Another issue: perhaps the increase in happiness with income (I) reported for I < 75k is due to the fact that jobs with higher income have nicer work conditions and colleagues. If this is true it should be possible to make people happier without paying them more. Just arrange matters so that their jobs are more pleasant.

It was always puzzling to me as a child that the people doing the hardest work are paid the least money. (hard in the sense that it physically drains you, and your whole body aches)