With all else being equal, I'm willing to believe that more intelligence would lead to greater income. But if you're claiming that IQ is the sole/main predictor for income that'd be a hard [citation needed] in my opinion.
In fact I recall a study (I saw on HN I believe) that showed that IQ is only correlated up to a certain value (not very high, I.e. Lower upper middle class IIRC) but then becomes quite uncorrelated. This certainly matches my anecdotal evidence that most of the rich kids during school were not very smart. The smart ones were typically the kids from academic middle class households.