Borderline personality disorder may be characterized by the following signs and symptoms:

- Markedly disturbed sense of identity

- Frantic efforts to avoid real or imagined abandonment and extreme reactions to such

- Splitting ("black-and-white" thinking)

- Impulsivity and impulsive or dangerous behaviours

- Intense or uncontrollable emotional reactions that often seem disproportionate to the event or situation

- Unstable and chaotic interpersonal relationships

- Self-damaging behavior

- Distorted self-image

- Dissociation

- Frequently accompanied by depression, anxiety, anger, substance abuse, or rage

The most distinguishing symptoms of BPD are marked sensitivity to rejection or criticism, and intense fear of possible abandonment. Overall, the features of BPD include unusually intense sensitivity in relationships with others, difficulty regulating emotions, and impulsivity. Other symptoms may include feeling unsure of one's personal identity, morals, and values; having paranoid thoughts when feeling stressed; dissociation and depersonalization; and, in moderate to severe cases, stress-induced breaks with reality or psychotic episodes.

People with HPD are usually high-functioning, both socially and professionally. They usually have good social skills, despite tending to use them to manipulate others into making them the center of attention. HPD may also affect a person's social and/or romantic relationships, as well as their ability to cope with losses or failures. They may seek treatment for clinical depression when romantic (or other close personal) relationships end.

Individuals with HPD often fail to see their own personal situation realistically, instead dramatizing and exaggerating their difficulties. They may go through frequent job changes, as they become easily bored and may prefer withdrawing from frustration (instead of facing it). Because they tend to crave novelty and excitement, they may place themselves in risky situations. All of these factors may lead to greater risk of developing clinical depression.

Additional characteristics may include:

- Exhibitionist behavior

- Constant seeking of reassurance or approval

- Excessive sensitivity to criticism or disapproval

- Pride of own personality and unwillingness to change, viewing any change as a threat

- Inappropriately seductive appearance or behavior of a sexual nature

- Using somatic symptoms (of physical illness) to garner attention

- A need to be the center of attention

- Low tolerance for frustration or delayed gratification

- Rapidly shifting emotional states that may appear superficial or exaggerated to others

- Tendency to believe that relationships are more intimate than they actually are

- Making rash decisions

- Blaming personal failures or disappointments on others

- Being easily influenced by others, especially those who treat them approvingly

- Being overly dramatic and emotional

Some people with histrionic traits or personality disorder change their seduction technique into a more maternal or paternal style as they age.

Premenstrual dysphoric disorder (PMDD) occurs in 3–8 percent of women. Symptoms begin 5–11 days before menstruation and cease a few days after it begins. Symptoms may include marked mood swings, irritability, depressed mood, feeling hopeless or suicidal, a subjective sense of being overwhelmed or out of control, anxiety, binge eating, difficulty concentrating, and substantial impairment of interpersonal relationships. People with PMDD typically begin to experience symptoms in their early twenties, although many do not seek treatment until their early thirties.

Although some of the symptoms of PMDD and BPD are similar, they are different disorders. They are distinguishable by the timing and duration of symptoms, which are markedly different: the symptoms of PMDD occur only during the luteal phase of the menstrual cycle, whereas BPD symptoms occur persistently at all stages of the menstrual cycle. In addition, the symptoms of PMDD do not include impulsivity.

Avoidant individuals often choose jobs of isolation so that they do not have to interact with the public regularly, due to their anxiety and fear of embarrassing themselves in front of others. Some with this disorder may fantasize about idealized, accepting, and affectionate relationships, due to their desire to belong. Individuals with the disorder tend to describe themselves as uneasy, anxious, lonely, unwanted and isolated from others. They often feel themselves unworthy of the relationships they desire, so they shame themselves from ever attempting to begin them.

People with avoidant personality disorder are preoccupied with their own shortcomings and form relationships with others only if they believe they will not be rejected. Loss and social rejection are so painful that these individuals will choose to be alone rather than risk trying to connect with others (see rejection sensitivity). They often view themselves with contempt, while showing an increased inability to identify traits within themselves that are generally considered as positive within their societies.

