Ace one mental test and you'll probably beat average on nearly all the others
In 1904, Charles Spearman noticed that people's scores across wildly different mental tests, math, vocabulary, spatial reasoning, were almost always positively correlated. He invented factor analysis to show this "positive manifold" could be explained by one underlying factor he called g, general intelligence. Over a century later, that positive correlation between unrelated cognitive tests remains one of the most reliably replicated findings in all of psychology.