Animal hoarding is keeping a higher-than-usual number of animals as domestic pets without ability to properly house or care for them, while at the same time denying this inability. Compulsive hoarding can be characterized as a symptom of mental disorder rather than deliberate cruelty towards animals. Hoarders are deeply attached to their pets and find it extremely difficult to let the pets go. They typically cannot comprehend that they are harming their pets by failing to provide them with proper care. Hoarders tend to believe that they provide the right amount of care for them. The American Society for the Prevention of Cruelty to Animals provides a "Hoarding Prevention Team", which works with hoarders to help them attain a manageable and healthy number of pets.

Compulsive hoarding in its worst forms can cause fires, unsanitary conditions (such as rat and roach infestations), and other health and safety hazards.

Listed below are possible symptoms hoarders may experience:

- They hold onto a large number of items that most people would consider useless or worthless, such as:

- Junk mail, old catalogs, magazines, and newspapers

- Worn out cooking equipment

- Things that might be useful for making crafts

- Clothes that might be worn one day

- Broken things or trash

- "Freebies" or other promotional products

- Their home is cluttered to the point where many parts are inaccessible and can no longer be used for intended purposes. For example:

- Beds that cannot be slept in

- Kitchens that cannot be used for food preparation

- Tables, chairs, or sofas that cannot be used for dining or sitting

- Unsanitary bathrooms

- Tubs, showers, and sinks filled with items and can no longer be used for washing or bathing.

- Their clutter and mess is at a point where it can cause illness, distress, and impairment. As a result, they:

- Do not allow visitors in, such as family and friends, or repair and maintenance professionals, because the clutter embarrasses them

- Are reluctant or unable to return borrowed items

- Keep the shades drawn so that no one can look inside

- Get into a lot of arguments with family members regarding the clutter

- Are at risk of fire, falling, infestation, or eviction

- Often feel depressed or anxious due to the clutter

Compulsive decluttering is a pattern of behavior that is characterized by an excessive desire to discard objects in one's home and living areas. Other terms for such behavior includes obsessive compulsive spartanism. The homes of compulsive declutterers are often empty. It is the antonym of compulsive hoarding.

The DSM-5 diagnostic criteria for hoarding disorder are:

Understanding the age of onset of hoarding behavior can help develop methods of treatment for this “substantial functional impairment”. Hoarders pose danger to not only themselves, but others as well. The prevalence of compulsive hoarding in the community has been estimated at between 2% and 5%, significantly higher than the rates of OCD, panic disorder, schizophrenia, and other disorders.

751 people were chosen for a study in which people self-reported their hoarding behavior. Of these individuals, most reported the onset of their hoarding symptoms between the ages of 11 and 20 years old, with 70% reporting the behaviors before the age of 21. Fewer than 4% of people reported the onset of their symptoms after the age of 40. The data shows that compulsive hoarding usually begins early, but often does not become more prominent until after age 40. Different reasons have been given for this, such as the prominence of family presence early in life and the extent of limits and facilitates they have on removing clutter. The understanding of early onset hoarding behavior may help in the future to better distinguish hoarding behavior from “normal” childhood collecting behaviors.

A second key part of this study was to determine if stressful life events are linked to the onset of hoarding symptoms. Similar to self-harming, traumatized persons may create "a problem" for themselves in order to avoid their real anxiety or trauma. Facing their real issues may be too difficult for them, so they "create" a kind of "artificial" problem (in their case, hoarding) and prefer to battle with it rather than determine, face, or do something about their real anxieties. Hoarders may suppress their psychological pain by "hoarding." The study shows that adults who hoard report a greater lifetime incidence of having possessions taken by force, forced sexual activity as either an adult or a child, including forced intercourse, and being physically handled roughly during childhood, thus proving traumatic events are positively correlated with the severity of hoarding. For each five years of life the participant would rate from 1 to 4, 4 being the most severe, the severity of their hoarding symptoms. Of the participants, 548 reported a chronic course, 159 an increasing course and 39 people, a decreasing course of illness. The incidents of increased hoarding behavior were usually correlated to five categories of stressful life events.

