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

Intrusive thoughts may involve violent obsessions about hurting others or themselves. They can be related to primarily obsessional obsessive compulsive disorder. These thoughts can include harming a child; jumping from a bridge, mountain, or the top of a tall building; urges to jump in front of a train or automobile; and urges to push another in front of a train or automobile. Rachman's survey of healthy college students found that virtually all of them had intrusive thoughts from time to time, including:

- causing harm to elderly people

- imagining or wishing harm upon someone close to oneself

- impulses to violently attack, hit, harm or kill a person, small child, or animal

- impulses to shout at or abuse someone, or attack and violently punish someone, or say something rude, inappropriate, nasty, or violent to someone.

These thoughts are part of being human, and need not ruin quality of life. Treatment is available when the thoughts are associated with OCD and become persistent, severe, or distressing.

A variant of aggressive intrusive thoughts is L'appel du vide, or the call of the void. Sufferers of "L'appel du vide" generally describe the condition as manifesting in certain situations, normally as a wish or brief desire to jump from a high location.

Primarily cognitive obsessive-compulsive disorder (also commonly called "primarily obsessional OCD", purely obsessional OCD, Pure-O, OCD without overt compulsions or with covert compulsions) is a lesser-known form or manifestation of OCD. For people with primarily obsessional OCD, there are fewer observable compulsions, compared to those commonly seen with the typical form of OCD (checking, counting, hand-washing, etc.). While ritualizing and neutralizing behaviors do take place, they are mostly cognitive in nature, involving mental avoidance and excessiverumination. Primarily obsessional OCD often takes the form of intrusive thoughts of a distressing or violent nature.

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.

Many people experience the type of bad or unwanted thoughts that people with more troubling intrusive thoughts have, but most people can dismiss these thoughts. For most people, intrusive thoughts are a "fleeting annoyance". Psychologist Stanley Rachman presented a questionnaire to healthy college students and found that virtually all said they had these thoughts from time to time, including thoughts of sexual violence, sexual punishment, "unnatural" sex acts, painful sexual practices, blasphemous or obscene images, thoughts of harming elderly people or someone close to them, violence against animals or towards children, and impulsive or abusive outbursts or utterances. Such bad thoughts are universal among humans, and have "almost certainly always been a part of the human condition".

When intrusive thoughts occur with obsessive-compulsive disorder (OCD), patients are less able to ignore the unpleasant thoughts and may pay undue attention to them, causing the thoughts to become more frequent and distressing. The thoughts may become obsessions which are paralyzing, severe, and constantly present, and can range from thoughts of violence or sex to religious blasphemy. Distinguishing them from normal intrusive thoughts experienced by many people, the intrusive thoughts associated with OCD may be anxiety provoking, irrepressible, and persistent.

How people react to intrusive thoughts may determine whether these thoughts will become severe, turn into obsessions, or require treatment. Intrusive thoughts can occur with or without compulsions. Carrying out the compulsion reduces the anxiety, but makes the urge to perform the compulsion stronger each time it recurs, reinforcing the intrusive thoughts. According to Lee Baer, suppressing the thoughts only makes them stronger, and recognizing that bad thoughts do not signify that one is truly evil is one of the steps to overcoming them. There is evidence of the benefit of acceptance as an alternative to suppression of intrusive thoughts. A study showed that those instructed to suppress intrusive thoughts experienced more distress after suppression, while patients instructed to accept the bad thoughts experienced decreased discomfort. These results may be related to underlying cognitive processes involved in OCD. However, accepting the thoughts can be more difficult for persons with OCD. In the 19th century, OCD was known as "the doubting sickness"; the "pathological doubt" that accompanies OCD can make it harder for a person with OCD to distinguish "normal" intrusive thoughts as experienced by most people, causing them to "suffer in silence, feeling too embarrassed or worried that they will be thought crazy".

The possibility that most patients suffering from intrusive thoughts will ever act on those thoughts is low. Patients who are experiencing intense guilt, anxiety, shame, and upset over these thoughts are different from those who actually act on them. The history of violent crime is dominated by those who feel no guilt or remorse; the very fact that someone is tormented by intrusive thoughts and has never acted on them before is an excellent predictor that they will not act upon the thoughts. Patients who are not troubled or shamed by their thoughts, do not find them distasteful, or who have actually taken action, might need to have more serious conditions such as psychosis or potentially criminal behaviors ruled out. According to Lee Baer, a patient should be concerned that intrusive thoughts are dangerous if the person does not feel upset by the thoughts, or rather finds them pleasurable; has ever acted on violent or sexual thoughts or urges; hears voices or sees things that others do not see; or feels uncontrollable irresistible anger.

