That's 'high IQ'->'higher income'. You are implying 'higher income'->'high IQ', of which there are multitudes of counterexamples.
I know this is HN and we love to point and shout the names of informal fallacies, but maybe learning some formal ones would be useful, too.
If you are going to make a blanket statement in your thesis then pull back from its correctness then best not to spout the original as hard truth