdominal muscles. A correlation has been shown between the readiness to laugh and a lower incidence of coronary artery disease. Is it reasonable to speculate that smiling is a means of communication but that laughing is a form of relaxation?

Further speculation is fuelled by the little understood reasons behind why we cry out when in pain. This is more than just exhalation or a cry for help as it does, in some way, seem to alleviate the pain. It would be interesting to try to correlate crying out with pain, laughing loudly and the body's production of pain-relieving endorphins. We know that endorphins flood the body when it experiences pain. Is this process boosted by crying out? Does a loud laugh have a similar effect and is it our personal way of achieving a "fix"?

Some people seem able to cry as a means of expressing sadness and self pity but for most it is the product of pain, pleasure or frustration. Clinical studies show that such tears contain derivatives of adrenaline, a hormone secreted in response to stress. The tears act as a safety valve excreting stress hormones when their levels get too high. This prevents potentially catastrophic rises in blood pressure.

Sweat, a close relative of tears, has a similar dual action. We secrete two types of sweat. A relatively clear liquid is produced to reduce body temperature by evaporation--this is the odourless fluid that drips from our bodies in response to exercise. The other type of sweat is caused by stress and worry and results in the smelly shirt armpit that is familiar after a trying day at work.

With babies, their crying is not true crying at all. It is just a shout for attention which is not accompanied by tears--until they become frustrated by a lack of response and their excess adrenaline needs a route out of the body.

J Stott , Barry South Glamorgan

One of your correspondents maintains that smiling is a voluntary relaxation of the facial muscles. This is not so.

Smiling is due to contraction of several muscles attached to the corners of the mouth: the incisivus labii superioris, levator anguli oris and buccinator muscles, which comprise most of the orbicularis oris muscle and, in smiling, are assisted by the risorius muscle. Contraction of the risorius alone produces an unpleasant sardonic grin.

All these muscles are supplied by the facial (seventh cranial) nerve from its nucleus in the pons varolii in the brainstem. The only muscle which relaxes is the opposing depressor ang-uli oris--as this contracts it pulls the corner of the mouth downwards. The action of laughing requires the contraction of the diaphragm and abdominal muscles.

R Harrison , Kent

Tears may fulfil many mysterious functions, but to suggest that they act as a safety valve for excreting excess circulating adrenaline is incorrect. If it were, the body would have to secrete the adrenaline in tears at a higher concentration than that found in the circulation.

Without a highly specialised organ that is able to concentrate blood solutes, the maximum concentration of anything that could be secreted is equivalent to its circulating concentration. In reality, some "filtration" occurs and the concentrations of blood-borne substances found in tears and sweat are consider00ably lower than those found in the blood. Clearly then, the excretion of adrenaline at a concentration less than, or equivalent to, that found in the blood would not lead to any net decrease in its circulating concentration.

The body is capable of regulating blood solutes by concentration and excretion--this is one of the specialised roles of the kidney. However, no equivalent structure exists for the production of tears. Many people, primarily through acculturation, do not cry readily. While it may be argued that crying can represent a therapeutic response to stress, the failure to do so, as suggested by your correspondent, certainly does not lead to acute hypertensive crisis.

Don Cameron , Adelaide, South Australia