In Europe, the rate of problem gambling is typically 0.5 to 3 percent. The "British Gambling Prevalence Survey 2007", conducted by the United Kingdom Gambling Commission, found approximately 0.6 percent of the adult population had problem gambling issues—the same percentage as in 1999. The highest prevalence of problem gambling was found among those who participated in spread betting (14.7%), fixed odds betting terminals (11.2%) and betting exchanges (9.8%). In Norway, a December 2007 study showed the amount of present problem gamblers was 0.7 percent.

Most research has focused on adult population or on college students, but little is known about epidemiology of behavioral addictions in adolescence. A study conducted by Villella "et al" looked at a group of students and the prevalence of various addictions. His results showed exercise addiction was the second most prevalent, after compulsive buying.

High risk groups that appear to be addicted to exercise include athletes in sports encouraging thinness or appearance standards, young and middle-age women, and young men.

A gambler who does not receive treatment for pathological gambling when in his or her desperation phase may contemplate suicide. Problem gambling is often associated with increased suicidal ideation and attempts compared to the general population.

Early onset of problem gambling increases the lifetime risk of suicide. However, gambling-related suicide attempts are usually made by older people with problem gambling. Both comorbid substance use and comorbid mental disorders increase the risk of suicide in people with problem gambling. A 2010 Australian hospital study found that 17% of suicidal patients admitted to the Alfred Hospital's emergency department were problem gamblers. In the United States, a report by the National Council on Problem Gambling showed approximately one in five pathological gamblers attempt suicide. The council also said that suicide rates among pathological gamblers were higher than any other addictive disorder.

Specific genes predispose people to drug addiction—it's one of the behavioral disorders most strongly corrolated with genetic makeup—but environmental factors, especially trauma and mistreatment in childhood, also correlate strongly with addiction. . Examples such as physical or sexual abuse, and unpredictable expectations and behavior of parents, increase a person's risk for developing addiction.

Sensation-seeking has been studied as having a strong relationship with addictive personalities.

Individuals with exercise addiction may put exercise above family and friends, work, injuries, and other social activities. If not identified and treated, an exercise addiction may lead to a significant decline in one's health.

Some scholars believe that codependency is not a negative trait, and does not need to be treated, as it is more likely a healthy personality trait taken to excess. Codependency in nonclinical populations has some links with favourable characteristics of family functioning.

Stan Katz states that codependence is over-diagnosed, and that many people who could be helped with shorter-term treatments instead become dependent on long-term self-help programs. The language of, symptoms of, and treatment for codependence derive from the medical model suggesting a disease process underlies the behavior. However, there is no evidence that codependence is caused by a disease process.

In their book, “Attached.”, Dr. Amir Levine and Rachel S. F. Heller, address what they call the “codependency myth” by asserting that attachment theory is a more scientific and helpful model for understanding and dealing with attachment in adults.

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.

Internet addiction disorder, more commonly called problematic Internet use (PIU), refers to excessive Internet use that interferes with daily life.

Individual differences in women's estrogen cycles may be related to the expression of BPD symptoms in female patients. A 2003 study found that women's BPD symptoms were predicted by changes in estrogen levels throughout their menstrual cycles, an effect that remained significant when the results were controlled for a general increase in negative affect.

Another growing area is social media addiction. Psychology researchers surveyed 253 undergraduate students at the University of Albany and found that not only is social media (particularly Facebook) itself potentially addictive, those who use it may also be at greater risk for substance abuse.

Socioeconomic status has also been looked at as a potential cause for personality disorders. There is a strong association with low parental/neighborhood socioeconomic status and personality disorder symptoms. In a recent study comparing parental socioeconomic status and a child's personality, it was seen that children who were from higher socioeconomic backgrounds were more altuistic, less risk seeking, and had overall higher IQs. These traits correlate with a low risk of developing personality disorders later on in life. In a study looking at female children who were detained for disciplinary actions found that psychological problems were most negatively associated with socioeconomic problems. Furthermore, social disorganization was found to be inversely correlated with personality disorder symptoms.

