When people talk about smarts, Intelligence Quotient or IQ always seems to come up.

People love to bring up how Einstein had a genius-level IQ of 160. And to join Mensa, you need to have an IQ of at least 130.

But is IQ even a good way to measure intelligence? Well, that depends on how you define intelligence. IQ scores may be a useful shorthand to talk about education strategies for big groups of people, like when discussing public policy.

But IQ can be affected by a lot of factors, even things as subjective as your motivation while taking the test. The first sort of IQ test was invented by the French psychologist Alfred Binet in the early 1900s. A law in 1882, aimed at egalitarianism, said that any healthy child had to go to school and learn the basics, like: reading, writing, arithmetic, history, public policy, and the natural sciences.

The law even included special consideration for children with disabilities, like deafness or blindness. But the French government acknowledged that not every kid would able to keep up with the normal curriculum, for lots of possible reasons. So, Binet and other psychologists were commissioned to create a standardized test to measure how different kids handled their schoolwork.

Along with Théodore Simon, Binet developed the Binet-Simon test – in which children would answer a series of questions until they couldn't anymore. That way, kids could be grouped in classes with students with similar scores, instead of relying on their age or the subjective judgments of teachers. In the next decade or so, this scale was revised for use with both kids and adults and renamed the Stanford-Binet Intelligence Scales.

This popular IQ test is still used today, along with other standardized tests that are meant to measure learning ability – sometimes defined as how quickly and easily we learn new things. On early versions of the test, IQ was calculated by taking a person's score on a standardized test, dividing it by their chronological age, and then multiplying the result by 100. In more modern versions, you're basically ranked against other test takers.

The scores of a group of people are scaled so that 100 is the average, and your IQ score is determined based on where you are in relation to that average. But here's the thing: whether or not IQ tests actually measure your intelligence depends on how you're defining intelligence. In simple definitions, intelligence is the ability to learn new things or adapt to new situations.

But the definition can also include the ability to use logic or reason, or to think abstractly. These definitions are all focused on intellectual capacity, which is how intelligence is defined by the American Psychological Association. And they don't include other kinds of intelligence, like social or emotional intelligence, or things like creativity, or self-awareness.

The Stanford-Binet test, for instance, focuses on testing five main categories of information: baseline knowledge, basic mathematics, visualizing objects in space, working memory, and fluid reasoning – or the ability to solve new problems. So, depending on what you're trying to understand about someone, IQ tests might be useful, or they might be a waste of time. It also turns out that your IQ score can be affected by a lot of different things – and because intelligence is so complex, we're not sure how strongly different factors might affect it.

There's some evidence that says cognitive abilities are somewhat heritable, meaning there might be some kind of genetic component to IQ. But it's not that simple! Recent studies have shown that IQ tests are affected by motivation.

For example, one 2011 meta-analysis found that people who are offered cash if they do well on an IQ test scored higher than people who weren't offered anything. Like, up to 20 points higher for just a 10 dollar reward. That's a huge effect!

And we know that motivation can play a role in other things, like your grades and your career path, that could be wrongly chalked up to just an IQ score. IQ also seems to be affected by environmental factors. Cultural values can influence your IQ scores.

For example, a kid who grows up in a community that prizes storytelling might do better on verbal sections of the test, or problems that require you to remember and reuse information. How much education you get – and the quality of that education – may have an effect, too. Kids who miss school because it's hard for them to get there, or who attend schools without many resources, tend to score lower than their peers.

Even your family environment can affect your IQ, like whether you grow up in a low-income household or whether you experience a lot of trauma as a kid. So, like a lot of things, IQ seems to result from a mix of nature and nurture. There are just so many factors that affect your learning ability as you grow up – from the environment you develop in before you're born, to things like education opportunities and family dynamics.

But psychologists seem to agree that one thing that seems to help people with learning and academic achievement is thinking about intelligence as a thing that can change. So IQ tests aren't anywhere near perfect or comprehensive, but they can help us predict how people might learn in the near future, which can make a difference in the support they receive. For instance, IQ scores can affect diagnosis of intellectual disability, which can inform public policy about education programs to support different students.

It's understandable why it's valuable to have a standard way to sort-of measure intelligence, like when it comes to making these general policy decisions. But it's also easy to see why IQ tests have been surrounded in controversy, too. There's a lot we don't understand about intelligence, and a lot that an IQ score can't tell us about a person or groups of people.

So while IQ can be a useful shorthand in some cases, it is not something would set in stone and do not let a number define you. Thanks for watching this episode of SciShow Psych, brought to you by our patrons on Patreon! If you'd like to support all the SciShow channels, you can go to patreon.com/scishow.

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