A 2007 study found that 78% of a clinical sample of OCD patients had intrusive images. Most people who suffer from intrusive thoughts have not identified themselves as having OCD, because they may not have what they believe to be classic symptoms of OCD, such as handwashing. Yet, epidemiological studies suggest that intrusive thoughts are the most common kind of OCD worldwide; if people in the United States with intrusive thoughts gathered, they would form the fourth-largest city in the US, following New York City, Los Angeles, and Chicago.

The prevalence of OCD in every culture studied is at least 2% of the population, and the majority of those have obsessions, or bad thoughts, only; this results in a conservative estimate of more than 2 million sufferers in the United States alone (as of 2000). One author estimates that one in 50 adults have OCD and about 10–20% of these have sexual obsessions. A recent study found that 25% of 293 patients with a primary diagnosis of OCD had a history of sexual obsessions.

The cause of OCPD is unknown. However, it is believed to involve a combination of genetic and environmental factors. Under the genetic theory, people with a form of the DRD3 gene will probably develop OCPD and depression, particularly if they are male. But genetic concomitants may lie dormant until triggered by events in the lives of those who are predisposed to OCPD. These events could include trauma faced during childhood, such as physical, emotional, or sexual abuse, or other psychological trauma. Under the environmental theory, OCPD is a learned behavior.

The prevalence of scrupulosity is speculative. Available data do not permit reliable estimates, and available analyses mostly disregard associations with age or with gender, and have not reliably addressed associations with geography or ethnicity. Available data suggest that the prevalence of obsessive–compulsive disorder does not differ by culture, except where prevalence rates differ for all psychiatric disorders. No association between OCD and depth of religious beliefs has been demonstrated, although data are scarce. There are large regional differences in the percentage of OCD patients who have religious obsessions or compulsions, ranging from 0–7% in countries like the U.K. and Singapore, to 40–60% in traditional Muslim and orthodox Jewish populations.

Some evidence based on brain lesion case studies also suggests that the anterior ventromedial prefrontal and cingulate cortices may be involved in abnormal hoarding behaviors, but sufferers of such injuries display less purposeful behavior than other individuals who hoard compulsively, thus making the involvement of these brain structures unclear. Other neuropsychological factors that have been found to be associated with individuals exhibiting hoarding behaviors include slower and more variable reaction times, increased impulsivity, and decreased spatial attention. A study comparing neural activity in hoarders, people with OCD, and a control group when deciding to throw possessions away found that when hoarders were trying to decide to throw away their own possessions, they had lower activity in the anterior cingulate cortex and insula regions of the brain. The study suggested this lower activity was related to "problems in identifying the emotional significance of a stimulus, generating appropriate emotional response, or regulating affective state during decision making." Hoarders had normal levels of activity in those regions when making decisions about possessions that did not belong to them.

Not only are there significant health risks associated with compulsive hoarding, but scientists are also trying to pinpoint how significant the interference is with occupational and social functioning in a hoarder's daily life. In a pool of compulsive hoarders, 42% found their behavior problematic to the 63% of their family and friends who saw the behavior as problematic. The findings suggest that individuals who hoard may exhibit impaired sensitivity to their own and others’ emotions, and conversely, relate the world around them by forming attachments to possessions rather than to people. Lower emotional intelligence among hoarding patients may also impact their ability to discard and organize their possessions. With such detrimental characteristics, comprehensive research has been performed to find a cure. Although this is ongoing research, most investigations have found that only a third of patients who hoard show an adequate response to these medications and therapeutic interventions.

With the modifications to the DSM, insurance coverage for treatments will change as well as special education programs.

Unwanted thoughts by mothers about harming infants are common in postpartum depression. A 1999 study of 65 women with postpartum major depression by Katherine Wisner "et al." found the most frequent aggressive thought for women with postpartum depression was causing harm to their newborn infants. A study of 85 new parents found that 89% experienced intrusive images, for example, of the baby suffocating, having an accident, being harmed, or being kidnapped.