Elements of the family and social environment may also play a role in the development and maintenance of conduct disorder. For instance, antisocial behavior suggestive of conduct disorder is associated with single parent status, parental divorce, large family size, and young age of mothers. However, these factors are difficult to tease apart from other demographic variables that are known to be linked with conduct disorder, including poverty and low socioeconomic status. Family functioning and parent-child interactions also play a substantial role in childhood aggression and conduct disorder, with low levels of parental involvement, inadequate supervision, and unpredictable discipline practices reinforcing youth's defiant behaviors.

Peer influences have also been related to the development of antisocial behavior in youth, particularly peer rejection in childhood and association with deviant peers. Peer rejection is not only a marker of a number of externalizing disorders, but also a contributing factor for the continuity of the disorders over time. Hinshaw and Lee (2003) also explain that association with deviant peers has been thought to influence the development of conduct disorder in two ways: 1) a “selection” process whereby youth with aggressive characteristics choose deviant friends, and 2) a “facilitation” process whereby deviant peer networks bolster patterns of antisocial behavior. In a separate study by Bonin and colleagues, parenting programs were shown to positively affect child behavior and reduce costs to the public sector.

There is a strong correlation between child abuse, especially child sexual abuse, and development of BPD. Many individuals with BPD report a history of abuse and neglect as young children, but causation is still debated. Patients with BPD have been found to be significantly more likely to report having been verbally, emotionally, physically, or sexually abused by caregivers of either gender. They also report a high incidence of incest and loss of caregivers in early childhood. Individuals with BPD were also likely to report having caregivers of both sexes deny the validity of their thoughts and feelings. Caregivers were also reported to have failed to provide needed protection and to have neglected their child's physical care. Parents of both sexes were typically reported to have withdrawn from the child emotionally and to have treated the child inconsistently. Additionally, women with BPD who reported a previous history of neglect by a female caregiver and abuse by a male caregiver were significantly more likely to experience sexual abuse by a non-caregiver.

It has been suggested that children who experience chronic early maltreatment and attachment difficulties may go on to develop borderline personality disorder. Writing in the psychoanalytic tradition, Otto Kernberg argues that a child's failure to achieve the developmental task of and failure to overcome splitting might increase the risk of developing a borderline personality. A child's inability to tolerate delayed gratification at age four does not predict later development of BPD.

Child abuse and neglect consistently show up as risk factors to the development of personality disorders in adulthood. A study looked at retrospective reports of abuse of participants that had demonstrated psychopathology throughout their life and were later found to have past experience with abuse. In a study of 793 mothers and children, researchers asked mothers if they had screamed at their children, and told them that they did not love them or threatened to send them away. Children who had experienced such verbal abuse were three times as likely as other children (who did not experience such verbal abuse) to have borderline, narcissistic, obsessive-compulsive or paranoid personality disorders in adulthood. The sexually abused group demonstrated the most consistently elevated patterns of psychopathology. Officially verified physical abuse showed an extremely strong correlation with the development of antisocial and impulsive behavior. On the other hand, cases of abuse of the neglectful type that created childhood pathology were found to be subject to partial remission in adulthood.

The development of conduct disorder is not immutable or predetermined. A number of interactive risk and protective factors exist that can influence and change outcomes, and in most cases conduct disorder develops due to an interaction and gradual accumulation of risk factors. In addition to the risk factors identified under cause, several other variables place youth at increased risk for developing the disorder, including child physical abuse and prenatal alcohol abuse and maternal smoking during pregnancy. Protective factors have also been identified, and most notably include high IQ, being female, positive social orientations, good coping skills, and supportive family and community relationships.

