Some children are born thrill-seekers who make their parents old before their time. While it may be tempting to dismiss the urge for hyper-stimulation as a behavioural problem, evidence is emerging of a "type-T personality" which is biologically based.

The thrill-seeking "type-T" is easily bored and craves constant variety, challenge and change. The term was invented by Dr Frank Farley of Temple University in Philadelphia, who believes that children inherit some predisposition from their parents. But in order for the predisposition to be expressed, these children have to grow up in an environment which permits unconventional thinking and behaviour.

Such children tend to be brimming with energy and highly self-confident - so much so that they feel totally in control of events. They are fearless risk-takers - the types that become skiers, racing-car drivers, mountain climbers, entrepreneurs and, yes, dot.com millionaires.

"They like to make up their own minds and to impose their own rules on themselves, not have them imposed by others. They have great inner strength. And they believe life is not worth living, is not meaningful, if they are not tested," says Farley.

If you have such a child, try to relax a little and let your child teach you something about facing challenge and fear.