Jan. 30 (Bloomberg) — The debate over President Barack Obama's proposal to raise the minimum wage turns on the credibility of an economic tenet popularized by Milton Friedman: "There's no such thing as a free lunch."

Since states began raising their wage floors two decades ago, economists have focused on the patchwork of laws in a debate about the impact — or lack thereof. Some studies argue that moderate increases have little or no ill effects on hiring and therefore will boost household income and spending. Others call the laws job killers that hurt unskilled and low-paid workers, the very people they're meant to help.

The sides square off again as Obama and other Democrats push for an increase in the $7.25-an-hour federal minimum. In his annual State of the Union address this week, Obama announced a wage increase for the lowest-paid federal workers and contract employees, and prodded state and city governments to take similar action if Congress doesn't.