When mortgage default rates started to climb earlier this year many experts thought the damage would be confined to the minority of issuers that had binged on subprime lending. It hasn’t turned out that way. Consumer spending is off the credit crunch is spreading and the housing market is in a slump. ”The degree and the extent of the harm done were not expected. It surprised me. But the fact that this would end badly was not surprising ” says Wharton real estate and finance professor Susan Wachter. Wachter and colleague Richard K. Green trace the evolution of the home-financing market over the past decades in a new paper titled ”The Housing Finance Revolution.”