The work, which pulls together studies on 50,470 twins from 212 different IQ studies, is published today in the Nature journal. By statistical analysis, it quashes the idea that genes determine intelligence more than environment does - that is, that nature overwhelms nurture.
The work, led by Bernard Devlin, of the University of Pittsburgh, Pennsylvania, analysed the fact that the similarity in IQ between a pair of twins is not easily accounted for by their shared genes and living environment. Identical twins have identical sets of genes, and often identical rearing. Non-identical twins have the different genes, but the same rearing. Siblings have different genes and, generally, different rearing. When data was analysed about the IQ similarity of non-twin siblings reared apart, as well as twins reared apart, variations in IQ appeared that could not be accounted for by genes or home environment.
In the case of similarity between twins this amounted to 20 per cent, and in the case of non-twin siblings 5 per cent. The only explanation was the effect of what happened to the foetus in the womb. While it is known that alcohol, smoking and poor diet can produce unhealthy babies, previous research has not been able to show a significant link between a mother's lifestyle and her unborn child's IQ.
It also contradicts the work of controversial academics such as Charles Murray and Richard Herrnstein, who said in their book The Bell Curve that IQ was so strongly determined by genes that increased breeding of low-IQ people would lead to a growing "under-class". The new work found that "IQ heritability" - that is, the proportion of variation in IQ which can be attributed to the effects of genes - was less than 50 per cent.
This is too small to produce an "under-class" effect. Commenting, Matt McGue, of the Institute of Human Genetics at the University of Minnesota, said it means that "... By the third or fourth generation, descendants of gifted individuals are not much more likely to be gifted than are descendants of ordinary people."
Prof Devlin said more thorough clinical research was needed. "Some studies point to nutrition; some studies suggest that birth weight is important, but it could be that the effects in the womb are subtle and small, and it's going to be difficult to detect them without some very careful studies ...If intervention can improve a child's intelligence before birth it might lead to an improvement in society's IQ generally, and a lot of people would think that's a good thing."Reuse content