Photo cred: my dad

Here's a definition of intelligence that lots of psychologists can get behind: 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 [...]

Intelligence, so defined, can be measured, and intelligence tests measure it well. Intelligence sounds pretty great. Who doesn't want to "catch on" and "make sense"? Hell, "figuring out" what to do is pretty much all of life! Naturally, people with more of this mental horsepower must live happier lives.

So smarter people are happier, right? Well, this meta-analysis says no. Another says maybe a teeny tiny bit. This large, nationally-representative study from the UK finds that people who score the lowest on an intelligence test are a little less happy than everyone else, but that's pretty much it. I also pulled data from the General Social Survey, which includes (a) a short vocabulary test that seems to correlate reasonably well with longer intelligence tests (you can try it here), and (b) a simple measure of happiness: "Taken all together, how would you say things are these days-would you say that you are very happy, pretty happy, or not too happy?" Across 50 years of data and 30,346 people, the folks who scored higher on the vocab test were a tiny bit less happy (r = -.06, p < .001).

Maybe our tests are bad. The psychological study of intelligence has a long, bleak history of racism and prejudice against poor people ("three generations of imbeciles are enough"), so we should be skeptical coming in. Psychologists have been trying to construct bias-free tests for a long time, but it's hard. Plus, people score higher on IQ tests when you pay them for performance, so what looks like a test of intelligence may in part be a test of how hard you're willing to try. But even if intelligence tests only measure something like "ability to succeed in an unfair society" or "willingness to try hard," it only deepens the mystery. Shouldn't those people end up with happier lives, however unfair that may be? And the tests likely do tap something more than just privilege and effort. There's plenty of skepticism toward intelligence tests in psychology, but even the biggest skeptics agree that IQ can predict things like how well you do in school and what kind of job you get, even accounting for all the criticisms. So why doesn't it also predict living a life that you like? I think there's one guy to blame for this big mystery, and his name is Charles Spearman. Way back in 1904, Spearman noticed something weird: the same kids who did well in one subject in school tended to do well in other subjects, too. The correlations were never perfect, of course, but they were pretty darn high, even across subjects that seemed pretty different from each other, like French and math. How come? Spearman figured there must be some general mental ability that humans use to solve all kinds of problems. He later wrote: This continued tendency to success of the same person throughout all variations of both form and subject-matter-that is to say throughout all conscious aspects of cognition whatever-appears only explicable by some factor lying deeper than the phenomena of consciousness. Helpfully, he also drew us a picture: Intelligence! This is, I think, exactly where everything went wrong with the study of intelligence for the next 119 years. It's not that Spearman's results were inaccurate-in fact, they've been replicated over and over. At this point, pretty much every paper on intelligence has to start out like this review from 2006: In the study of intelligence, one empirical phenomenon is well established: Test scores on cognitive tasks show a positive manifold, that is, they are invariably positively intercorrelated, albeit to varying degrees. This implies that people who score well on one cognitive test are likely to score well on other cognitive tests. The positive manifold is a robust phenomenon. Spearman's stats were sound, but his interpretation was wrong. He did not, as he claimed, observe a "continued tendency to success throughout all variations of both form and subject-matter," nor has anybody else. It merely looks as if we've varied all the forms and the subject-matters because we have the wrong theory about what makes them different. We think tests of math, vocabulary, French, music, etc. are all different because some are about words and others are about numbers and others are about sounds. But psychology, like all sciences, is all about discovering the differences between seemingly similar things, and discovering the similarities between seemingly different things. If psychologists ever had to march into battle, a good candidate for our crests may be the famous Müller-Lyer illusion, the two lines that look like they're different lengths but aren't: It strikes fear into the hearts of the economists! Just like those lines, I think all of our various tests of intelligence aren't as different as they seem. They're all full of problems that have a few important things in common: There are stable relationships between the variables. There's no disagreement about whether the problems are problems, or whether they've been solved. There have clear boundaries; there is a finite amount of relevant information and possible actions. The problems are repeatable. Although the details may change, the process for solving the problems does not. I think a good name for problems like these is well-defined. Well-defined problems can be very difficult, but they aren't mystical. You can write down instructions for solving them. And you can put them on a test. In fact, standardized tests items must be well-defined problems, because they require indisputable answers. Matching a word to its synonym, finding the area of a trapezoid, putting pictures in the correct order-all common tasks on IQ tests-are well-defined problems. Spearman was right that people differ in their ability to solve well-defined problems. But he was wrong that well-defined problems are the only kind of problems. "Why can't I find someone to spend my life with?" "Should I be a dentist or a dancer?" and "How do I get my child to stop crying?" are all important but poorly defined problems. "How can we all get along?" is not a multiple-choice question. Neither is "What do I do when my parents get old?" And getting better at rotating shapes or remembering state capitals is not going to help you solve them. We all share some blame with Spearman, of course, because everybody talks about smarts as if they're one thing. Google "smartest people in the world" and most of the results will be physicists, mathematicians, computer scientists, and chess masters. These are all difficult problems, but they are well-defined, and that makes it easy to rank people. The best chess player in the world is the one who can beat everybody else. The best mathematician is the one who can solve the problems that nobody else could solve. That makes it seem like the best chess players and mathematicians are not just the smartest in their fields, but the smartest in the whole world. There is, unfortunately no good word for "skill at solving poorly defined problems." Insight, creativity, agency, self-knowledge-they're all part of it, but not all of it.