According to the latest version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), hoarding disorder is described as a pattern of compulsive behaviour involving accumulating numerous possessions that are often of little value to anyone else. While we all have possessions that we really don't need (hence the popularity of garage sales), compulsive hoarders take the problem to extremes that can even endanger their lives. Older hoarders, who may also have problems with dementia, can develop a more severe form of hoarding behaviour, known as Diogenes syndrome, also known as senile squalor disorder, and these cases can include compulsive hoarding of garbage, along with severe self-neglect, unsafe living conditions, social withdrawal, apathy, and a lack of self-restraint leading to grim consequences for hoarders and their families.
Media stories of compulsive hoarders whose bodies have been found in their home, virtually buried under tons of newspapers, garbage, etc. are not uncommon and some hoarders have even become media celebrities. Most people diagnosed with a hoarding disorder are also likely to have related mental health problems, including chronic depression, generalized anxiety, pbsessive-compulsive disorder (OCD), and social anxiety. Hoarders often report serious problems with family, friends, and local authorities that can aggravate their symptoms. Even the children of hoarders can develop long-term emotional problems that persist well after they have left the family home.
Symptoms of hoarding disorder can include:
Items that can be hoarded include junk mail, old clothing, newspapers, broken objects, or other mementoes. Even animals can be hoarded which can lead to serious health problems, both for the hoarder and the animals being kept under unsafe conditions. Hoarding can begin early in life, with hoarding behaviours seen in children as young as twenty-two months, though the hoarding tends to worsen over time and can persist well into old age. Affecting both males and females (though more often seen in males), compulsive hoarding is believed to affect about two to six percent of the population, according to community surveys in the United States and Europe. While long believed to be a form of obsessive-compulsive disorder, hoarding disorder is often more difficult to treat and is far more likely to cause problems with social disability and work.
The actual cause of hoarding disorder is also controversial. While Freud suggested that hoarding behaviour stemmed from anal retention and unusually harsh toilet training, more modern theories focus on cognitive-behavioural factors such as information-processing deficits, behavioural avoidance, and distorted beliefs about objects stemming from trauma and other early childhood problems. There have also been numerous studies looking at different biological factors linked to human hoarding. This includes neuropsychological evidence showing greater cognitive problems in hoarders, i.e. impaired executive functioning, selective attention, and decision making. Brain imaging studies have found significant differences in brain activation for specific regions of the brain between hoarders, people with OCD, and human controls. The anterior ventromedial prefrontal and cingulate cortices have been identified as being important in different types of hoarding behaviour although there doesn't appear to be a specific brain mechanism involved.
Since compulsive hoarding is often seen along with other mental conditions such as OCD and depression, researchers looking at biochemical factors linked to hoarding have had difficulty finding medications that can offer effective treatment. While selective serotonin reuptake inhibitors have no effect on hoarding behaviour, some research suggests that stimulants and cognitive enhancers such as Aricept may help curb some of the symptoms. As for genetic factors, there does seem to be evidence that hoarding can occur in people who are closely related (including the well-known case of the Collyer brothers) though it is still unclear whether this is due to heredity or environment. There also appears to be evidence linking compulsive hoarding to chromosone 14—which has also been linked to disorders such as Alzheimer's disease and certain types of mental impairments.
In recent years, many researchers have pointed out the similarity between hoarding behaviour in humans with the type of hoarding often seen in different animal species. These include different species of primates, carnivores, rodents, birds, and insects though the kind of hoarding seen in animals can seem very different from the compulsive hoarding in humans. Still, the development of an animal model of hoarding behaviour can provide insight into the different brain regions associated with compulsive hoarding and possible new treatment methods.
But how valid is the animal model of hoarding? A comprehensive overview published in the Review of General Psychology takes a comprehensive look at the evidence and makes some suggestions about how animal models could be used to study hoarding behaviour in humans. Written by Jennifer Andrews-McClymont of Stephens College and Scott Lilienfeld and Marshall P. Duke of Emory University, the review covers the different genetic, biochemical, and neurological studies comparing human and animal hoarding behaviour. Though the authors recognize the difficulty in comparing behaviours across different species, there are enough similarities to be able to make some conclusions about why hoarding behaviour happens in both humans and animals.
At least some bird species such as corvids (including crows and ravens) and parids (including chickadees) have been seen hoarding non-food items in the same way they typically store food. This suggests that birds can develop abnormal hoarding behaviour that resembles compulsive hoarding in humans in many ways. Hoarding behaviour in rodents, including gerbils, rats, and mice, can also appear pathological since they can hoard inedible (and often shiny) objects rather than food. Rats are also more prone to increased hoarding behaviour as a result of being exposed to fear-provoking situations or being deprived of food as infants, possibly as a way of feeling more secure. Brain studies in rats suggest that areas of the brain linked to reward can influence hoarding behaviour. Damage to parts of the hypothalamus and other brain regions can lead to increased hoarding in rats as well as changes in dopamine and other neurotransmitters. Despite the similar behaviour however, the differences between human and rat brains makes direct comparison extremely difficult.
Research into hoarding behaviour in different species of primates is much more scarce. While primates are humans' closest genetic relatives, the hoarding seen in several monkey species bears little resemblance to the compulsive hoarding seen in humans. Still, brain studies suggest that damage to the same brain regions associated with hoarding in humans can lead to impairments in normal primate hoarding behaviour. At this point, the consensus seems to be that matching non-human primate hoarding to human compulsive hoarding will require far more research before any real progress is made.
As for the question of whether animal models can help explain compulsive hoarding in humans, there is no simple answer. Though hoarding behaviour occurs across numerous animal species, from insects to primates, linking animal hoarding to the often bizarre hoarding seen in many humans may not be possible. While the closest match seems to be with hoarding behaviour in rats,the many differences between human and rat brains means that direct comparison can only go so far. The many different kinds of hoarding seen in animals and the different factors that can shape hoarding behaviour over time will likely need far more research before a real combined model can be developed.
If a true evolutionary model of hoarding behaviour becomes accepted, it could mean a radical new approach for treating compulsive behaviour and recognizing how it is caused. For now, dealing with compulsive hoaders will continue to pose a problem that seems to defy simply solutions.