Digital hoarding (also known as e-hoarding) is excessive acquisition and reluctance to delete electronic material no longer valuable to the user. The behavior includes the mass storage of digital artifacts and the retainment of unnecessary or irrelevant electronic data. The term is increasingly common in pop culture, used to describe the habitual characteristics of compulsive hoarding, but in cyberspace. As with physical space in which excess items are described as "clutter" or "junk," excess digital media is often referred to as "digital clutter."

Trichotillomania is classified as compulsive picking of hair of the body. It can be from any place on the body that has hair. This picking results in bald spots. Most people who have mild Trichotillomania can overcome it via concentration and more self-awareness.

Those that suffer from compulsive skin picking have issues with picking, rubbing, digging, or scratching the skin. These activities are usually to get rid of unwanted blemishes or marks on the skin. These compulsions also tend to leave abrasions and irritation on the skin. This can lead to infection or other issues in healing. These acts tend to be prevalent in times of anxiety, boredom, or stress.

Compulsive behavior is defined as performing an act persistently and repetitively without it necessarily leading to an actual reward or pleasure. Compulsive behaviors could be an attempt to make obsessions go away. The act is usually a small, restricted and repetitive behavior, yet not disturbing in a pathological way. Compulsive behaviors are a need to reduce apprehension caused by internal feelings a person wants to abstain from or control. A major cause of the compulsive behaviors is said to be obsessive–compulsive disorder (OCD). "The main idea of compulsive behavior is that the likely excessive activity is not connected to the purpose to which it appears directed." Furthermore, there are many different types of compulsive behaviors including, shopping, hoarding, eating, gambling, trichotillomania and picking skin, checking, counting, washing, sex, and more. Also, there are cultural examples of compulsive behavior.

Bibliomania is not to be confused with bibliophilia, which is the usual love of books and is not considered a clinical psychological disorder.

Other abnormal behaviours involving books include book-eating (bibliophagy), compulsive book-stealing (bibliokleptomania), and book-burying (bibliotaphy).

Frank Tallis, a researcher in the topic of love and lovesickness, suggests in his 2005 article that lovesickness occurs when one is “truly, madly, deeply” in love and should be taken more seriously by medical professionals. Similarly, health experts agree that lovesickness has been known to kill and the diagnosis process should be taken more seriously. Symptoms of lovesickness are usually misdiagnosed for various other diseases or mental health issues such as OCD, this is because love sickness is less commonly recognized as a mental health issue in itself even though lovesickness is an extremely common, widespread disease.

Tallis includes a list of common symptoms of love sickness:

- Mania - an abnormally elevated mood or inflated self-esteem

- Depression

- Nausea

- Tearfulness

- Insomnia, which may lead to fatigue

- Lack of concentration

- Loss of appetite or overeating

- Hopelessness and/or helplessness

- Stress - high blood pressure, pain in chest and heart, acute insomnia; sometimes brought on by a "crush"

- Obsessive-Compulsive disorder - Preoccupation and hoarding valueless but superstitiously resonant items

- Psychologically created physical symptoms, such as upset stomach, change in appetite, insomnia, dizziness, and confusion

- Chronic neck pain, body tremors, intrusive thoughts, frequent flashbacks.

- Rapid mood swings

According to Tallis, many symptoms of being lovesick can be categorized under the DSM-IV (Diagnostic and Statistical Manual of Mental Disorders) and the ICD-10 (International Classification of Diseases). Obsessive-Compulsive disorder (OCD) is a symptom of lovesickness because it includes a preoccupation, this would include constantly checking one's cellphone, Facebook, the hoarding of valueless items, etc. A further study conducted by Italian Psychiatrist Donatella Marazitti found that when people fall in love their estimated serotonin levels drop to levels found in patients with OCD, this level is significantly lower than that of an average or healthy person.

The terms compulsive shopping, compulsive buying, and compulsive spending are often used interchangeably, but the behaviors they represent are in fact distinct. (Nataraajan and Goff 1992) One may buy without shopping, and certainly shop without buying: of compulsive shoppers, some 30% described the act of buying itself as providing a buzz, irrespective of the goods purchased.

