But poor borrowers are often too inexperienced and too harried to understand what they are being charged, experts said. In Mexico City, Maria Vargas has borrowed larger and larger amounts from Compartamos over 20 years to expand her T-shirt factory to 25 sewing machines from 5. She is hazy about what interest rate she actually pays, though she considers it high.

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“The interest rate is important, but to be honest, you can get so caught up in work that there is no time to go fill out paperwork in another place,” she said. After several loans, now a simple phone call to Compartamos gets her a check the next day, she said. Occasionally, interest rates spur political intervention. In Nicaragua, President Daniel Ortega, outraged that interest rates there were hovering around 35 percent in 2008, announced that he would back a microfinance institution that would charge 8 to 10 percent, using Venezuelan money.

There were scattered episodes of setting aflame microfinance branches before a national “We’re not paying” campaign erupted, which was widely believed to be mounted secretly by the Sandinista government. After the courts stopped forcing small borrowers to repay, making international financial institutions hesitant to work with Nicaragua, the campaign evaporated.

A Push for More Transparency

The microfinance industry is pushing for greater transparency among its members, but says that most microlenders are honest, with experts putting the number of dubious institutions anywhere from less than 1 percent to more than 10 percent. Given that competition has a pattern of lowering interest rates worldwide, the industry prefers that approach to government intervention. Part of the problem, however, is that all kinds of institutions making loans plaster them with the “microfinance” label because of its do-good reputation.

Damian von Stauffenberg, who founded an independent rating agency called Microrate, said that local conditions had to be taken into account, but that any firm charging 20 to 30 percent above the market was “unconscionable” and that profit rates above 30 percent should be considered high.

Mr. Yunus says interest rates should be 10 to 15 percent above the cost of raising the money, with anything beyond a “red zone” of loan sharking. “We need to draw a line between genuine and abuse,” he said. “You will never see the situation of poor people if you look at it through the glasses of profit-making.”

Yet by that measure, 75 percent of microfinance institutions would fall into Mr. Yunus’s “red zone,” according to a March analysis of 1,008 microlenders by Adrian Gonzalez, lead researcher at the Mix. His study found that much of the money from interest rates was used to cover operating expenses, and argued that tackling costs, as opposed to profits, could prove the most efficient way to lower interest rates.