The study shows that before the housing boom in the mid-2000s, many mortgage loans had special features that made it easier for people to borrow money. These features led to a rise in house prices. The researchers used data from different counties to see how these special mortgages affected the housing market. They found that areas with a lot of these special mortgages saw a big increase in house prices. This happened because more people were able to get loans, especially those with lower credit scores.