麻豆传媒

GENES play a surprisingly powerful role in determining our mental abilities. A study of elderly twins has found that after eighty years or more of nurture, nature increasingly shines through.

The result comes as a surprise to most scientists, who had expected that a lifetime鈥檚 experience would gradually wash out any role that genes might have played in determining mental abilities. 鈥淚t was an open question whether genetic factors are at all important later in life,鈥� says Robert Plomin, a behavioural geneticist at the Institute of Psychiatry in London.

To answer this question, Plomin and his colleagues looked at 240 pairs of Swedish twins鈥�110 identical and 130 same-sex fraternal鈥攁ll of whom were at least 80 years old and mentally alert. Each twin was given a battery of tests to measure verbal and spatial ability, memory and general cognitive ability. By comparing the similarity in scores between identical twin pairs (who share 100 per cent of their genes) to that between fraternal pairs (who have at most 50 per cent of their genes in common) the researchers were able to distinguish between the effects of genes and environment.

In last week鈥檚 Science (vol 276, p 1560), the team reports that despite the many years of experience gleaned by the subjects, genetic differences still account for 62 per cent of the variation in general cognitive ability among the 480 individuals鈥攁bout the same level that earlier studies had found for much younger adults. 鈥淭his tells us things are a lot different than we thought,鈥� comments Jared Jobe, a psychologist at the National Institute on Aging near Washington DC.

However, the new result does not imply that mental abilities are rigidly fixed by our genes or that experience is irrelevant in shaping them as we grow older, say both Jobe and Plomin. The remaining 38 per cent not explained by heredity leaves plenty of room for life鈥檚 events to leave their mark on mental acuity.

Experience may play a second, more subtle role as well, says Plomin. Other studies have shown that the genetic contribution to mental ability actually seems to increase from about 20 per cent during infancy to 50 per cent in adolescence, then to 60 per cent by adulthood. Plomin thinks this paradox arises because individuals drift toward using the skills they鈥檙e naturally best at. Someone who has a slight genetic propensity for verbal skills, for example, might tend to read more throughout childhood and adult life.

鈥淚t鈥檚 not a case of new genes kicking in,鈥� Plomin says. 鈥淲e create an environment that鈥檚 correlated with our genetic propensity. If that鈥檚 the case, little genetic differences can push us in different directions, and become larger as we go through life.鈥�

Now that behavioural geneticists know that genes affect mental ability at all ages, they will be able to search more vigorously for the genes that lie at the root of intelligence, says Plomin. For example, he notes, the gene for apolipoprotein E鈥攚hich is known to affect the risk of getting Alzheimer鈥檚鈥攎ay be a good candidate for determining general cognitive ability as well.