The result of a test can be predictive of job performance simply because the result of /any/ test would be predictive of it, because it shows you're willing and

The result of a test can be predictive of job performance simply because the result of /any/ test would be predictive of it, because it shows you're willing and able to take and complete tests. This does not mean the score on a specific test is "real". Minimal example: if you gave a test to a living person and a corpse, only one of them would finish it, and that one would also have a higher income. Also note the "highest intelligence" jobs don't always have the highest income, like academia, but rather involve taking lots of tests as a prerequisite.