An individual's IQ score — long-held as the standard measure of human intelligence — is not a valid way of assessing brainpower, say Canadian researchers.
A team from Western University is debunking the concept of general intelligence, saying that there is no single component that can account for how a person performs various mental and cognitive tasks.
Instead, human intelligence is made up of multiple and distinct components, each of which must be looked at independently.
The study, published today in the journal Neuron, included the largest online intelligence survey on record, which recruited more than 100,000 participants.
"The uptake was astonishing," said Adrian M. Owen, the project's senior investigator. "We expected a few hundred responses, but thousands and thousands of people took part, including people of all ages, culture and creeds from every corner of the world."
The survey, which was open to anyone in the world with an internet connection, asked respondents to complete 12 cognitive tests that tapped into memory, reasoning, attention and planning abilities.
The results showed that how people performed at the tests could only be explained with at least three distinct components: short-term memory, reasoning and verbal ability.
No single measure, such as an intelligence quotient, or IQ score, could account for how well, or how poorly, people did.
The researchers also used a brain-scanning technique known as functional magnetic resonance imaging (fMRI) to look at the brains of a select group of participants as they performed different tasks.
They found that each cognitive component related to distinct circuits in the brain, supporting the idea of multiple specialized brain systems, each one with its own capacity.
The study also looked at respondents' backgrounds and lifestyle habits and found links between brain function and factors such as age and gender.
"Regular brain training didn't help people's cognitive performance at all, yet aging had a profound negative effect on both memory and reasoning abilities," said Owen.
Other factors were smoking, anxiety and the tendency to play videogames.
"Intriguingly, people who regularly played computer games did perform significantly better in terms of both reasoning and short-term memory," said Adam Hampshire, one of the study's authors.
"And smokers performed poorly on the short-term memory and the verbal factors, while people who frequently suffer from anxiety perform badly on the short-term memory factor in particular," Hampshire added.
The concept of a general intelligence factor dates back to at least 1904, when psychologist Charles Spearman suggested that there was a correlation between seemingly unrelated tasks, such as memorization, reading and performing arithmetic.
He called this link the 'g' factor, or general factor, and proposed that is accounted for an individual's performance across different mental tasks. Various intelligence tests, using a wide variety of methods, were developed throughout the 20th century as a way to evaluate children, students, military recruits and even potential hires.
Intelligence testing has often been criticized for various reasons, such as being culturally biased or for having invalid methodologies.
Yet it has been generally accepted that general intelligence does exist, even if it is difficult to measure.
"We have shown categorically that you cannot sum up the difference between people in terms of one number, and that is really what is important here," said Owen, adding that further tests still need to be done.
"Now we need to go forward and work out how we can assess the differences between people, and that will be something for future studies."