The first ever genetic analysis of people with extremely high
intelligence has revealed small but important genetic differences
between some of the brightest people in the United States and the
general population.
The King's College London study selected 1,400 high-intelligence
individuals from the Duke University Talent Identification Program.
Representing the top 0.03 per cent of the 'intelligence
distribution', these individuals have an IQ of 170 or more -
substantially higher than that of Nobel Prize winners, who have an
average IQ of around 145.
Genetic research on intelligence consistently indicates that
around half of the differences between people can be explained by
genetic factors. This study's unique design, which focused on the
positive end of the intelligence distribution and compared
genotyping data against more than 3,000 people from the general
population, greatly enhanced the study's power to detect genes
responsible for the heritability of intelligence.
Researchers analysed single nucleotide polymorphisms (SNPs),
which are DNA differences (polymorphisms) between individuals in
the 3 billion nucleotide base pairs of DNA - steps in the spiral
staircase of the double helix of DNA that make up the human genome.
Each SNP represents a difference in a single nucleotide base pair,
and these SNPs account for inherited differences between people,
including intelligence. The study focused, for the first time, on
rare, functional SNPs - rare because previous research had only
considered common SNPs and functional because these are SNPs that
are likely to cause differences in the creation of proteins.
The researchers did not find any individual protein-altering
SNPs that met strict criteria for differences between the
high-intelligence group and the control group. However, for SNPs
that showed some difference between the groups, the rare allele was
less frequently observed in the high intelligence group. This
observation is consistent with research indicating that rare
functional alleles are more often detrimental than beneficial to
intelligence.
Professor Robert Plomin from King's College London said: "Rare
functional alleles do not account for much on their own but in
combination, their impact is significant.
"Our research shows that there are not genes for genius.
However, to have super-high intelligence you need to have many of
the positive alleles and importantly few of the negative rare
effects, such as the rare functional alleles identified in our
study."
The researchers also analysed genome-wide similarity to explore
the genetic architecture of intelligence.
Professor Plomin added: "Previous research suggests that common
SNPs in total account for around 25 per cent of the variance in
intelligence. The question we asked, for the first time, was - how
much will these functional variants account for? We found that the
functional SNPs in our study explain around 17 per cent of the
differences between people in intelligence."
The authors acknowledge that environmental influences also have
an impact, often interacting with genetic factors. Professor Plomin
said: "Clearly super-bright people such as those in our study are
more likely to select environments conducive to their genetic
propensity, so they might have grown up reading books that present
intellectual problems or be more likely to attend a
university."
Professor Michael Simpson of King's College London, said: "Our
study demonstrates the challenges in identifying specific genetic
variants that contribute to this complex trait, but provides
potential insight into its genetic architecture that will inform
future studies."
The paper, A genome-wide analysis of putative functional and exonic variation
associated with extremely high intelligence, is
published in Molecular Psychiatry.