- Hypersensitivity to rejection and criticism

- Self-imposed social isolation

- Extreme shyness or anxiety in social situations, though the person feels a strong desire for close relationships

- Avoids physical contact because it has been associated with an unpleasant or painful stimulus

- Feelings of inadequacy

- Drastically reduced or absent self-esteem

- Self-loathing, autophobia or self-harm

- Mistrust of others or oneself; exhibits heightened self-doubt

- Emotional distancing related to intimacy

- Highly self-conscious

- Self-critical about their problems relating to others

- Heightened attachment-related anxiety, which may include a fear of abandonment

- Problems in occupational functioning

- Lonely self-perception, although others may find the relationship with them meaningful

- Feeling inferior to others

- Substance abuse and/or dependence

- In some extreme cases, agoraphobia

- Uses fantasy as a form of escapism to interrupt painful thoughts

Antisocial personality disorder is defined by a pervasive and persistent disregard for morals, social norms, and the rights and feelings of others. Individuals with this personality disorder will typically have no compunction in exploiting others in harmful ways for their own gain or pleasure and frequently manipulate and deceive other people, achieving this through wit and a facade of superficial charm or through intimidation and violence. They may display , think lowly and negatively of others, and lack remorse for their harmful actions and have a callous attitude to those they have harmed. Irresponsibility is a core characteristic of this disorder: they can have significant difficulties in maintaining stable employment as well as fulfilling their social and financial obligations, and people with this disorder often lead exploitative, unlawful, or parasitic lifestyles.

Those with antisocial personality disorder are often impulsive and reckless, failing to consider or disregarding the consequences of their actions. They may repeatedly disregard and jeopardize their own safety and the safety of others and place themselves and others in danger. They are often aggressive and hostile and display a disregulated temper and can lash out violently with provocation or frustration. Individuals are prone to substance abuse and addiction, and the abuse of various psychoactive substances is common in this population. These behaviors lead such individuals into frequent conflict with the law, and many people with ASPD have extensive histories of antisocial behavior and criminal infractions stemming back before adulthood.

Serious problems with interpersonal relationships are often seen in those with the disorder. Attachments and emotional bonds are weak, and interpersonal relationships often revolve around the manipulation, exploitation, and abuse of others. While they generally have no problems in establishing relationships, they may have difficulties in sustaining and maintaining them. Relationships with family members and relatives are often strained due to their behavior and the frequent problems that these individuals may get into.

While antisocial personality disorder is a mental disorder diagnosed in adulthood, it has its precedent in childhood. The DSM-5's criteria for ASPD require that the individual have conduct problems evident by the age of 15. Persistent antisocial behavior as well as a lack of regard for others in childhood and adolescence is known as conduct disorder and is the precursor of ASPD. About 25-40% of youths with conduct disorder will be diagnosed with ASPD in adulthood.

Conduct disorder (CD) is a disorder diagnosed in childhood that parallels the characteristics found in ASPD and is characterized by a repetitive and persistent pattern of behavior in which the basic rights of others or major age-appropriate norms are violated. Children with the disorder often display impulsive and aggressive behavior, may be callous and deceitful, and may repeatedly engage in petty crime such as stealing or vandalism or get into fights with other children and adults. This behavior is typically persistent and may be difficult to deter with threat or punishment. Attention deficit hyperactivity disorder (ADHD) is common in this population, and children with the disorder may also engage in substance abuse." CD is differentiated from oppositional defiant disorder (ODD) in that children with ODD do not commit aggressive or antisocial acts against other people, animals, and property, though many children diagnosed with ODD are subsequently rediagnosed with CD.

