A common genetic variant may be partly to blame for poor reading ability, research suggests.
The variant, carried by more than one in seven people, has already been associated with dyslexia.
Tests by the University of Oxford found people carrying the key sequence tended to perform worse than average in tests of their reading ability.
But the study, published in the American Journal of Psychiatry, found no impact on general intelligence.
Previous research has identified at least six candidate genes that appear to influence the odds of developing dyslexia, a learning difficulty which affects the development of literacy and language skills.
The most likely of the candidates - implicated in at least three separate studies - is a gene called KIAA0319, which lies on chromosome six.
Previous work by the Oxford team, based at the Wellcome Trust Centre of Human Genetics, also suggested the gene may be involved in dyslexia.
Their work highlighted a sequence of DNA - called a haplotype - which included part of the key gene.
In the latest study, the Oxford team examined the link between this haplotype and reading abilility in a sample of 6,000 seven to nine-year old children taking part in a major long-term study known as the Avon Longitudinal Study of Parents and Children (ALSPAC).
Lead researcher Dr Silvia Paracchini said: "On average, people carrying this common genetic variant tended to perform poorly on tests of reading ability.
"However, it's important to note that this is only true for reading ability and not for IQ, so it doesn't appear to be connected to cognitive impairment."
Dr Paracchini said the effect of the genetic variant seemed to be small - but was statistically signfiicant.
Impact on foetus
The Oxford team has previously shown that the same haplotype is linked to reduced activity of the KIAA0319 gene during development of the foetus.
In effect, it acts like a dimmer switch, reducing the power of the gene to do its normal job as the foetus grows.
This affects development of the cerebral cortex, the area of the brain responsible for thought processes.
In animal studies, switching off KIAA0319 affects neuronal migration, the process that enables nerve cells created in the inner layer of the cerebral cortex area to migrate outwards to their final destination.
Dr Paracchini said: "This is clearly only part of the jigsaw puzzle that explains why some people have poorer reading ability than others or develop dyslexia.
"There are likely to be many other contributing factors, but our research provides some valuable clues.
"We need to carry out studies into the exact role that this gene plays in brain development and how this affects people's reading ability."
Professor Margaret Snowling, vice president of the British Dyslexia Association, said other genes and environmental factors probably also played a key role in determining reading ability.
And she stressed that some people were able to compensate and go on to successful careers even though they carried the gene variation.