So along that strip from around 0 to 40k where it is pretty much horizontal, for every dollar you earn, you basically sacrifice a dollar that you would have gotten from welfare/medicare/food stamps. So if you make 20k a year and someone tells you if you work extra hours you can make 30k, you would shrug because it would cost you that same 10k in extra taxes/loss of welfare. The fact that the line has a constant slope after 50k means that once you're in that range, you're pretty much taxed at a constant pace, which is surprising until you see that this is a family of 3, so 150k/annum is not rich by the US' standards. I would expect that line to flatten out a little farther down?