Two developmental courses for CD have been identified based on the age at which the symptoms become present. The first is known as the "childhood-onset type" and occurs when conduct disorder symptoms are present before the age of 10 years. This course is often linked to a more persistent life course and more pervasive behaviors, and children in this group express greater levels of ADHD symptoms, neuropsychological deficits, more academic problems, increased family dysfunction, and higher likelihood of aggression and violence. The second is known as the "adolescent-onset type" and occurs when conduct disorder develops after the age of 10 years. Compared to the childhood-onset type, less impairment in various cognitive and emotional functions are present, and the adolescent-onset variety may remit by adulthood. In addition to this differentiation, the DSM-5 provides a specifier for a callous and unemotional interpersonal style, which reflects characteristics seen in psychopathy and are believed to be a childhood precursor to this disorder. Compared to the adolescent-onset subtype, the childhood onset subtype, especially if callous and unemotional traits are present, tends to have a worse treatment outcome.

A mnemonic that can be used to remember the characteristics of histrionic personality disorder is shortened as "PRAISE ME":

- Provocative (or seductive) behavior

- Relationships are considered more intimate than they actually are

- Attention-seeking

- Influenced easily by others or circumstances

- Speech (style) wants to impress; lacks detail

- Emotional lability; shallowness

- Make-up; physical appearance is used to draw attention to self

- Exaggerated emotions; theatrical

Personality disorders (PD) are a class of mental disorders characterized by enduring maladaptive patterns of behavior, cognition, and inner experience, exhibited across many contexts and deviating markedly from those accepted by the individual's culture. These patterns develop early, are inflexible, and are associated with significant distress or disability. The definitions may vary somewhat, according to source. Official criteria for diagnosing personality disorders are listed in the "Diagnostic and Statistical Manual of Mental Disorders" (DSM) and the of the "International Classification of Diseases" (ICD). The DSM-5 lists personality disorders in the same way as other mental disorders, rather than on a separate 'axis', as previously.

Personality, defined psychologically, is the set of enduring behavioral and mental traits that distinguish between individual humans. Hence, personality disorders are defined by experiences and behaviors that differ from societal norms and expectations. Those diagnosed with a personality disorder may experience difficulties in cognition, emotiveness, interpersonal functioning, or impulse control. In general, personality disorders are diagnosed in 40–60% of psychiatric patients, making them the most frequent of psychiatric diagnoses.

Personality disorders are characterized by an enduring collection of behavioral patterns often associated with considerable personal, social, and occupational disruption. Personality disorders are also inflexible and pervasive across many situations, largely due to the fact that such behavior may be ego-syntonic (i.e. the patterns are consistent with the ego integrity of the individual) and are therefore perceived to be appropriate by that individual. This behavior can result in maladaptive coping skills and may lead to personal problems that induce extreme anxiety, distress, or depression. These behaviour patterns are typically recognized in adolescence, the beginning of adulthood or sometimes even childhood and often have a pervasive negative impact on the quality of life.

Many issues occur with classifying a personality disorder. Because the theory and diagnosis of personality disorders occur within prevailing cultural expectations, their validity is contested by some experts on the basis of inevitable subjectivity. They argue that the theory and diagnosis of personality disorders are based strictly on social, or even sociopolitical and economic considerations.

In the American Psychiatric Association's DSM-5, schizotypal personality disorder is defined as a "pervasive pattern of social and interpersonal deficits marked by acute discomfort with, and reduced capacity for, close relationships as well as by cognitive or perceptual distortions and eccentricities of behavior, beginning by early adulthood and present in a variety of contexts."

At least five of the following symptoms must be present: ideas of reference, strange beliefs or magical thinking, abnormal perceptual experiences, strange thinking and speech, paranoia, inappropriate or constricted affect, strange behavior or appearance, lack of close friends, and excessive social anxiety that does not abate and stems from paranoia rather than negative judgments about self. These symptoms must not occur only during the course of a disorder with similar symptoms (such as schizophrenia or autism spectrum disorder).

Persons with narcissistic personality disorder (NPD) are characterized by their persistent grandiosity, excessive need for admiration, and a personal disdain for, and lack of empathy for other people. As such, the person with NPD usually displays the behaviors of arrogance, a sense of superiority, and actively seeks to establish abusive power and control over other people. Narcissistic personality disorder is a condition different from self-confidence (a strong sense of self); people with NPD typically value themselves over other persons to the extent that they openly disregard the feelings and wishes of others, and expect to be treated as superior, regardless of their actual status or achievements. Moreover, the person with narcissistic personality disorder usually exhibits a fragile ego (Self-concept), an inability to tolerate criticism, and a tendency to belittle others in order to validate their own superiority.

