The Republican presidential candidates -- particularly Mitt Romney -- have been under pressure to release their tax returns. The topic has gotten plenty of play in the GOP debates this year. It's not the first time the issue has come up in a presidential campaign, however. Although tax returns are private, most presidents since the 1970s have chosen to release them.

In honor of Presidents Day, here's a look back at how much some past presidents made, and how they earned it. All numbers come from tax returns courtesy of the Tax History Project.

Barack Obama
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2008 income: $2,656,902

Barack Obama's yearly earnings, though still high, have taken a considerable dip since he took office. Most of his revenue comes from the sale of his books, Dreams From My Father and The Audacity of Hope, as well as Of Thee I Sing: A Letter to My Daughters, a children's book.

Earlier on HuffPost: