There are several proposals about income tax rates in the future. Most of those proposals have a "threshold" of $200,000 for single taxpayers and $250,000 for joint returns.The new, additional 3.8 percent Medicare Surtax starts Jan. 1, 2013, for taxpayers with adjusted gross income above those thresholds who have investment income.What I don't understand is why the joint return threshold is not twice what it is for a single person.If a single person has a allowable Adjusted Gross Income of $200,000 before the 3.8 percent surtax may apply, why wouldn't a couple filing a joint return have a threshold of …