This striking conclusion is the handiwork of scientists who have uncovered a close correlation between the size of a pensioner's cranium and the results of intelligence and memory tests. In other words, when it comes to brainpower, size does matter.

And for people with small heads, the results of the study - to be published in next month's issue of Brain - are particularly bleak, for the unit found that people with the smallest heads had a fivefold increased risk of suffering cognitive decline compared with those with the largest heads.

The team began their studies on a sample of 215 men and women aged between 75 and 80. Each was given an initial IQ test and a memory test, and was then asked to retake these three years later. The decline in each person's cognitive powers was carefully calculated.

The team then measured each individual's head circumference and searched their medical records for midwives' notes on which their head size at birth were recorded.

The results showed a clear link between head size and loss of memory and cognitive ability in later life. The bigger the head, the less the decline. However, Martyn stressed that his study found that those born with a small head were not automatically doomed to early senility. It is subsequent brain growth in infancy and childhood that is the crucial criterion, he discovered.

In other words, being born with a big head is far less important than having good growth as you progress through childhood. During the first year of life, babies' brains double in size, and by the time they are six, their brain weight has tripled. These are the crucial years for laying down brain cells and neural connections.

'Proper nourishment in early life and providing a stimulating intellectual environment are vital for achieving good brain growth and development and this lasts through life. In other words, brain growth in childhood is important not only in determining how bright you become but how bright you stay,' said Martyn.

'That is the real message from this study: that we have to ensure infants and children are brought up in conditions that optimise brain growth - partly to provide us with lots of bright young adults but also to reduce risk of decline in higher mental function in old age.'

The Southampton study also provides the best evidence yet uncovered for demonstrating the importance of head size. According to Martyn, a person's head circumference is closely correlated with brain volume and that in turn is related to mental ability. For example, researchers from the University of Western Ontario who measured the width of the heads of brothers aged 20 to 35 and then put them through a series of intelligence and cognition tests, found that greater width pointed to greater intelligence.

In addition, a link with brain size and Alzheimer's disease was suggested by a study published several years ago in the British Journal of Psychiatry. The bigger the skull size, the less likely a person seems to suffer from the disease. Now the Southampton unit has provided careful measurements to back these findings - and to suggest a route for reducing the problem.