CBD is frequently comorbid with mood, anxiety, substance abuse and eating disorders. People who score highly on compulsive buying scales tend to understand their feelings poorly and have low tolerance for unpleasant psychological states such as bad moods. Onset of CBD occurs in the late teens and early twenties and is generally chronic. CBD is similar to, but distinguished from, OCD hoarding and mania. Compulsive buying is not limited to people who spend beyond their means; it also includes people who spend an inordinate amount of time shopping or who chronically think about buying things but never purchase them. Promising treatments for CBD include medication such as selective serotonin reuptake inhibitors (SSRIs), and support groups such as Debtors Anonymous.

The "DSM-IV-TR" provides the following criteria for the diagnosis of a specific phobia:

The book "Phobias" defines a panic attack as "a sudden terror lasting at least a few minutes with typical manifestations of intense fear". These manifestations may include palpitations, sweating, trembling, difficulty breathing, the urge to escape, faintness or dizziness, dry mouth, nausea and/or several other symptoms. As with other specific phobias, patients suffering from cynophobia may display a wide range of these reactions when confronted with a live dog or even when thinking about or presented with an image (static or filmed) of a dog. Furthermore, classic avoidance behavior is also common and may include staying away from areas where dogs might be (i.e., a park), crossing the street to avoid a dog, or avoiding the homes of friends and/or family who own a dog.

Bibliomania can be a symptom of obsessive–compulsive disorder which involves the collecting or even hoarding of books to the point where social relations or health are damaged.

Cynophobia (from the "kýōn" "dog" and "phóbos" "fear") is the fear of dogs. Cynophobia is classified as a specific phobia, under the subtype "animal phobias". According to Dr. Timothy O. Rentz of the Laboratory for the Study of Anxiety Disorders at the University of Texas, animal phobias are among the most common of the specific phobias and 36% of patients who seek treatment report being afraid of dogs or cats. Although snakes and spiders are more common animal phobias, cynophobia is especially debilitating because of the high prevalence of dogs (for example, there are an estimated 25 million stray dogs in India, and an estimated 62 million pet dogs in the United States) and the general ignorance of dog owners to the phobia. The "Diagnostic and Statistical Manual of Mental Disorders" ("DSM-IV-TR") reports that only 12% to 30% of those suffering from a specific phobia will seek treatment.

Zoophobia or animal phobia is a class of specific phobias to particular animals, or an irrational fear or even simply dislike of any non-human animals.

Examples of specific zoophobias would be entomophobias, such as that of bees (apiphobia). Fears of spiders (arachnophobia), birds (ornithophobia) and snakes (ophidiophobia) are also common. See the article at -phobia for the list of various phobias. Sigmund Freud mentioned that an animal phobia is one of the most frequent psychoneurotic diseases among children.

Zoophobia is not the sensible fear of dangerous or threatening animals, such as wild dogs (example: wolves, dingoes, and coyotes), big cats, bears or venomous snakes. It is a phobia of animals that causes distress or dysfunction in the individual's everyday life.

Perhaps the strongest psychological model put forward to explain animal hoarding is obsessive–compulsive disorder (OCD). An overwhelming sense of responsibility for something is characteristic of people with OCD, who then take unrealistic measures to fulfill their perceived duty. Animal hoarders often feel a strong sense of responsibility to take care of and protect animals, and their solution—that of acquiring as many animals as they possibly can—is unrealistic. Further, the hoarding of inanimate objects, practiced by a majority of animal hoarders, is a fairly common occurrence in people with OCD. These connections between animal hoarding and obsessive–compulsive disorder suggest that OCD may be a useful model in explaining animal hoarding behavior. However, this theory has also been refuted by some; Dr. Akimitsu Yokoyama theorizes that animal hoarding could be explained using Asperger syndrome.

Species dysphoria is the experience of dysphoria, sometimes including clinical lycanthropy (delusion or hallucination of one's self as an animal) and dysmorphia (excessive concern over one's body image), associated with the feeling that one's body is of the wrong species. Earls and Lalumière (2009) describe it as "the sense of being in the wrong (species) body... a desire to be an animal". Outside of psychological literature, the term is common within the otherkin and therian communities. The phenomenon is sometimes experienced in the context of sexual arousal to the image of one's self as an animal.

Digital hoarding stems from a variety of individual traits and habits, corporate conditions, and societal trends:

- Some individuals experience anxiety when faced with disposing of digital items, particularly if they fear losing something important.

