How do you figure? If I have the choice of increasing my labor by 20% (by some arbitrary metric, like labor hours per week) to increase my post-tax income by 20%, I might decide that is worth it and go for the higher income. If, on the other hand, I am currently at the top end of my current tax bracket, and the next highest bracket means that my 20% increase in labor will only increase my post-tax income by 10%, I might decide that isn't worth it and thus not go for the higher income.