Research

Here's an article I came across from Social Work Today which goes into the psychology a bit, the current research (emphasis added by me):

Hoarding treatment begins with a person acknowledging the underlying problems that fuel hoarding behavior. The reasons for hoarding differ depending on the person, and treatment is challenging because people who hoard often do not realize their behavior is a problem or they are socially isolated and ashamed due to the condition of their home. Sometimes, they resist treatment because they think it will involve simply clearing out the clutter.

“Just cleaning out the stuff does not address the underlying psychological issues,” Shulman says. “It’s not a clutter problem; it’s a perception/thinking problem.”
According to Steketee, no single condition causes hoarding. Contributing factors or stressors may include the following:

• being raised in a chaotic home or one with confusing family context, or moving frequently;
• cognitive processing issues that affect decision making and problem solving;
• attention-deficit disorder;
• anxiety and/or depression;
• excessive guilt about waste; and
• genetics and family history because hoarding behavior runs in families.

[...]

“Hoarding may induce feelings of safety and security and/or reinforce identity,” adds Steketee.

The exact causes of hoarding are still uncertain, and research on the physiology and psychology of hoarding is ongoing. Geneticists are working to identify genetic loci related to hoarding behavior, says Steketee. An imaging study found that cerebral blood flow in a patient with OCD and severe hoarding exhibited a certain pattern during the most severe hoarding symptoms, and the pattern changed when the patient’s hoarding behaviors improved (Ohtsuchi, Matsuo, Akimoto, & Watanabe, 2010). A review of epidemiological, neurobiological, and treatment studies concluded that compulsive hoarding may be a discrete disorder with its own diagnostic criteria (Pertusa et al., 2010).

I kind of hate that word "hoarding" - sounds so awful and messy and sickening. I think of that first step, about acknowledging the underlying problems, but honestly, who would ever want to admit to having that?