Some women may develop symptoms of OCD during pregnancy or the postpartum period. Postpartum OCD occurs mainly in women who may already have OCD, perhaps in a mild or undiagnosed form. Postpartum depression and OCD may be comorbid (often occurring together). And though physicians may focus more on the depressive symptoms, one study found that obsessive thoughts did accompany postpartum depression in 57% of new mothers.

Wisner found common obsessions about harming babies in mothers experiencing postpartum depression include images of the baby lying dead in a casket or being eaten by sharks; stabbing the baby; throwing the baby down the stairs; or drowning or burning the baby (as by submerging it in the bathtub in the former case or throwing it in the fire or putting it in the microwave in the latter). Baer estimates that up to 200,000 new mothers with postpartum depression each year may develop these obsessional thoughts about their babies; and because they may be reluctant to share these thoughts with a physician or family member, or suffer in silence and fear they are "crazy", their depression can worsen.

Intrusive fears of harming immediate children can last longer than the postpartum period. A study of 100 clinically depressed women found that 41% had obsessive fears that they might harm their child, and some were afraid to care for their children. Among non-depressed mothers, the study found 7% had thoughts of harming their child—a rate that yields an additional 280,000 non-depressed mothers in the United States with intrusive thoughts about harming their children.

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.

Like other forms of OCD, psychological and biological factors are believed to play a role in the development and maintenance of ROCD. In addition to the maladaptive ways of thinking and behaving identified as important in OCD, models of ROCD suggest that over-reliance on intimate-relationships or the perceived value of the partner for a person's feelings of self-worth and fear of abandonment (also see attachment theory) may increase vulnerability and maintain ROCD symptoms.

Quality of life is reduced across all domains in OCD. While psychological or pharmacological treatment can lead to a reduction of OCD symptoms and an increase in QoL, symptoms may persist at moderate levels even following adequate treatment courses, and completely symptom-free periods are uncommon. In pediatric OCD, around 40% still have the disorder in adulthood, and around 40% qualify for remission.

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.

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.

Complications of late Parkinson's disease may include a range of impulse-control disorders, including eating, buying, compulsive gambling and sexual behavior. One study found that 13.6% of Parkinson's patients exhibited at least one form of ICD. There is a significant co-occurrence of pathological gambling and personality disorder, and is suggested to be caused partly by their common "genetic vulnerability". The degree of heritability to ICD is similar to other psychiatric disorders including substance abuse disorder. There has also been found a genetic factor to the development of ICD just as there is for substance abuse disorder. The risk for subclinical PG in a population is accounted for by the risk of alcohol dependence by about 12-20% genetic and 3-8% environmental factors. There is a high rate of comorbidity between ADHD and other impulse-control disorders.

Estimates for the prevalence of OCPD in the general population range from 2.1% to 7.9%. A large U.S. study found a prevalence rate of 7.9%, making it the most common personality disorder. Men are diagnosed with OCPD about twice as often as women. It may occur in 8–9% of psychiatric outpatients.

Kleptomania is characterized by an impulsive urge to steal purely for the sake of gratification.

In the U.S. the presence of kleptomania is unknown but has been estimated at 6 per 1000 individuals. Kleptomania is also thought to be the cause of 5% of annual shoplifting in the U.S. If true, 100,000 arrests are made in the U.S. annually due to kleptomaniac behavior.

Anxiety disorder, the most common mental illness in the United States, affects 40 million people, ages 10 and older; this accounts for 18% of the U.S. population. Most people suffering from anxiety disorder report some form of racing thoughts symptom

The prevalence of OCD in every culture studied is at least 2% of the population, and the majority of those have obsessions, or racing thoughts. With these reportings, estimates of more than 2 million people in the United States (as of 2000) suffer from racing thoughts.

Those suffering from primarily obsessional OCD might appear normal and high-functioning, yet spend a great deal of time ruminating, trying to solve or answer any of the questions that cause them distress. Very often, Pure O sufferers are dealing with considerable guilt and anxiety. Ruminations may include trying to think about something 'in the right way' in an attempt to relieve this distress.

