genes play a more important role than the environment in determining mental abilities of humans, even at a mature age, say researchers from the us , the uk and Sweden. Most behavioural scientists are surprised with the findings since it had generally been accepted that an individual's experience in different environments has a major role in determining mental abilities ( Science , Vol 276, No 5318).

A team of researchers led by Robert Plomin, a behavioural geneticist at the Institute of Psychiatry in London, conducted a two-year study on 240 pairs of Swedish twins out of which 110 were identical and 130 same sex fraternal. All the people were above 80 but were mentally alert. Tests were conducted to measure verbal and spatial ability, memory and general cognitive ability.

Despite years of experience gathered by the subjects, it was found that genetic differences accounted for nearly 62 per cent of the variation in mental ability in the 480 individuals. The rest was attributed to education, exposure to stress, socio-economic status, geography, nutrition, diseases and other environmental factors that shape life. Earlier studies conducted on younger adults had shown that genetic influence on cognitive ability remains constant throughout life.

However, the present findings do not suggest that mental abilities are determined rigidly by genes. In nearly 38 per cent people variations in cognitive abilities could not be explained by genes alone, showing that there is plenty of scope for the environment to leave its imprint.

Experience may also play a subtle role in the growth and development of intelligence. Studies have shown that the contribution of genes in mental ability varies from 20 per cent during infancy to 50 per cent during adolescence, rising to 60 per cent in adulthood. According to Plomin, this paradox arises because individuals drift towards using the skills that they are naturally born with and those which they are best at. If some--one is born with a genetic inclination for verbal skills, the person is likely to read more throughout childhood and adult life.

The findings also clarify the links between genes and intelligence. The fact that genetic influence on mental ability varies with age would encourage scientists to search for genes that determine intelligence. Psychologists have long assumed that the relative importance of genes declines when other factors of intelligence, such as experience, increase.

By identifying the gene sites that are active in cognitive functions, it would be possible to develop chemical means to activate genes that exert desirable effects or deactivate the factors which hasten cognitive decay. The study has an immediate application in the treatment of the dreaded Alzheimer's disease, where identification and regulation of genes could help millions of those affected with the disease.