- Many digital hoarders don't know how to organize their digital content or aren't in the habit of doing so, and they lack a methodology for determining which content is worth keeping.

- Keeping all of one's digital files requires less time and effort than evaluating and deleting them.

- Many businesses rely on email correspondence for decision-making and formal approvals, so employees are often careful to keep work emails in case they are needed to verify a decision later.

- Data storage devices are now so large and inexpensive that individuals and companies often do not feel the need to save data selectively.

- The widespread availability and rapid dissemination of open content on the Internet makes it easier for users to obtain digital media, which can accumulate more quickly than ever.

- Since digital media do not take up physical space, they're less likely to be perceived as clutter, and users can more easily forget the extent of what they own.

- Unlike many physical items, electronic content does not die or decay on its own; users must consciously choose to delete it.

The main observed symptoms of OCPD are (1) preoccupation with remembering past events, (2) paying attention to minor details, (3) excessive compliance with existing social customs, rules or regulations, (4) unwarranted compulsion to note-taking, or making lists and schedules, and (5) rigidity of one's own beliefs, or (6) showing unreasonable degree of perfectionism that could eventually interfere with completing the task at hand.

OCPD's symptoms may cause varying level of distress for varying length of time (transient, acute, or chronic), and may interfere with the patient's occupational, social, and romantic life.

Boanthropy is a psychological disorder in which a human believes himself or herself to be a bovine.

Lovesickness refers to an informal affliction that describes negative feelings associated with rejection, unrequited love or the absence of a loved one. It can manifest as physical as well as mental symptoms. It is not to be confused with the condition of being lovestruck, which refers to the physical and mental symptoms associated with falling in love. The term lovesickness is rarely used in medical or psychological fields.

Many people believe lovesickness was created as an explanation for longings, but it can be associated with depression and various mental health problems.

Obsessive–compulsive personality disorder (OCPD) is a personality disorder characterized by a general pattern of concern with orderliness, perfectionism, excessive attention to details, mental and interpersonal control, and a need for control over one's environment, at the expense of flexibility, openness to experience, and efficiency. Workaholism and miserliness are also seen often in those with this personality disorder. Persons affected with this disorder may find it hard to relax, always feeling that time is running out for their activities, and that more effort is needed to achieve their goals. They may plan their activities down to the minute—a manifestation of the compulsive tendency to keep control over their environment and to dislike unpredictable things as things they cannot control.

The cause of OCPD is unknown. This is a distinct disorder from obsessive–compulsive disorder (OCD), and the relation between the two is contentious. Some (but not all) studies have found high comorbidity rates between the two disorders, and both may share outside similaritiesrigid and ritual-like behaviors, for example. Hoarding, orderliness, and a need for symmetry and organization are often seen in people with either disorder. Attitudes toward these behaviors differ between people affected with either of the disorders: for people with OCD, these behaviors are unwanted and seen as unhealthy, being the product of anxiety-inducing and involuntary thoughts, while for people with OCPD they are egosyntonic (that is, they are perceived by the subject as rational and desirable), being the result of, for example, a strong adherence to routines, a natural inclination towards cautiousness, or a desire to achieve perfection.

OCPD occurs in about 2–8% of the general population and 8–9% of psychiatric outpatients. The disorder occurs more often in men.

Affected individuals believe that they are in the process of transforming into an animal or have already transformed into an animal. It has been associated with the altered states of mind that accompany psychosis (the that typically involves delusions and hallucinations) with the transformation only seeming to happen in the mind and behavior of the affected person.

A study on lycanthropy from the McLean Hospital reported on a series of cases and proposed some diagnostic criteria by which lycanthropy could be recognised:

- A patient reports in a moment of lucidity or reminiscence that they sometimes feel as an animal or have felt like one.

- A patient behaves in a manner that resembles animal behavior, for example howling, growling, or crawling.

According to this criteria, either a delusional belief in current or past transformation or behavior that suggests a person thinks of themselves as transformed is considered evidence of clinical lycanthropy. The authors note that, although the condition seems to be an expression of psychosis, there is no specific diagnosis of mental or neurological illness associated with its behavioral consequences.