The DSM-5 indicates that persons with NPD usually display some or all of the following symptoms, typically without the commensurate qualities or accomplishments:

1. Grandiosity with expectations of superior treatment from other people

2. Fixated on fantasies of power, success, intelligence, attractiveness, etc.

3. Self-perception of being unique, superior, and associated with high-status people and institutions

4. Needing continual admiration from others

5. Sense of entitlement to special treatment and to obedience from others

6. Exploitative of others to achieve personal gain

7. Unwilling to empathize with the feelings, wishes, and needs of other people

8. Intensely envious of others, and the belief that others are equally envious of them

9. Pompous and arrogant demeanor

Narcissistic personality disorder usually develops in adolescence or during early adulthood. It is not uncommon for children and adolescents to display "some" traits similar to those of NPD, but such occurrences usually are transient, and do not meet the criteria for a diagnosis of NPD. True symptoms of NPD are pervasive, apparent in varied situations, and rigid, remaining consistent over time. The NPD symptoms must be sufficiently severe to the degree that significantly impairs the person's capabilities to develop meaningful human relationships. Generally, the symptoms of NPD also impair the person's psychological abilities to function, either at work, or school, or important social settings. The DSM-5 indicates that the traits manifested by the person must substantially differ from cultural norms, in order to qualify as symptoms of NPD.

Theodore Millon proposes two subtypes of schizotypal. Any individual with schizotypal personality disorder may exhibit either one of the following somewhat different subtypes "("Note that Millon believes it is rare for a personality with one pure variant, but rather a mixture of one major variant with one or more secondary variants):"

Both diagnostic systems provide a definition and six criteria for a general personality disorder. These criteria should be met by all personality disorder cases before a more specific diagnosis can be made.

The ICD-10 lists these general guideline criteria:

- Markedly disharmonious attitudes and behavior, generally involving several areas of functioning; e.g. affectivity, arousal, impulse control, ways of perceiving and thinking, and style of relating to others;

- The abnormal behavior pattern is enduring, of long standing, and not limited to episodes of mental illness;

- The abnormal behavior pattern is pervasive and clearly maladaptive to a broad range of personal and social situations;

- The above manifestations always appear during childhood or adolescence and continue into adulthood;

- The disorder leads to considerable personal distress but this may only become apparent late in its course;

- The disorder is usually, but not invariably, associated with significant problems in occupational and social performance.

The ICD adds: "For different cultures it may be necessary to develop specific sets of criteria with regard to social norms, rules and obligations."

In DSM-5, any personality disorder diagnosis must meet the following criteria:

- An enduring pattern of inner experience and behavior that deviates markedly from the expectations of the individual’s culture. This pattern is manifested in two (or more) of the following areas:

1. Cognition (i.e., ways of perceiving and interpreting self, other people, and events).

2. Affectivity (i.e., the range, intensity, lability, and appropriateness of emotional response).

3. Interpersonal functioning.

4. Impulse control.

- The enduring pattern is inflexible and pervasive across a broad range of personal and social situations.

- The enduring pattern leads to clinically significant distress or impairment in social, occupational, or other important areas of functioning.

- The pattern is stable and of long duration, and its onset can be traced back at least to adolescence or early adulthood.

- The enduring pattern is not better explained as a manifestation or consequence of another mental disorder.

- The enduring pattern is not attributable to the physiological effects of a substance (e.g., a drug of abuse, a medication) or another medical condition (e.g., head trauma).

People with NPD tend to exaggerate their skills and accomplishments as well as their level of intimacy with people they consider to be high-status. Their sense of superiority may cause them to monopolize conversations and to become impatient or disdainful when others talk about themselves. In the course of a conversation, they may purposefully or unknowingly disparage or devalue the other person by overemphasizing their own success. When they are aware that their statements have hurt someone else, they tend to react with contempt and to view it as a sign of weakness. When their own ego is wounded by a real or perceived criticism, their anger can be disproportionate to the situation, but typically, their actions and responses are deliberate and calculated. Despite occasional flare-ups of insecurity, their self-image is primarily stable (i.e., overinflated).