In psychology, relationship obsessive–compulsive disorder (ROCD) is a form of obsessive-compulsive disorder focusing on intimate relationships (whether romantic or non-romantic). Such obsessions can become extremely distressing and debilitating, having negative impacts on relationships functioning.

Obsessive-compulsive disorder comprises thoughts, images or urges that are unwanted, distressing, interfere with a person's life and that are commonly experienced as contradicting a persons' beliefs and values. Such intrusive thoughts are frequently followed by compulsive behaviors aimed at "neutralizing" the feared consequence of the intrusions and temporarily relieve the anxiety caused by the obsessions. Attempts to suppress or "neutralize" obsessions increase rather than decrease the frequency and distress caused by the obsessions.

Common obsessive themes include fear of contamination, fears about being responsible for harming the self or others, doubts, and orderliness. However, people with OCD can also have religious and sexual obsessions. Some people with OCD may experience obsessions relating to the way they feel in an ongoing relationship or the way they felt in past relationships (ROCD). Repetitive thought about a person's feelings in intimate relationships may occur in the natural course of the relationship development; however, in ROCD such preoccupations are unwanted, intrusive, chronic and disabling.

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.

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.

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.

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.

OCD can present with a wide variety of symptoms. Certain groups of symptoms typically occur together. These groups are sometimes viewed as dimensions or clusters that may reflect an underlying process. The standard assessment tool for OCD, the Yale–Brown Obsessive Compulsive Scale (Y-BOCS), has 13 predefined categories of symptoms. These symptoms fit into three to five groupings. A meta analytic review of symptom structures found a four factor structure(grouping) to be most reliable. The observed groups included a "symmetry factor", a "forbidden thoughts factor", a "cleaning factor" and a "hoarding factor". The "symmetry factor" correlated highly with obsessions related to ordering, counting, symmetry as well as repeating compulsions. The "forbidden thoughts factor" correlated highly with intrusive and distressing thoughts of a violent, religious or sexual nature. The "cleaning factor" correlated highly with obsessions about contamination and compulsions related to cleaning. The "hoarding factor" only involved hoarding related obsessions and compulsions, and was identified as being distinct from other symptom groupings.

While OCD has been considered a homogenous disorder from a neuropsychological perspective, many of the putative neuropsychological deficits may be due to comorbid disorders. Furthermore, some subtypes have been associated with improvement in performance on certain tasks such as pattern recognition(washing subtype) and spatial working memory(obsessive thought subtype). Subgroups have also been distinguished by neuroimaging findings and treatment response. Neuroimaging studies on this have been too few, and the subtypes examined have differed too much to draw any conclusions. On the other hand, subtype dependent treatment response has been studied, and the hoarding subtype has consistently responded least to treatment.

In scrupulosity, a person's obsessions focus on moral or religious fears, such as the fear of being an evil person or the fear of divine retribution for sin. Although it can affect nonreligious people, it is usually related to religious beliefs. In the strict sense, not all obsessive–compulsive behaviors related to religion are instances of scrupulosity: strictly speaking, for example, scrupulosity is not present in people who repeat religious requirements merely to be sure that they were done properly.

Some people with OCD exhibit what is known as "overvalued ideas". In such cases, the person with OCD will truly be uncertain whether the fears that cause them to perform their compulsions are irrational or not. After some discussion, it is possible to convince the individual that their fears may be unfounded. It may be more difficult to do ERP therapy on such people because they may be unwilling to cooperate, at least initially. There are severe cases in which the person has an unshakeable belief in the context of OCD that is difficult to differentiate from psychotic disorders.

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.

Kleptomania or klopemania is the inability to refrain from the urge for stealing items and is usually done for reasons other than personal use or financial gain. First described in 1816, kleptomania is classified in psychiatry as an impulse control disorder. Some of the main characteristics of the disorder suggest that kleptomania could be an obsessive-compulsive spectrum disorder.

The disorder is frequently under-diagnosed and is regularly associated with other psychiatric disorders, particularly anxiety and eating disorders, and alcohol and substance abuse. Patients with kleptomania are typically treated with therapies in other areas due to the comorbid grievances rather than issues directly related to kleptomania.

Over the last 100 years, a shift from psychotherapeutic to psychopharmacological interventions for kleptomania has occurred. Pharmacological treatments using selective serotonin reuptake inhibitors (SSRIs), mood stabilizers and opioid receptor antagonists, and other antidepressants along with cognitive behavioral therapy, have yielded positive results.

