An international team of scientists has said that the largest brain study of its kind has found a gene linked to intelligence, a small piece in the puzzle as to why some people are smarter than others

A variant of this gene can tilt the scales in favour of a higher intelligence, study leader Paul Thompson said. Searching for a genetic explanation for brain disease, the scientists stumbled upon a minute variant in a gene called HMGA2 among people who had larger brains and scored higher on standardised IQ tests. Thompson dubbed it an intelligence gene and said it was likely that many more such genes were yet to be discovered. The variant occurs on HMGA2 where there is just a single change in the permutation of the four letters of the genetic code. DNA comprises four basic chemicals called A (for adenine), C (cytosine), T (thymine) and G (guanine), strung together in different combinations along a double helix. In this case, the researchers found that people with a double "C" and no "T" in a specific section of the HMGA2 gene had bigger brains on average. "It is a strange result, you wouldn't think that something as simple as one small change in the genetic code could explain differences in intelligence worldwide," said Thompson, a neurologist at the University of California at Los Angeles. The discovery came in a study of brain scans and DNA samples from more than 20,000 people from North America, Europe and Australia, of European ancestry. People who received two Cs from their parents, a quarter of the population, scored on average 1.3 points higher than the next group - half of the population with only one C in this section of the gene. The last quarter of people, with no Cs, scored another 1.3 points lower. "The effect is small," said Thompson, but "would be noticeable on a (IQ) test ... (it) may mean you get a couple more questions correct. It wouldn't be an enormous change. Even so, it would help our brain resist cognitive decline later in life." The research was conducted by more than 200 scientists from 100 institutions worldwide, working together on a project called Enigma. Thompson said other studies have implicated some genes in IQ, but this was the first to link a common gene to brain size. The team found that every T in place of a C represented a 0.6 percent smaller brain - equal to more than a year's worth of brain loss through the normal aging process. John Williams, head of neuroscience and mental health at the Wellcome Trust, a British charitable foundation which backs biomedical research, said that the findings paved the way for further research into structural changes which occur in disorders such as dementia, autism and schizophrenia.