There's still a lot we don't know about compulsive hoarding, including what causes it. Hoarding's classification as its own disease is a relatively recent event — it was first labeled a separate disease in 2013 in the 5th edition of the "Diagnostic and Statistical Manual of Mental Disorders," the guide used by medical and psychiatric professionals (and insurance companies) to diagnose mental health problems. This classification could attract research funding and also help compulsive hoarders get treatment covered by their health insurance.
"As research developed we found many characteristics of hoarding behaviors that suggested it was really a separate condition, both in the presentation of symptoms and poor response to treatments known to work well for OCD," Hale says. "The current line of thinking from the research is that hoarding is a neuropsychiatric condition linked to processing challenges — trouble with the connections and functionality of emotional, visual, and organizational areas of the brain."
Each study into hoarding gives us another clue as to the underlying cause. Hoarding probably has a genetic component, since research has found that hoarding behavior is part of other illnesses that are definitely known to be genetic, and compulsive hoarders are likely to have family members who are also compulsive hoarders [source: Gothelf]. There are even correlations between hoarding and certain chromosome defects [source: Samuels]. Another study found that a certain type of brain lesion seemed to account for the onset of compulsive hoarding in some subjects [source: Anderson].
Hoarding seems to be the result of some kind of short circuit of a normal human behavior. We all attach value and meaning to material items and derive a sense of security from them. Compulsive hoarding is this same behavior gone completely out of control. There's an evolutionary reason that we tend to acquire things and want to keep them, which can be expressed mathematically (the cost of acquiring the items weighed against the cost of not acquiring them).
The non-math version is this: Imagine that an animal can either save enough food to last through a short winter or enough to last through a long winter. Gathering the extra food for a long winter carries additional risk, so short winter animals have a greater chance of surviving. However, eventually there will be a long winter, which will kill off all the animals who only had enough food for a short winter. The "save up for a long winter" evolutionary trait, therefore, will be passed on to future generations [source: Bergstrom].
There are more complicated versions of the scenario (such as a genetic trait that causes long or short winter strategies to occur randomly in the same animal from year to year), but the bottom line is that long-term acquisition and saving strategies tend to have greater evolutionary success, and humans have received that trait from countless generations of our mammalian ancestors. In some humans, the trait goes haywire.