What possesses someone to save every scrap of paper that s ever come into his home? What compulsions drive a woman like Irene, whose hoarding cost her her marriage? Or Ralph, whose imagined uses for castoff items like leaky old buckets almost lost him his house? Or Jerry and Alvin, wealthy twin bachelors who filled up matching luxury apartments with countless pieces of fine art, not even leaving themselves room to sleep?

Randy Frost and Gail Steketee were the first to study hoarding when they began their work a decade ago; they expected to find a few sufferers but ended up treating...