Another American scientist, Robert Plomin, has been working with 25, 000 pairs of identical and non-identical twins in Sweden.

ECONOMIST: WHAT WE LEARN FROM TWINS: The mirror of your soul | The

Mr Plomin has also set up a study working with 10, 000 pairs of identical and non-identical twins in Britain.

ECONOMIST: WHAT WE LEARN FROM TWINS

"The nature-nurture controversy is never more contentious than when it concerns the genetics of intelligence, " wrote Robert Plomin, a psychologist at King's College in London, in a commentary accompanying the study, in which Dr. Plomin wasn't involved.

WSJ: Intelligence Changes Linked to Genes

"If you can identify kids who are going to have trouble learning, you can intervene" early on in their lives, through special schooling or other programs, says Robert Plomin, a professor of behavioral genetics at King's College, London, who is involved in the BGI project.

WSJ: The Search for the Genetics of Intelligence