However, a mere correlation between a particular risk factor and a later developmental outcome (such as conduct disorder) cannot be taken as definitive evidence for a causal link. Co-variation between two variables can arise, for instance, if they represent age-specific expressions of similar underlying genetic factors. For example, the tendency to smoke during pregnancy (SDP) is subject to substantial genetic influence (D'Onofrio et al., 2007), as is conduct disorder. Thus, the genes that dispose the mother to SDP may also dispose the child to CD following mitotic transmission. Indeed, Rice et al. (2009) found that in mother-fetus pairs that were not genetically related (by virtue of in-vitro fertilisation), no link between SDP and later conduct problems arose. Thus, the distinction between causality and correlation is an important consideration.

Over the past decade, the concept of Internet addiction has grown in terms of acceptance as a legitimate clinical disorder often requiring treatment. Researchers are divided over whether Internet addiction is a disorder on its own or a symptom of another underlying disorder. There is also debate over whether it should be classified as an impulse-control disorder or an obsessive-compulsive disorder rather than an addiction.

While the existence of Internet addiction is debated, self-proclaimed sufferers are resorting to the courts for redress. In one American case ("Pacenza v. IBM Corp."), the plaintiff argued he was illegally dismissed from his employment in violation of the Americans with Disabilities Act because of Internet addiction triggered by Vietnam War-related post-traumatic stress disorder (PTSD). The case was dismissed by the United States District Court for the Southern District of New York and affirmed on appeal to the United States Court of Appeals, Second Circuit in 2010 (case summarized in Glaser & Carroll, 2007).

About 25% of users fulfill Internet addiction criteria within the first six months of using the Internet. Many individuals initially report feeling intimidated by the computer but gradually feel a sense of "competency and exhilaration from mastering the technology and learning to navigate the applications quickly by visual stimulation" (Beard 374). The feeling of exhilaration can be explained by the way IAD sufferers often describe themselves as: bold, outgoing, open-minded, intellectually prideful, and assertive.

According to a study by Kathy Scherer, a psychologist from the University of Texas at Austin, "13% of college internet users fit the criteria for Internet addicts" (Scherer 1997). In her study, Scherer enlisted the help of 531 college students. She discovered that "72% of the Internet addicted students were men" (Scherer 1997).

The China Communist Youth League claimed in 2007 that over 17% of Chinese citizens between 13 and 17 were addicted to the Internet.

Public concern, interest in, and the study of, Internet over use can be attributed to the fact that it has become increasingly difficult to distinguish between the online and offline worlds. The Internet has tremendous potential to affect the emotions of humans and in turn, alter our self-perception and anxiety levels.

According to Maressa Orzack, director of the Computer Addiction Study at Harvard University's McLean Hospital, between 5% and 10% of Web surfers suffer some form of Web dependency.

According to the Center for Internet Addiction Recovery (whose director is Kimberly S. Young, a researcher who has lobbied for the recognition of net abuse as a distinct clinical disorder), "Internet addicts suffer from emotional problems such as depression and anxiety-related disorders and often use the fantasy world of the Internet to psychologically escape unpleasant feelings or stressful situations." More than half are also addicted to alcohol, drugs, tobacco, or sex.

Mark Griffiths states that "[t]he way of determining whether nonchemical (i.e., behavioral) addictions are addictive in a nonmetaphorical sense is to compare them against clinical criteria for other established drug-ingested addictions", and although his data is dated, and may no longer represent average Internet use accurately, Griffiths comes to the conclusion that the Internet does meet that criteria for addiction in a small number of users.

Scientists have found that compulsive Internet use can produce morphological changes in the structure of the brain. A study which analyzed Chinese college students who had been classified as computer addicts by the study designers and who used a computer around 10 hours a day, 6 days a week, found reductions in the sizes of the dorsolateral prefrontal cortex, rostral anterior cingulate cortex, supplementary motor area and parts of the cerebellum compared to students deemed "not addicted" by the designers. It has been theorized that these changes reflect learning-type cognitive optimizations for using computers more efficiently, but also impaired short-term memory and decision-making abilities—including ones in which may contribute to the desire to stay online instead of be in the real world.

Patricia Wallace PhD, Senior Director, Information Technology and CTY Online, at the Johns Hopkins University Center for Talented Youth argues that based on the case histories that have surfaced, no one denies that excessive involvement with certain psychological spaces on the net can have serious effects on a person's life. She explains that, at a large university in New York, the dropout rate among freshmen newcomers rose dramatically as their investment in computers and Internet access increased, and the administrators learned that 43% of the dropouts were staying up all night on the Internet.

Dependent personality disorder occurs in about 0.6% of the general population. The disorder is diagnosed more often in females than males; however, research suggests that this is largely due to behavioural differences in interviews and self-reporting rather than a difference in prevalence between the sexes. A 2004 twin study suggests a heritability of 0.81 for developing dependent personality disorder. Because of this, there is significant evidence that this disorder runs in families. Children and adolescents with a history of anxiety disorders and physical illnesses are more susceptible to acquiring this disorder.

Behavioral addiction is a form of addiction that involves a compulsion to engage in a rewarding non-drug-related behavior – sometimes called a natural reward – despite any negative consequences to the person's physical, mental, social or financial well-being. A gene transcription factor known as ΔFosB has been identified as a necessary common factor involved in both behavioral and drug addictions, which are associated with the same set of neural adaptations in the reward system.

Another theory is that there may be shared risk factors that can lead to both substance abuse and mental illness. Mueser hypothesizes that these may include factors such as social isolation, poverty, lack of structured daily activity, lack of adult role responsibility, living in areas with high drug availability, and association with people who already misuse drugs.

Other evidence suggests that traumatic life events, such as sexual abuse, are associated with the development of psychiatric problems and substance abuse.

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.

An addictive behavior is a behavior, or a stimulus related to a behavior (e.g., sex or food), that is both rewarding and reinforcing, and is associated with the development of an addiction. Addictions involving addictive behaviors are normally referred to as behavioral addictions.

Codependency is a type of dysfunctional helping relationship where one person supports or enables another person's drug addiction, alcoholism, gambling addiction, poor mental health, immaturity, irresponsibility, or under-achievement. Among the core characteristics of codependency, the most common theme is an excessive reliance on other people for approval and a sense of identity.

Given its grassroots origin, the precise definition of codependency varies based on the source but can be generally characterized as a subclinical and situational or episodic behavior similar to that of dependent personality disorder. In its broadest definition, a codependent is someone who cannot function from their innate self and whose thinking and behavior is instead organized around another person, or even a process, or substance. In this context, people who are addicted to a substance, like drugs, or a process, like gambling or sex, can also be considered codependent. In its most narrow definition, it requires one person to be physically or psychologically addicted, such as to heroin, and the second person to be psychologically dependent on that behavior. Some users of the codependency concept use the word as an alternative to using the concept of dysfunctional families, without statements that classify it as a disease.

As demonstrated by the chart below, numerous studies have examined factors which mediate substance abuse or dependence. In these examples, the predictor variables lead to the mediator which in turn leads to the outcome, which is always substance abuse or dependence. For example, research has found that being raised in a single-parent home can lead to increased exposure to stress and that increased exposure to stress, not being raised in a single-parent home, leads to substance abuse or dependence. The following are some, but by no means all, of the possible mediators of substance abuse.

As demonstrated by the chart below, numerous studies have examined factors which moderate substance abuse or dependence. In these examples, the moderator variable impacts the level to which the strength of the relationship varies between a given predictor variable and the outcome of substance abuse or dependence. For example, there is a significant relationship between psychobehavioral risk factors, such as tolerance of deviance, rebelliousness, achievement, perceived drug risk, familism, family church attendance and other factors, and substance abuse and dependence. That relationship is moderated by familism which means that the strength of the relationship is increased or decreased based on the level of familism present in a given individual.

