Intellectual heavyweights are born, not made. Scientists have found that the bigger the baby the brainier it is likely to turn out to be.
Tests on 4 000 people born in 1946 carried out at intervals over their lives have shown that bigger babies tend to grow up brighter, do better at school and are still cleverer in adulthood than their punier peers.
The finding lends new credence to the theory that brain development is dependent on nutrition in the womb.
Previous research has shown newborns weighing less than 2,5kg, do less well on later measures of cognitive performance than those of normal weight.
The new research, by a team from University College London, shows that the link holds across the whole of the normal birth weight range, from the smallest babies to the largest.
Heavier babies were consistently brighter than lighter ones up to a maximum weight at birth of 5kg.
The difference may be linked with the size of the brain. Bigger babies have bigger heads, bigger brains and are brighter. As the head is the heaviest part of a baby, brainy babies weigh more.
The participants were given cognitive tests at ages 8, 11, 15, 26 and 43.
These included measurements of reading ability, arithmetic skills, non-verbal reasoning, memory, thinking-speed and concentration.
Even at age 26, those who were bigger babies performed better than those who were smaller at birth, after correcting for other factors such as social class, mother's education and birth order.
Birth order is a key factor affecting cognitive ability as first born children tend to be brighter than subsequent children. Researchers believe this is because parents tend to expend greater effort and investment in the education of the first born than in subsequent children.
By age 43, however, the effects of birth weight had mostly disappeared, say the researchers in the British Medical Journal.
"Adult environmental influences, particularly educational and occupational attainment, may overshadow perinatal factors by middle life."
Marcus Richards, who led the study, said growth hormones circulating in the foetus may be important for mental and physical development.
He said parents of small babies should not despair that they would never become Nobel prize winners.
"There are many postnatal factors which affect the outcome, and the extent to which parents are involved in their children's education is a major one."