Conventional wisdom holds that low-down payment mortgages approved for borrowers who really couldn’t afford home ownership costs, and who ultimately defaulted on their loans, contributed much to the implosion of the financial markets. But the Urban Institute compared default rates on loans with down payments of 3-5 percent and 5-10 percent, and found that there wasn’t much difference between them.   Credit history is a far more accurate indicator of a borrower’s ability to repay, the study concluded. The study comes as Fannie Mae and Freddie Mac are again accepting loans with down payments of less than 5 percent. “This analysis tells us that there is likely to be minimal impact on default rates as low-down payment GSE lending gravitates towards borrowers with otherwise strong credit profiles,” the study concluded.