To the extent that people are pathologically narcissistic, they can be controlling, blaming, self-absorbed, intolerant of others' views, unaware of others' needs and the effects of their behavior on others, and insist that others see them as they wish to be seen. Narcissistic individuals use various strategies to protect the self at the expense of others. They tend to devalue, derogate, insult and blame others, and they often respond to threatening feedback with anger and hostility. Since the fragile ego of individuals with NPD is hypersensitive to perceived criticism or defeat, they are prone to feelings of shame, humiliation and worthlessness over minor or even imagined incidents. They usually mask these feelings from others with feigned humility or by isolating themselves socially, or they may react with outbursts of rage, defiance, or by seeking revenge. The merging of the "inflated self-concept" and the "actual self" is seen in the inherent grandiosity of narcissistic personality disorder. Also inherent in this process are the defense mechanisms of denial, idealization and devaluation.

According to the DSM-5, "Many highly successful individuals display personality traits that might be considered narcissistic. Only when these traits are inflexible, maladaptive, and persisting and cause significant functional impairment or subjective distress do they constitute narcissistic personality disorder." Due to the high-functionality associated with narcissism, some people may not view it as an impairment in their lives. Although overconfidence tends to make individuals with NPD ambitious, it does not necessarily lead to success and high achievement professionally. These individuals may be unwilling to compete or may refuse to take any risks in order to avoid appearing like a failure. In addition, their inability to tolerate setbacks, disagreements or criticism, along with lack of empathy, make it difficult for such individuals to work cooperatively with others or to maintain long-term professional relationships with superiors and colleagues.

Avoidant personality disorder (AvPD) is a Cluster C personality disorder. Those affected display a pattern of severe social anxiety, social inhibition, feelings of inadequacy and inferiority, extreme sensitivity to negative evaluation, and avoidance of social interaction despite a strong desire for intimacy. The behavior is usually noticed by early adulthood and occurs in most situations.

People with AvPD often consider themselves to be socially inept or personally unappealing and avoid social interaction for fear of being ridiculed, humiliated, rejected or disliked. They generally avoid becoming involved with others unless they are certain they will be liked. As the name suggests, the main coping mechanism of those with avoidant personality disorder is avoidance of feared stimuli. Both childhood emotional neglect (in particular, the rejection of a child by one or both parents) and peer group rejection are associated with an increased risk for its development; however, it is possible for AvPD to occur without any notable history of abuse or neglect.

While some scientists claim the exact causes for the disorder are unknown, others found that parents of avoidant children seemed to have difficulty with their own negative emotions. Some researchers have also theorized that certain cases of AvPD may occur when individuals with innately high sensory processing sensitivity (characterized by deeper processing of physical and emotional stimuli, alongside high levels of empathy) are raised in abusive, negligent or otherwise dysfunctional environments, which inhibits their ability to form secure bonds with others.

The likely course and outcome of mental disorders varies and is dependent on numerous factors related to the disorder itself, the individual as a whole, and the social environment. Some disorders are transient, while others may be more chronic in nature.

Even those disorders often considered the most serious and intractable have varied courses i.e. schizophrenia, psychotic disorders, and personality disorders. Long-term international studies of schizophrenia have found that over a half of individuals recover in terms of symptoms, and around a fifth to a third in terms of symptoms and functioning, with some requiring no medication. At the same time, many have serious difficulties and support needs for many years, although "late" recovery is still possible. The World Health Organization concluded that the long-term studies' findings converged with others in "relieving patients, carers and clinicians of the chronicity paradigm which dominated thinking throughout much of the 20th century."

