Something called g. Which is basically rounds to our intuition of how smart you are. It is one the best predictors of job performance (up there with a work sample test), income, reaction time, how much schooling you'll get, how likely you are to commit crime, how long you'll live.

Imagine it like this, if we give someone a history test, a math test, a physics test, and english test. We'll notice how good you are at any given tests predicts how good you are at other tests. Even though math and history don't have any real overlap in terms of skills. They also found that doing well on some of these tests will better predict how well you'll do at others. For instance math and physics are typically more g-loaded so if someone does well on a math or physics test they are more likely to do well on an english test than the reverse. We call this quality of that predicts how well you're going to do at intellectual tasks g. And then we built a test to try to measure G and that's an IQ test, with IQ being the measurement of g.