Phil Mickelson © Harry How, Getty Images

Does Phil Mickelson have a legitimate beef when it comes to his taxes? The Hall of Fame golfer and one of the world's highest-paid athletes recently caused a stir when he threatened to move out of his native California -- and hinted at perhaps even retiring from golf -- because of his high state and federal tax rates.

Mickelson reportedly earned nearly $48 million last year. He was reacting in part to California's newly passed Proposition 30, which raises the state tax rate on people earning more than $1 million from 10.3% to 13.3%.

And as The Wall Street Journal recently reported, federal tax rates on income of more than $400,000 rose this year, from 35% to 39.6%. At the same time, according to the WSJ, "'Obamacare' levies a new 3.8% surtax on investment income and raises the Medicare tax by 0.9% on wages over $200,000. Limits on itemized deductions for high earners have also been reinstated, which will raise many athletes' marginal rates by one to two percentage points."

While "Lefty," as Mickelson is known, quickly backtracked and apologized for his comments, he also opened the door on something that a lot of affluent folks and their accountants already know: Rich people are not all equal when it comes to the taxes they pay.

"If you add up all the federal and you look at the disability and the unemployment and the Social Security and the state, my tax rate's 62, 63%," Mickelson said at the time.

Analysts believe his actual tax rate is closer to 50%, according to CNBC. But still, how does Mickelson end up paying about half his income in taxes when Mitt Romney, who's reportedly worth about $250 million, famously had a rate of around 14% on his 2011 taxes?

Or, to quote Salary.com, "How does a man worth hundreds of millions end up with a tax rate similar to that paid by a household earning $50,000 per year?"

One big difference is in their jobs. "If you look at someone like a professional athlete, they are earning their income through their services, through labor, similar to a construction worker," says Sharon Lassar, a director of the school of accountancy at the University of Denver's Daniels College of Business. "And income that is earned through labor is ordinary income that is subject to U.S. taxes today as high as 39.6%. That's after the legislation that was passed to deal with the fiscal cliff; it was lower before that."

In comparison, Romney's income likely came through capital gains -- the profits from selling assets such as real estate or stocks -- acquired through his partnership with firms that buy and sell other companies. "And the sale of those underlying companies is a capital gain; it generates a capital gain to the partnership," Lassar says.

Romney's income is taxed twice – first at the corporate level and a second time when reported on a personal income-tax return. As he said during his interview with "60 Minutes" last September, "One of the reasons why the capital-gains tax rate is lower is because capital has already been taxed once at the corporate level, as high as 35%."

The income Romney and others like him report on their federal tax returns comes mostly from capital gains, which currently are taxed at a high of 23.8%. But last year, before the fiscal-cliff legislation, they were taxed at a maximum of 15%. "And that's why Mitt Romney's tax rate was just under 15%," Lassar says, "because he also had charitable contributions and other items which reduced his taxable income."

"Essentially, the (federal) policy is this: If you have income from capital, we're going to tax it at a preferential rate," Joseph Newpol, a professor of law, taxation and financial planning at Bentley University in Waltham, Mass., explained to Salary.com. The government's rationale for that lower capital-gains tax rate, he says, is "to encourage people to invest in stocks, bonds and things like that because that's good for the economy, it creates jobs."

Romney also came under fire during his presidential campaign for not providing enough of his tax returns for public scrutiny. But Lassar says those returns wouldn't have provided much political ammunition to Romney's opponents, because they cannot really explain the tangled way that investment income is earned and taxed.

Others look at the discrepancy between what a top athlete and a world-class investor pay in taxes in a more pragmatic way.

"The Mitt-Lefty paradox has a simple explanation: In America, we tax work. And highly," Edward McCaffery, a professor of law, economics and political science at the University of Southern California, said recently on CNN.com. "We do not tax capital or wealth much at all. Indeed, if you have wealth already, taxes are essentially optional."

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