Tolerance of pain varies among individuals, in the same person from day to day, between the sexes and among nations. One of its most puzzling features is its variability. An injury on the sports field that hardly interrupts play may prove disabling if it occurs on a country walk. Among individuals, studies show that pain thresholds vary more than ten- fold, yet pain serves the same function in all of us - to warn of injury and trigger escape.
So why the variability? Doctors at St Thomas's hospital compared reactions in over 600 pairs of female twins - female, because they were the ones who volunteered for the study. They found that environmental influences proved more important in shaping their response to pain than genetic factors.
The doctors, from the Twin Research Unit at St Thomas's, tested each volunteer with a spring-loaded instrument like a giant plunger which was used to apply increasing pressure to a spot in the centre of their foreheads about the size of a five pence piece. The volunteers were asked to call out when they felt pain and the pressure was measured.
Half the pairs of twins in the study were identical with the same genetic make-up and half were non-identical, as genetically different as ordinary sisters. All had been reared together as twins in the same households, so shared the same upbringing and home environment.
The results, published this month in the medical journal Pain, showed a range in pain thresholds from half a kilo per square centimetre to 7.5 kilos, with most tolerating between two and four kilos. Each pair of twins had similar scores suggesting that their experience of pain was determined by something in their family background - but there was no difference between the identical and non-identical sets; indicating that genetic similarity had no influence. That left environmental factors - the influence of home and family.
Dr Tim Spector, director of the unit, said: "Looking at twins in this manner enables us to highlight the importance of shared environmental factors, such as family background, as opposed to genetic factors, in determining a person's pain threshold. Clearly a person's family can have a great influence on their attitude to pain."
"Were the parents protective or dismissive when the child injured itself? Did the child get a reward or a clip round the ear? The parent's example would have been important, too. Was the Dad, for instance, always taking time off work and complaining or was he the stoical type who never complained?"
Identical twins Gillian Sonin and Judith Magnus were active, outdoor children who took knocks and bruises in their stride. Now both piano teachers at Mill Hill school in north London they believe the level-headed approach adopted by their parents has shaped their own responses to pain. Both scored close to the average 2.5 kilos on the pain threshold measure.
Gillian said: "We were a pair of tomboys, always out on our bikes, falling in stinging nettles or going over the handlebars, like most children. Our parents made no undue fuss when we were hurt."
Judith added: "I took the top off my finger once, trapping it in a bicycle chain when I was four or five. I remember it very well. I wasn't frightened, I was just fascinated looking at it."
Sheila Bannister and Maureen Noad, non-identical twins, remember their mother being firm but sympathetic. "She would be cruel to be kind," says Maureen. Neither had any difficulty coping with childbirth, although Maureen remembers crying out for her mother in the final stages. "But I never screamed the place down like some of these young mothers do."
Studies suggest men have higher pain thresholds than women although women claim childbirth would crack any man's stiff upper lip. Pain thresholds tend to rise with age. "Whether that is because older people have more pain or get less reward for putting up with it we do not know, Dr Spector said.
Northern Europeans tend to be more stoical than people from Mediterranean countries, whose culture encourages emotional expression. But is stoicism the right quality to cultivate in yourself and your family? Not necessarily, according to Dr Spector.
" In some ways stoicism can help you keep a job and get through life. But in others it may be a disadvantage. Arthritis is an area where the traditional stoic patient is treated later than they should have been. Complaining can be good for you."