Stuff, stuff and more stuff. Many of us love to buy and keep things, even when the items are not useful. About 70 percent of children amass collections of favored objects, such as coins, dolls or baseball cards; many adults do the same. People often regard possessions as extensions of themselves and become attached to them accordingly.

Yet in rare cases, the habit of gathering and retaining things reaches unhealthy extremes, culminating in hoarding disorder, a condition that is poorly understood. Many laypeople believe that clinical hoarders are too lazy to discard their junk or that they enjoy living with it. Neither of those assumptions appears to be true. Moreover, most experts have long assumed that extreme hoarding is a variant of obsessive-compulsive disorder (OCD), even though most recent research suggests otherwise. Instead the ailment may stem from an exaggerated version of a basically adaptive tendency to accumulate materials that are important to us.

Distinct Pathology

Nikolai Gogol's 1842 novel, Dead Souls, featured a character named Plyushkin, a landowner who saved almost everything he found. Sigmund Freud regarded hoarding as a symptom of what he termed the “anal character,” purportedly stemming from overly harsh toilet training. (Few psychologists today share this view.) In the early and mid-1990s, however, hoarding increasingly came to be recognized as a serious clinical problem. Systematic research criteria for pathological hoarding, introduced in 1993 by psychologist Randy O. Frost of Smith College, spread awareness of the malady, as did a parade of television documentaries and reality shows, such as Hoarders, Clean House and Hoarding: Buried Alive.

Until recently, most mental health professionals regarded pathological hoarding as a subtype of OCD. Hoarding was considered a compulsion—a repeated, ritualized action intended to ward off anxiety, such as checking the stove repeatedly to make sure that it is turned off. According to a 2010 review by psychologist David Mataix-Cols of King's College London, however, 80 percent or more of people who engage in extreme hoarding do not meet criteria for OCD. For example, many do not experience the obsessions—recurrent or intrusive thoughts, images or impulses—that are widespread in OCD. Moreover, hoarders tend to be poorer, older, and more prone to mood and anxiety disorders than those with OCD; they are also less likely than OCD sufferers to be aware that they are disordered.

In recognition of these differences, the fifth edition of the American Psychiatric Association's diagnostic manual (DSM-5), published this past May, for the first time included pathological hoarding as a distinct condition. According to this volume, “hoarding disorder” is characterized by extreme and enduring difficulties parting with possessions, even if they have no tangible value. The afflicted have powerful urges to retain items or become very upset about tossing them out. Their home or workplace is filled with so much clutter that the space is unusable—and their problems seriously impair their everyday functioning or cause distress. Before diagnosing hoarding disorder, clinicians must rule out medical conditions that can lead to hoarding. For instance, in a 1998 study psychiatrist Jen-Ping Hwang of the Veterans Administration of Taipei and his colleagues found that 23 percent of patients with dementia displayed clinically significant hoarding behavior.

Hoarding disorder appears to be present in between 2 and 5 percent of the population, making it more prevalent than schizophrenia. It afflicts men and women in about equal numbers. People most often hoard books, magazines, newspapers and clothes; in some cases, they accrue scores of shirts, pants and dresses that have never been removed from their packaging. More rarely, individuals stockpile animals. In one case in 2010 authorities found more than 150 cats living in a home in Powell, Wyo. Animal hoarders tend to be more psychologically impaired than other hoarders and live in more squalid conditions, according to a 2011 article by Frost and his colleagues.

Deadly Business

Hoarding can be a serious, even deadly, business. The clutter may reach such proportions that living spaces become essentially uninhabitable, and patients may need to construct narrow tunnels or “goat paths” to get from one location to another. In a 2008 study psychologist David Tolin of the Institute of Living in Hartford, Conn., and his co-workers reported that 2 percent of hoarders had been evicted because of their mess. In a 2009 investigation, psychology student Gregory Lucini and his colleagues at the Worcester Polytechnic Institute Project Center in Melbourne, Australia, revealed that hoarding contributed to 24 percent of preventable deaths in house fires. In other cases, hoarders have been smothered to death by their clutter; this past April a 68-year-old New Jersey woman was found dead underneath piles of rotting garbage, clothing, tote bags and other possessions.

No one knows for sure why hoarders hoard. One clue to the condition, however, is that they often report a powerful emotional attachment to objects; some may imbue them with humanlike qualities, such as feelings, while recognizing that doing so is irrational. In other cases, hoarders insist on maintaining old items, such as clothing, “just in case.” Hoarding runs in families; in a 1993 study by Frost and psychology student Rachel C. Gross, now a professor at American University, 85 percent of pathological hoarders described one or more first-degree relatives (parents, children, siblings) as “pack rats”; this percentage significantly exceeded that of nonhoarders. In a 2009 study of more than 5,000 twin pairs, psychologist Alessandra C. Iervolino of King's College London and her collaborators found that this family pattern is genetically influenced; they estimated the heritability of severe hoarding at 50 percent.

Hoarding may have evolutionary origins. The behavior is present in a host of species, including honeybees, crows, rodents and monkeys, as psychologist Jennifer G. Andrews-McClymont, now at Morehouse College, and her colleagues pointed out in a 2013 review. This observation raises the possibility that the condition reflects a naturally selected urge to stockpile resources for times of scarcity.

Help for Hoarders

Hoarding disorder is challenging to treat, but some types of cognitive-behavior therapy can reduce its severity, according to a 2007 literature review by Tolin and his colleagues. The treatment focuses on altering irrational beliefs about the value of objects and providing supervised practice with organizing and discarding things. This intervention is not a panacea, however, given that many people with hoarding disorder do not complete their “homework,” which typically involves rearranging and tossing out clutter.

The limited treatment options for hoarders partly reflect our relatively poor understanding of this serious ailment. With the formal recognition of hoarding disorder in DSM-5, however, research into the causes of pathological hoarding will likely increase and, along with it, the promise of more effective therapies.