Body dysmorphic disorder is defined by an obsession with an imagined defect in physical appearance, and compulsive rituals in an attempt to conceal the perceived defect. Typical complaints include perceived facial flaws, perceived deformities of body parts and body size abnormalities. Some compulsive behaviors observed include mirror checking, ritualized application of makeup to hide the perceived flaw, excessive hair combing or cutting, excessive physician visits and plastic surgery. Body dysmorphic disorder is not gender specific and onset usually occurs in teens and young adults.

Sexual obsessions are obsessions with sexual activity. In the context of obsessive-compulsive disorder (OCD), these are extremely common, and can become extremely debilitating, making the person ashamed of the symptoms and reluctant to seek help. As preoccupation with sexual matters, however, does not only occur as a symptom of OCD, they may be enjoyable in other contexts (i.e. sexual fantasy).

The following conditions have been hypothesized by various researchers as existing on the spectrum.

However, recently there is a growing support for proposals to narrow down this spectrum to only include body dysmorphic disorder, hypochondriasis, tic disorders, and trichotillomania.

Some of the fundamental components of kleptomania include recurring intrusive thoughts, impotence to resist the compulsion to engage in stealing, and the release of pressure following the act. These symptoms suggest that kleptomania could be regarded as an obsessive-compulsive type of disorder.

People diagnosed with kleptomania often have other types of disorders involving mood, anxiety, eating, impulse control, and drug use. They also have great levels of stress, guilt, and remorse, and privacy issues accompanying the act of stealing. These signs are considered to either cause or intensify general comorbid disorders. The characteristics of the behaviors associated with stealing could result in other problems as well, which include social segregation and substance abuse. The many types of other disorders frequently occurring along with kleptomania usually make clinical diagnosis uncertain.

There is a difference between ordinary theft and kleptomania: "ordinary theft (whether planned or impulsive) is deliberate and is motivated by the usefulness of the object or its monetary worth," whereas with kleptomania, there "is the recurrent failure to resist impulses to steal items even though the items are not needed for personal use or for their monetary value."

Compulsive shopping or buying is characterized by a frequent irresistible urge to shop even if the purchases are not needed or cannot be afforded. The prevalence of compulsive buying in the U.S. has been estimated to be 2–8% of the general adult population, with 80–95% of these cases being females. The onset is believed to occur in late teens or early twenties and the disorder is considered to be generally chronic.

Scrupulosity is characterized by pathological guilt about moral or religious issues. It is personally distressing, objectively dysfunctional, and often accompanied by significant impairment in social functioning. It is typically conceptualized as a moral or religious form of obsessive–compulsive disorder (OCD), although this categorization is empirically disputable.

The term is derived from the Latin "scrupulum", a sharp stone, implying a stabbing pain on the conscience. Scrupulosity was formerly called "scruples" in religious contexts, but the word "scruples" now commonly refers to a troubling of the conscience rather than to the disorder.

As a personality trait, scrupulosity is a recognized diagnostic criterion for obsessive–compulsive personality disorder. It is sometimes called "scrupulousness", but that word properly applies to the positive trait of having scruples.

Many different types of medication can create/induce pure OCD in patients that have never had symptoms before. A new chapter about OCD in the DSM-5 (2013) now specifically includes drug-induced OCD.

Second generation atypical anti-psychotics, such as Olanzapine (Zyprexa), have been proven to induce de-novo OCD in patients.

The causes of racing thoughts are most often associated with anxiety disorders, but many influences can cause these rapid, racing thoughts. There are also many associated conditions, in addition to anxiety disorders, which can be classified as having secondary relationships with causing racing thoughts. The conditions most commonly linked to racing thoughts are bipolar disorder, anxiety disorder, attention deficit hyperactivity disorder, sleep deprivation, amphetamine dependence, and hyperthyroidism.

Racing thoughts may be experienced as background or take over a person's consciousness. Thoughts, music, and voices might be zooming through one's mind as they jump tangentially from one to the next. There also might be a repetitive pattern of voice or of pressure without any associated "sound". It is a very overwhelming and irritating feeling, and can result in losing track of time. In some cases, it may also be frightening to the person experiencing it, as there is a loss of control. If one is experiencing these thoughts at night when going to sleep, they may suddenly awaken, startled and confused by the very random and sudden nature of the thoughts.

Racing thoughts differs in manifestation according to the individual's perspective. These manifestations can vary from unnoticed or minor distractions to debilitating stress, preventing the sufferer from maintaining a thought.

Generally, racing thoughts are described by an individual who has had an episode where the mind uncontrollably brings up random thoughts and memories and switches between them very quickly. Sometimes they are related, as one thought leads to another; other times they seem completely random. A person suffering from an episode of racing thoughts has no control over his or her train of thought, and it stops them from focusing on one topic or prevents sleeping.