For example, an intrusive thought "I could just kill Bill with this steak knife" is followed by a catastrophic misinterpretation of the thought, i.e. "How could I have such a thought? Deep down, I must be a psychopath." This might lead a person to continually surf the Internet, reading numerous articles on defining psychopathy. This reassurance-seeking ritual will, ironically, provide no further clarification and could exacerbate the intensity of the search for the answer. There are numerous corresponding cognitive biases present, including thought-action fusion, over-importance of thoughts, and need for control over thoughts.

Despite how real and imposing the intrusive thoughts may be to an individual, the sufferer will probably never carry out actions related to these thoughts, even if one believes themselves capable of doing so. One of the reasons for this is because the person in question will go to extreme lengths to avoid circumstances which could trigger their intrusive thoughts.

The disorder is particularly easy to miss by many well-trained clinicians, as it closely resembles markers of generalized anxiety disorder and does not include observable, compulsive behaviors. Clinical "success" is reached when the sufferer becomes indifferent to the need to answer the question. While many clinicians will mistakenly offer reassurance and try to help their patient achieve a definitive answer (an unfortunate consequence of therapists treating primarily obsessional OCD as generalized anxiety disorder), this method only contributes to the intensity or length of the patient's rumination, as the neuropathways of the OCD brain will predictably come up with creative ways to "trick" the person out of reassurance, negating any temporary relief and perpetuating the cycle of obsessing.

The most effective treatment for primarily obsessional OCD appears to be cognitive-behavioral therapy. (more specifically exposure and response prevention (ERP)) as well as cognitive therapy (CT) which may or may not be combined with the use of medication, such as SSRIs. People suffering from OCD without overt compulsions are considered by some researchers more refractory towards ERP compared to other OCD sufferers and therefore ERP can prove less successful than CT.

ERP of Pure-O is theoretically based on the principles of classical conditioning and extinction. The spike often presents itself as a paramount question or disastrous scenario. A response that answers the spike in a way that leaves ambiguity is sometimes warranted. "If I don't remember what I had for breakfast yesterday my mother will die of cancer!" Using the antidote procedure, a cognitive response would be one in which the subject accepts this possibility and is willing to take the risk of his mother dying of cancer or the question recurring for eternity. No effort is expended in directly answering the question in an effort to find resolution. In another example, the spike would be, "Maybe I said something offensive to my boss yesterday." A recommended response would be, "Maybe I did. I'll live with the possibility and take the risk he'll fire me tomorrow." Using this procedure, it is imperative that the distinction be made between the therapeutic response and rumination. The therapeutic response does not seek to answer the question but to accept the uncertainty of the unsolved dilemma.

Acceptance and commitment therapy (ACT) is a newer approach that also is used to treat purely obsessional OCD, as well as other mental disorders such as anxiety and clinical depression. Mindfulness-based stress reduction (MBSR) may also be helpful for breaking out of the ruminative thinking process.

Trichotillomania is an impulse control disorder which causes an individual to pull out their hair from various parts of their body without a purpose. The cause for trichotillomania remains unknown. Like OCD trichotillomania isn’t a nervous condition but stress can trigger this habit. For some people pulling their hair out of boredom is normal, but that isn’t the case for someone that is dealing with trichotillomania. Emotions do not affect the behavior but these behaviors are more prevalent in those that suffer with depression.

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.

Hypochondria is currently considered a psychosomatic disorder, as in a mental illness with physical symptoms. Cyberchondria is a colloquial term for hypochondria in individuals who have researched medical conditions on the Internet. The media and the Internet often contribute to hypochondria, as articles, TV shows and advertisements regarding serious illnesses such as cancer and multiple sclerosis often portray these diseases as being random, obscure and somewhat inevitable. Inaccurate portrayal of risk and the identification of non-specific symptoms as signs of serious illness contribute to exacerbating the hypochondriac’s fear that they actually have that illness.

Major disease outbreaks or predicted pandemics can also contribute to hypochondria. Statistics regarding certain illnesses, such as cancer, will give hypochondriacs the illusion that they are more likely to develop the disease.

Overly protective caregivers and an excessive focus on minor health concerns have been implicated as a potential cause of hypochondriasis development.