Examples of mediators and moderators can be found in several empirical studies. For example, Pilgrim et al.’s hypothesized mediation model posited that school success and time spent with friends mediated the relationship between parental involvement and risk-taking behavior with substance use (2006). More specifically, the relationship between parental involvement and risk-taking behavior is explained via the interaction with third variables, school success and time spent with friends. In this example, increased parental involvement led to increased school success and decreased time with friends, both of which were associated with decreased drug use. Another example of mediation involved risk-taking behaviors. As risk-taking behaviors increased, school success decreased and time with friends increased, both of which were associated with increased drug use.

A second example of a mediating variable is depression. In a study by Lo and Cheng (2007), depression was found to mediate the relationship between childhood maltreatment and subsequent substance abuse in adulthood. In other words, childhood physical abuse is associated with increased depression, which in turn, in associated with increased drug and alcohol use in young adulthood. More specifically, depression helps to explain how childhood abuse is related to subsequent substance abuse in young adulthood.

A third example of a mediating variable is an increase of externalizing symptoms. King and Chassin (2008) conducted research examining the relationship between stressful life events and drug dependence in young adulthood. Their findings identified problematic externalizing behavior on subsequent substance dependency. In other words, stressful life events are associated with externalizing symptoms, such as aggression or hostility, which can lead to peer alienation or acceptance by socially deviant peers, which could lead to increased drug use. The relationship between stressful life events and subsequent drug dependence however exists via the presence of the mediation effects of externalizing behaviors.

An example of a moderating variable is level of cognitive distortion. An individual with high levels of cognitive distortion might react adversely to potentially innocuous events, and may have increased difficulty reacting to them in an adaptive manner (Shoal & Giancola, 2005). In their study, Shoal and Giancola investigated the moderating effects of cognitive distortion on adolescent substance use. Individuals with low levels of cognitive distortion may be more apt to choose more adaptive methods of coping with social problems, thereby potentially reducing the risk of drug use. Individuals with high levels of cognitive distortions, because of their increased misperceptions and misattributions, are at increased risk for social difficulties. Individuals may be more likely to react aggressively or inappropriately, potentially alienating themselves from their peers, thereby putting them at greater risk for delinquent behaviors, including substance use and abuse. In this study, social problems are a significant risk factor for drug use when moderated by high levels of cognitive distortions.

Comorbidity of addictive disorders and other psychiatric disorders, i.e., dual disorders, is very common and a large body of literature has accumulated demonstrating that mental disorders are strongly associated with substance use disorders. The 2011 USA National Survey on Drug Use and Health found that 17.5% of adults with a mental illness had a co-occurring substance use disorder; this works out to 7.98 million people. Estimates of co-occurring disorders in Canada are even higher, with an estimated 40-60% of adults with a severe and persistent mental illness experiencing a substance use disorder in their lifetime.

A study by Kessler et al. in the United States attempting to assess the prevalence of dual diagnosis found that 47% of clients with schizophrenia had a substance misuse disorder at some time in their life, and the chances of developing a substance misuse disorder was significantly higher among patients suffering from a psychotic illness than in those without a psychotic illness.

Another study looked at the extent of substance misuse in a group of 187 chronically mentally ill patients living in the community. According to the clinician's ratings, around a third of the sample used alcohol, street drugs, or both during the six months before evaluation.

Further UK studies have shown slightly more moderate rates of substance misuse among mentally ill individuals. One study found that individuals suffering from schizophrenia showed just a 7% prevalence of problematic drug use in the year prior to being interviewed and 21% reported problematic use some time before that.

Wright and colleagues identified individuals with psychotic illnesses who had been in contact with services in the London borough of Croydon over the previous 6 months. Cases of alcohol or substance misuse and dependence were identified through standardized interviews with clients and keyworkers. Results showed that prevalence rates of dual diagnosis were 33% for the use of any substance, 20% for alcohol misuse only and 5% for drug misuse only. A lifetime history of any illicit drug use was observed in 35% of the sample.