Around half of people initially diagnosed with bipolar disorder achieve syndromal recovery (no longer meeting criteria for the diagnosis) within six weeks, and nearly all achieve it within two years, with nearly half regaining their prior occupational and residential status in that period. However, nearly half go on to experience a new episode of mania or major depression within the next two years. Functioning has been found to vary, being poor during periods of major depression or mania but otherwise fair to good, and possibly superior during periods of hypomania in Bipolar II.

Schizoid personality disorder (SPD) is a personality disorder characterized by a lack of interest in social relationships, a tendency towards a solitary or sheltered lifestyle, secretiveness, emotional coldness, detachment, and apathy. Affected individuals may be unable to form intimate attachments to others and simultaneously demonstrate a rich, elaborate, and exclusively internal fantasy world.

SPD is not the same as schizophrenia or schizotypal personality disorder, but there is some evidence of links and shared genetic risk between SPD, other cluster A personality disorders, and schizophrenia. Thus, SPD is considered to be a "schizophrenia-like personality disorder".

Critics argue that the definition of SPD is flawed due to cultural bias and that it does not constitute a mental disorder but simply an avoidant attachment style requiring more distant emotional proximity. If that is true, then many of the more problematic reactions these individuals show in social situations may be partly accounted for by the judgments commonly imposed on people with this style. However, it is important to note that impairment is mandatory for any behaviour to be diagnosed as a personality disorder. SPD seems to satisfy this criterion because it is linked to negative outcomes. These include a significantly compromised quality of life, smaller GAF scores even after 15 years and one of the lowest levels of "life success" of all personality disorders (measured as "status, wealth, and successful relationships").

Schizoid personality disorder is a poorly studied disorder, and there is little clinical data on SPD because it is rarely encountered in clinical settings. The effectiveness of psychotherapeutic and pharmacological treatments for the disorder have yet to be empirically and systematically investigated.

Personality disorder not otherwise specified (also known as personality disorder NOS or PDNOS) is a DSM-IV Axis II personality disorder.

The DSM-5 does not have an equivalent to Personality Disorder NOS. However Personality disorder-trait specified (PD-TS) remains under consideration for future revisions. The DSM 5 "Unspecified Disorder" is not a personality disorder, it is used to enhance specificity of an existing disorder or it is an emergency diagnosis unto itself (i.e. Unspecified Mental Disorder, 300.9), without being attached to another disorder.

The "Diagnostic and Statistical Manual of Mental Disorders" is a widely used manual for diagnosing mental disorders. DSM- 5 still includes schizoid personality disorder with the same criteria as in DSM-IV.

According to the DSM, those with SPD may often be unable to, or will rarely express aggressiveness or hostility, even when provoked directly. These individuals can seem vague or drifting about their goals and their lives may appear directionless. Others view them as indecisive in their actions, self-absorbed, absentminded and detached from their surroundings ("not with it" or "in a fog"). Excessive daydreaming is often present. In cases with severe defects in the capacity to form social relationships, dating and marriage may not be possible.

This diagnosis may be given when no other personality disorder defined in the DSM fits the patient's symptoms.

Four personality disorders were excluded from the main body of the latest version of the DSM (DSM-IV-TR) but this diagnosis may be used instead. The four excluded personality disorders are:

- Sadistic personality disorder

- Self-defeating personality disorder

- Depressive personality disorder

- Passive–aggressive personality disorder

It is a requirement of DSM-IV that a diagnosis of any personality disorder also satisfies a set of Diagnostic criteria.

Cluster B personality disorders are a categorization of personality disorders as defined in the DSM-IV and DSM-5.

Cluster B personality disorders are characterized by dramatic, overly emotional or unpredictable thinking or behavior and manipulative, exploitative interactions with others. They include antisocial personality disorder, borderline personality disorder, histrionic personality disorder and narcissistic personality disorder.

The British National Health Service has described those with this disorder as someone who, "struggles to relate to others. As a result, they show patterns of behaviour most would regard as dramatic, erratic and threatening or disturbing."

There are four recognized Cluster B personality disorders:

- Antisocial personality disorder (DSM-IV code 301.7): a pervasive disregard for the law and the rights of others.

- Borderline personality disorder (DSM-IV code 301.83): extreme "black and white" thinking, instability in relationships, self-image, identity and behavior often leading to self-harm and impulsivity.

