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Legendary feats of intelligence — Ken Jennings winning 74 consecutive "Jeopardy!" games — have their match in astonishing acts of stupidity, like a would-be robber who dons a mask without remembering to cut eyeholes.
Quite a gulf exists between the extremes in innate human intelligence. Yet establishing a clear biological basis for why some people are smarter than others has so far proven tricky. Even the concept of intelligence as a quantifiable, explainable phenomenon has only been recently settled.
"A generation ago, people were arguing over the definition of intelligence, and that argument is now done," said Richard Haier, professor emeritus in the School of Medicine at the University of California, Irvine. "Intelligence is something that's real and exists, and it can be measured."
To this end, neuroscience has made great strides in illuminating the brain structures and mechanisms that account for intelligence. As progress continues, the differences that underlie brilliance and its opposite should come to light. [10 Ways to Keep Your Mind Sharp]
The best-known index of smarts is an intelligence quotient (IQ) test, which probes spatial ability, memory, speed of processing information and more. "In the last hundred years of studying intelligence, there have been pretty constant results that several different intelligence factors, like these, relate to how smart people are," said Haier.
In the most popular IQ tests, the median score is set at 100, with standard deviations of 15, meaning that 95 percent of people have an IQ score between 70 and 130. According to this standard bell curve, 2.5 percent of the population extends into intellectual giftedness on the high end or mental retardation on the low end.
When discussing intelligence, researchers tend to cite a general factor of intelligence, or g, the common factor across a battery of intelligence tests. "Think of the really smart kids you knew in school — they were kind of generally smart, not just in one subject," said Haier. "It's that general ability that's reflected in the g score."
IQ scores and g scores are good estimates of overall mental prowess, Haier said, but they do not indicate if someone is very skilled in a particular task.
Both extremes in the same person
Indeed, some of the most amazing displays of intelligence come from "savants" who, like Dustin Hoffman's number-whiz character in "Rain Man," can barely tie their own shoelaces.
"There are savants who have extraordinary specific mental abilities," said Haier. "They excel on one factor, but they could be literally mentally retarded."
Researchers have not yet figured out what biologically grants savants their profound skills. On the other side of the equation, however, many kinds of obvious brain abnormalities, either genetic defects from birth or injury, can blunt or destroy one's capacity for thought.
Identifying the seat of intelligence
Such damage has helped pinpoint what brain regions govern discrete mental functions. Along with studies of healthy brains, the global view that has emerged is that intelligence does not spring from a single fount, but lies in the connections between key districts of the mind.
Numerous studies support a model of intelligence known as parietal-frontal interaction theory, or P-FIT, developed by Haier. According to P-FIT, a network of areas in the brain located in the frontal and parietal lobes uniquely process information in each individual. This idiosyncratic network gives rise to our personal talents and deficits. [10 Things You Didn't Know About the Brain]
Notably, greater volumes of gray matter, which consists of neuron cell bodies where computing takes place, correlate with higher intelligence test scores. Also, having more white matter — connections between brain cells made of long, fat-coated, or myelinated axons — between crucial gray matter areas means faster communication in P-FIT networks, boosting test scores.
Although it sounds like a bigger brain equals beefed-up intelligence, this is not the case. "There are many steps between simply measuring how thick a region of the brain is and understanding its function," said John Duncan, a cognitive neuroscientist with the Medical Research Council, a U.K. organization. Men naturally have larger brains than women, yet average intelligence test scores are nearly identical regardless of gender, Duncan said.
A sphinxlike noodle
Despite these insights, neuroscience still has not advanced to the point where doctors can simply look at the 3 pounds of glop in our skulls and know if the brain is a supercharged one or not.
"There are many people who are classified as mentally retarded whose brains look normal in imaging or in autopsy," Haier said.
Nor can parsing someone's genetic code yet reveal the likelihood for developing brilliance. Intelligence, however, is heritable, with smart parents typically producing smart children and vice versa. IQ and g scores do not correlate 100 percent of the time, so the environment – early childhood experiences, diet, et cetera — must weigh in as well.
Notwithstanding the claims of "Baby Einstein" educational video makers, or the notion that listening to classical music makes us keener-witted, the jury is very much still out on these factors.
"Everybody would like to think there are environmental interventions to overcome biology. We know in medicine it's true – you can overcome certain genetic predispositions by exercise or changing diet," Haier told LiveScience. "But we haven't found such things on cognition."
With any luck, powerful new brain imaging technologies could soon unlock the secrets of savants, as well as run-of-the-mill eggheadedness and stupidity.
"The question of why are people smarter than others is a question we now have the scientific means to investigate," Haier said.
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