PMI Mortgage Insurance Co. has released a study that finds piggyback loans may pose risk to the mortgage system. The study, “Hidden Risks of Piggyback Lending,” focuses on risks to borrowers, lenders, and locates the highest concentrations of risky loans in regions most likely to depreciate. A piggyback loan is a second mortgage that is “piggybacked” on to a first to compensate for a smaller down payment. …