The study looked at why mortgage debt has decreased in recent years. By analyzing individual credit records, researchers found that the drop in mortgage debt is mainly due to fewer people taking on new debt, rather than more people paying off existing debt. Weak first-time homebuying and tight credit supply have limited debt accumulation. Most of the decrease in debt comes from financially distressed borrowers and defaults, with real estate investors playing a big role. Overall, borrowers are not paying off their balances more aggressively than before.