- Histrionic personality disorder (DSM-IV code 301.50): pervasive attention-seeking behavior including inappropriately seductive behavior and shallow or exaggerated emotions.

- Narcissistic personality disorder (DSM-IV code 301.81): a pervasive pattern of grandiosity, need for admiration, and a lack of empathy.

Dependent personality disorder (DPD), formerly known as asthenic personality disorder, is a personality disorder that is characterized by a pervasive psychological dependence on other people. This personality disorder is a long-term condition in which people depend on others to meet their emotional and physical needs, with only a minority achieving normal levels of independence.

The cause of dependent personality disorder is unknown. A study in 2012 estimated that between 55% and 72% of the risk of the condition is inherited from one's parents. The difference between a "dependent personality" and a "dependent personality disorder" is somewhat subjective, which makes diagnosis sensitive to cultural influences such as gender role expectations.

The following conditions commonly coexist (comorbid) with dependent personality disorder:

- mood disorders

- anxiety disorders

- adjustment disorder

- borderline personality disorder

- avoidant personality disorder

- histrionic personality disorder

There are no specific criteria for "externalizing behavior" or "externalizing disorders." Thus, there is no clear classification of what constitutes an externalizing disorder in the DSM-5. Attention-deficit/hyperactivity disorder (ADHD), oppositional defiant disorder (ODD), conduct disorder (CD), antisocial personality disorder(ASPD), pyromania, kleptomania, intermittent explosive disorder (IED), and substance-related disorders are frequently referred to as externalizing disorders. Disruptive mood dysregulation disorder has also been posited as an externalizing disorder, but little research has examined and validated it to date given its recent addition to the DSM-5, and thus, it is not included further herein.

Externalizing disorders often involve emotion dysregulation problems and impulsivity that are manifested as antisocial behavior and aggression in opposition to authority, societal norms, and often violate the rights of others. Some examples of externalizing disorder symptoms include, often losing one's temper, excessive verbal aggression, physical aggression to people and animals, destruction of property, theft, and deliberate fire setting. As with all DSM-5 mental disorders, an individual must have functional impairment in at least one domain (e.g., academic, occupational, social relationships, or family functioning) in order to meet diagnostic criteria for an externalizing disorder. Moreover, an individual's symptoms should be atypical for their cultural and environmental context and physical medical conditions should be ruled out before an externalizing disorder diagnosis is considered. Diagnoses must be made by qualified mental health professionals. DSM-5 classifications of externalizing disorders are listed herein, however, can also be used to classify externalizing disorders. More specific criteria and examples of symptoms for various externalizing disorders can be found in the DSM-5.

The definition and classification of mental disorders are key issues for researchers as well as service providers and those who may be diagnosed. For a mental state to classify as a disorder, it generally needs to cause dysfunction. Most international clinical documents use the term mental "disorder", while "illness" is also common. It has been noted that using the term "mental" (i.e., of the mind) is not necessarily meant to imply separateness from brain or body.

According to DSM-IV, a mental disorder is a psychological syndrome or pattern which is associated with distress (e.g. via a painful symptom), disability (impairment in one or more important areas of functioning), increased risk of death, or causes a significant loss of autonomy; however it excludes normal responses such as grief from loss of a loved one, and also excludes deviant behavior for political, religious, or societal reasons not arising from a dysfunction in the individual.

DSM-IV precedes the definition with caveats, stating that, as in the case with many medical terms, "mental disorder" "lacks a consistent operational definition that covers all situations", noting that different levels of abstraction can be used for medical definitions, including pathology, symptomology, deviance from a normal range, or etiology, and that the same is true for mental disorders, so that sometimes one type of definition is appropriate, and sometimes another, depending on the situation.

In 2013, the American Psychiatric Association (APA) redefined mental disorders in the DSM-5 as "a syndrome characterized by clinically significant disturbance in an individual's cognition, emotion regulation, or behavior that reflects a dysfunction in the psychological, biological, or developmental processes underlying mental functioning.”