It is common for serious illnesses or deaths of family members or friends to trigger hypochondria in certain individuals. Similarly, when approaching the age of a parent's premature death from disease, many otherwise healthy, happy individuals fall prey to hypochondria. These individuals believe they are suffering from the same disease that caused their parent's death, sometimes causing panic attacks with corresponding symptoms.

Family studies of hypochondriasis do not show a genetic transmission of the disorder. Among relatives of people suffering from hypochondriasis only somatization disorder and generalized anxiety disorder were more common than in average families. Other studies have shown that the first degree relatives of patients with OCD have a higher than expected frequency of a somatoform disorder (either hypochondriasis or body dysmorphic disorder).

Treatment is similar to that for other forms of obsessive–compulsive disorder. Exposure and response prevention (ERP), a form of behavior therapy, is widely used for OCD in general and may be promising for scrupulosity in particular. ERP is based on the idea that deliberate repeated exposure to obsessional stimuli lessens anxiety, and that avoiding rituals lowers the urge to behave compulsively. For example, with ERP a person obsessed by blasphemous thoughts while reading the Bible would practice reading the Bible. However, ERP is considerably harder to implement than with other disorders, because scrupulosity often involves spiritual issues that are not specific situations and objects. For example, ERP is not appropriate for a man obsessed by feelings that God has rejected and is punishing him. Cognitive therapy may be appropriate when ERP is not feasible. Other therapy strategies include noting contradictions between the compulsive behaviors and moral or religious teachings, and informing individuals that for centuries religious figures have suggested strategies similar to ERP. Religious counseling may be an additional way to readjust beliefs associated with the disorder, though it may also stimulate greater anxiety.

Little evidence is available on the use of medications to treat scrupulosity. Although serotonergic medications are often used to treat OCD, studies of pharmacologic treatment of scrupulosity in particular have produced so few results that even tentative recommendations cannot be made.

Treatment of scrupulosity in children has not been investigated to the extent it has been studied in adults, and one of the factors that makes the treatment difficult is the fine line the therapist must walk between engaging and offending the client.

Many people with sexual obsessions are alarmed that they seem to lose their sex drive. People with OCD may see this as evidence that they no longer have normal sexual attractions and are in fact deviant in some way. Some may wonder if medication is the answer to the problem. Medication is a double-edged sword. Drugs specifically for erectile dysfunction (i.e. Viagra, Cialis) are not the answer for people with untreated OCD. The sexual organs are working properly, but it is the anxiety disorder that interferes with normal libido.

Medications specifically for OCD (typically SSRI medications) will help alleviate the anxiety but will also cause some sexual dysfunction in about a third of patients. For many the relief from the anxiety is enough to overcome the sexual problems caused by the medication. For others, the medication itself makes sex truly impossible. This may be a temporary problem, but if it persists a competent psychiatrist can often adjust the medications to overcome this side-effect.

The prognosis varies on the severity of each case and utilization of treatment for each individual.

If these children are left untreated, they face risks such as poor results at school, avoidance of important social activities, and substance abuse. Children who have an anxiety disorder are likely to have other disorders such as depression, eating disorders, attention deficit disorders both hyperactive and inattentive.

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).

Compulsive talking goes beyond the bounds of what is considered to be a socially acceptable amount of talking. The two main factors in determining if someone is a compulsive talker are talking in a continuous manner, only stopping when the other person starts talking, and others perceiving their talking as a problem. Personality traits that have been positively linked to this compulsion include assertiveness, willingness to communicate, self-perceived communication competence, and neuroticism. Studies have shown that most people who are talkaholics are aware of the amount of talking they do, are unable to stop, and do not see it as a problem.

GAD runs in families and is six times more common in the children of someone with the condition.

While anxiety arose as an adaptation, in modern times it is almost always thought of negatively in the context of anxiety disorders. People with these disorders have highly sensitive systems; hence, their systems tend to overreact to seemingly harmless stimuli. Sometimes anxiety disorders occur in those who have had traumatic youths, demonstrating an increased prevalence of anxiety when it appears a child will have a difficult future. In these cases, the disorder arises as a way to predict that the individual’s environment will continue to pose threats.