More then 36% APR for anything more then a fraction of your weekly / monthly salary is likely to be crippling for most people and predatory. I can understand higher rates for very short term debts (i.e. a payday loan where the entire thing should be paid off in 1 month, but these lenders often try and have people turn over the principle into new loans).

IF someone's credit won't support a 20% interest rate then I don't think they should be leant money in the traditional sense. Either the purchase isn't required (in which case it shouldn't happen), or if its essential they should be supported by mechanisms to get people out of debt. Governments can make low value loans available to help here. In Australia, your entitled to a loan if your on benefits (centrelink) in which repayments are deducted from your welfare payments but charged no interest.