[lmm]

If you're of average IQ, an improvement of 0.2 standard deviations is about 8 percentiles of the population - the difference between being 15th in the class and being 12th or 13th. May not sound like much, but if we assume that translates directly into income (i.e. people of 10th percentile intelligence earn 10th percentile salaries) then that's the difference between a salary of $52k and $63k.

At the high end it's even more pronounced as the tail thins. Going from +2SD to +2.2SD is the difference between 98th percentile and 99th.

[notahacker]

Bearing in mind how loosely IQ and income are correlated IRL, and the opportunity cost of brain training vs devoting the equivalent time to job-specific learning, I'd say it's far from clear a 0.2 SD boost in adult IQ should lead to any change in income at all.

If it was 1 SD, the difference in ability would be rather more difficult not to notice.