Specializing in subprime lending has yielded above-average profitability for many banks that have chosen this strategy, but earnings are also generally more volatile and there’s been considerable fallout in the sector. Those are the conclusions of a new study by Chiwon Yom, an economist at the Federal Deposit Insurance Corp. who analyzed profitability of banks that have focused on three business specialties: credit cards, subprime lending and Internet banking. Of the three, credit card