TUESDAY, Aug. 9 (HealthDay News) -- New research indicates that up to half of human intelligence can be explained by genetics, but this involves small contributions by many different genes and not one overarching "smart" gene.
"We found that approximately half of individual differences ... in intelligence can be explained by genetics and across a great variety of genes," said Peter Visscher, co-author of a paper appearing in the Aug. 12 issue of the journal Molecular Psychiatry. "[And] this is likely to be an underestimate because we could only detect variation that is correlated with common DNA markers."
"There are huge numbers of genetic differences that make a difference to human intelligence," added study co-author Dr. Ian Deary, a professor of differential psychology at University of Edinburgh in Scotland. "It's like, along those huge stretches of DNA, there are many thousands of locations where there is a signal saying, 'some genetic difference near me makes a small contribution to people's thinking skills'."
Scientists have known that intelligence (or lack thereof) tends to run in families but it hasn't been clear how much of a role genetics plays. Nor has any single gene been strongly associated with "brains."
These authors did a genome-wide analysis on 3,511 unrelated adults which involved scanning data on about 550,000 single nucleotide polymorphisms, or SNPs.
According to the Human Genome Project, SNPs are one-letter changes in a DNA sequence and account for 90 percent of all genetic variations among humans.
After testing participants' skill on two types of intelligence -- knowledge and problem-solving -- researchers determined that 40 percent to 50 percent of the variation came from differences in genetic differences.
"This tells us that both genetic and environmental factors explain individual differences in intelligence, and roughly in equal quantities," said Visscher, who is a professor of statistical genetics at the Queensland Institute of Medical Research in Brisbane, Australia.
Deary is also looking at how people's intelligence stays the same or varies across the life span, from age 11 to 80.
"A lot of the research my team is devoted to is looking for the things that make people's intelligence age better or worse over their life course and one of those things is genetics," Deary said. "Genetic differences can be a cause of people's different malleability in their thinking skills."
While the research shows genes influence intelligence, genetic influence isn't always "stable and determined," Deary cautioned.
That leaves open the question of how much of a role educational opportunities and other environmental factors, such as parenting and teachers, may play in achievement, both in school and in life.
Added Visscher: "From my own perspective, I can say that whatever extra teaching and learning opportunities I would have had in childhood -- I had lots -- I doubt it very much that I could have become a skilled tradesman ... or an Einstein."
Indiana University has more on human intelligence.
SOURCES: Ian Deary, Ph.D., MBChB, professor, differential psychology, University of Edinburgh, Scotland; Peter Visscher, Ph.D., professor, statistical genetics, Queensland Institute of Medical Research, Brisbane, Australia; Aug. 12, 2011, Molecular Psychiatry
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