Your intelligence seems to be a trait that sticks with you for life, something that can hint at our potential for success in this world. It's fixed, yet also dynamic, capable of amplification—something that's safe to build your identity upon.
But that's not what IQ tests say. They are imperfect reflections of intelligence, and for most people don't tell you much at all.
Defining what we mean by intelligence is its own challenge. W. Joel Schneider, a psychologist at Illinois State University, calls it a "folk concept," one just vague enough to fit the context the user needs. Some people agree that there's an overarching thing called general intelligence, while others debate the existence of multiple intelligences. But most psychologists agree on the following definition for fluid intelligence, published in an op-ed in the Wall Street Journal in 1994:
Intelligence is a very general mental capability that, among other things, involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly and learn from experience. It is not merely book learning, a narrow academic skill, or test-taking smarts. Rather, it reflects a broader and deeper capability for comprehending our surroundings—"catching on," "making sense" of things, or "figuring out" what to do.
That is, in theory, what IQ tests measure. Most standard IQ tests, such as the Wechsler IQ test, measure a combination of reasoning, verbal ability, working memory, and processing speed. There's a certain amount of fluctuation built into the scores, which are scaled so that people of the same age are compared to one another. The average intelligence is 100; most people will fall between 80 and 120. People with a score below 70 qualify as having an intellectual disorder; high IQ is anything over 140, and "genius" level is 160 and up. The key is to have a knowledgeable person administering the test to keep the results as standard as possible.
But those scores don't necessarily equate to actual intelligence. "All [researchers] will tell you that IQ tests are an imperfect reflection of true intelligence—there are lots of factors that affect a person's inherent ability to perform on the test," says Wayne Silverman, a psychologist at the Kennedy Krieger Institute and Johns Hopkins University School of Medicine.
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Let's consider, first, things can have a temporary effect on cognition. An individual's score on an IQ test can change if there's something briefly altering their cognition, like if they have a cold or are, for some other reason, not totally clear-headed. "If you get me drunk and give me an IQ test, I will do very badly," says Richard Nisbett, a psychology professor at the University of Michigan.
Based on decades of research, scientists have generally concluded that a person's IQ score can also change over the course of her life (there are studies, even good ones, that contradict this, but this is where the bulk of the evidence lies).
What has become clearer in recent years, is that intelligence, like many facets of health and even complex diseases, is the result of a complex interaction of genetics and the environment. Several studies over the past five years have identified hundreds of genes that may be involved in intelligence. The results of IQ tests can even vary depending on a person's state of development, which is usually triggered by genetics—the greatest fluctuation usually happens during adolescence, when the brain is still developing.
But non-genetic factors can drastically affect IQ. There are elements of a person's childhood environment that can depress IQ scores later in life, such as poverty, poor at-home intellectual environment, and exposure to toxic chemicals such as lead. Repeated head injuries lower IQ in the long run. Some things can also raise them, such as hanging out with smarter people—moving a child from an impoverished household to a middle- or upper-class one can result in sizable gains in IQ scores.
"The brain seems to be rather like a muscle—the more you use it, the stronger it gets. That means you can upgrade your own intelligence all through life," intelligence researcher James Flynn told The Australian.
For an individual, it's easier to depress the score of IQ tests over a lifetime than to boost them, especially after adolescence. "You don't see many reports of significant increases in IQ unless someone had screwed up the testing," Silverman says.
Over the course of generations, however, people have in fact performed better on IQ tests, a phenomenon called the Flynn Effect. Some of the drivers of this trend include more education, less exposure to toxins, and the fact that more people work in cognitively demanding jobs, Flynn said in a 2013 TED talk.
Whether these changes in IQ scores document a change in actual intelligence is still a matter of debate, Nisbett says. And for most people who take IQ tests just for fun, the results probably don't really matter. Those scores do matter if the test is being used to assess someone for a cognitive disability. "There, IQ testing is pretty good at doing what it's supposed to do, which is to identify people with a significant cognitive impairment that is unlikely to change in the foreseeable future," Silverman says. Scores really matter in these cases, because variation between tests can make the difference between whether or not someone with an intellectual disability receives government benefits or not.
And though certainly intelligence helps people succeed in their careers, it's not necessarily the most important factor. As Warren Buffett famously said, "If you are in the investment business and have an IQ of 150, sell 30 points to someone else." The same is true for science, Nisbett says. "Being successful in science is partly sheer raw smarts, which is tabbed to a degree by IQ. But there are all sorts of other things, like achievement motivation, energy, and the ability to get along with other people. All have to be high to do well in science, but don't have to be super high." It's likely true for most other fields, too.
Researchers studying intelligence are still working to answer questions about whether you can boost an adult's IQ with brain training games like Lumosity. So far, studies seem to indicate that the answer is no, but as Nisbett points out, "the evidence isn't all in there yet. It's clearly controversial." Others are exploring questions about the role of early childhood education on long-term IQ, or exploring how to assess whether there are other forms of intelligence beyond general intelligence, such as social or emotional intelligence.
The role of environmental influence on IQ makes intelligence into a political or even moral issue—"Society has a moral obligation to see to it that every person has an environment that will allow that person to attain an intelligence that is consistent with a person's genetic potential," Nisbett says.
As for the tests themselves, they have to be redone every ten years so that the mean stays at 100, to keep up with the Flynn effect. So even if you're getting smarter, your score might not change so much after all.
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