DSM-IV Criteria

Clinical Lycanthropy is thought to be a cultural manifestation of schizophrenia due to the first 4 symptomatic criteria. The first criteria are delusions, and this fits clinical lycanthropy because a person believing that he or she turns into an animal is a delusion. The second symptom is hallucination, and people with clinical lycanthropy have vivid hallucinations of being an animal, and having traits that animal has, whether it be claws, fur, fangs, or whatever that particular animal has. The next symptom is disorganized speech, from a certain human, cultural perspective. The people who have the diagnosis of clinical lycanthropy often emit the sounds of the animal which they believe they become. So, if a person believes that he or she is a werewolf, they may begin to howl under the moon or sometimes even in the daylight. The last symptom that matches schizophrenia is grossly disorganized behavior. This is appropriate because individuals with clinical lycanthropy often act like the animal they believe they have become, including living outside and changing their diet.

It also seems that lycanthropy is not specific to an experience of human-to-wolf transformation; a wide variety of creatures have been reported as part of the shape-shifting experience. A review of the medical literature from early 2004 lists over thirty published cases of lycanthropy, only the minority of which have wolf or dog themes. Canines are certainly not uncommon, although the experience of being transformed into a hyena, cat, horse, bird or tiger has been reported on more than one occasion. Transformation into frogs, and even bees, has been reported in some instances. In Japan, transformation into foxes and dogs was usual (, ). A 1989 case study described how one individual reported a serial transformation, experiencing a change from human to dog, to horse, and then finally cat, before returning to the reality of human existence after treatment. There are also reports of people who experienced transformation into an animal only listed as "unspecified".

There is a case study of a psychiatric patient who had both clinical lycanthropy and Cotard delusion. The term "ophidianthropy" refers to the delusion that one has been transformed into a snake. Two case studies have been reported.

Intermittent explosive disorder or IED is a clinical condition of experiencing recurrent aggressive episodes that are out of proportion of any given stressor. Earlier studies reported a prevalence rate between 1%-2% in a clinical setting, however a study done by Coccaro and colleagues in 2004 had reported about 11.1% lifetime prevalence and 3.2% one month prevalence in a sample of a moderate number of individuals (n=253). Based on the study, Coccaro and colleagues estimated the prevalence of IED in 1.4 million individuals in the US and 10 million with lifetime IED.

Obsessive–compulsive disorder (OCD) is a mental disorder where people feel the need to check things repeatedly, perform certain routines repeatedly (called "rituals"), or have certain thoughts repeatedly. People are unable to control either the thoughts or the activities for more than a short period of time. Common activities include hand washing, counting of things, and checking to see if a door is locked. Some may have difficulty throwing things out. These activities occur to such a degree that the person's daily life is negatively affected. Often they take up more than an hour a day. Most adults realize that the behaviors do not make sense. The condition is associated with tics, anxiety disorder, and an increased risk of suicide.

The cause is unknown. There appear to be some genetic components with both identical twins more often affected than both non-identical twins. Risk factors include a history of child abuse or other stress inducing event. Some cases have been documented to occur following infections. The diagnosis is based on the symptoms and requires ruling out other drug related or medical causes. Rating scales such as the Yale–Brown Obsessive Compulsive Scale (Y-BOCS) can be used to assess the severity. Other disorders with similar symptoms include anxiety disorder, major depressive disorder, eating disorders, tic disorders, and obsessive–compulsive personality disorder.

Treatment involves counselling, such as cognitive behavioral therapy (CBT), and sometimes medication, typically selective serotonin reuptake inhibitors (SSRIs). CBT for OCD involves increasing exposure to what causes the problems while not allowing the repetitive behavior to occur. While clomipramine appears to work as well as SSRIs, it has greater side effects. Atypical antipsychotics may be useful when used in addition to an SSRI in treatment-resistant cases but are also associated with an increased risk of side effects. Without treatment, the condition often lasts decades.

Obsessive–compulsive disorder affects about 2.3% of people at some point in their life. Rates during a given year are about 1.2% and it occurs worldwide. It is unusual for symptoms to begin after the age of thirty-five, and half of people develop problems before twenty. Males and females are affected about equally. In English the phrase "obsessive–compulsive" is often used in an informal manner unrelated to OCD to describe someone who is excessively meticulous, perfectionistic, absorbed, or otherwise fixated.