The anarchist wave of extremist violence spread from Europe to the United States in the later 1800s. Similarities to recent Islamist extremist violence are evident. Borne of extreme income inequality, anarchist ideology advocated class warfare against capitalism and government oppression through violent revolution, including bombing and assassination [1,2].

Head injury and brain dysfunction are thought to be highly prevalent among mass murderers, with 10% a conservative estimate and considerably higher than in the general population. Brain injury may interact with adverse psychosocial factors to increase individual predisposition, suggested in the histories of Richard Speck, who sustained a head injury falling from a tree; Andrew Kehoe, who was in a coma for two weeks from a severe head injury sustained in a fall in early adulthood; and Charles Whitman, with severe headaches, changes in personality, and violent, intrusive ideation possibly from a large brain tumor found at autopsy [9,13].

Some widespread misperceptions and erroneous beliefs discussed in this course include [9,23,24,25,26]:

The perpetrator "snapped."

In this case, the premise is that nobody who reflected on such an act would engage in behavior so horrifying. Unlike impulsive violence, which is the most prevalent type overall, mass shootings almost always reflect targeted, or instrumental, violence. This subtype of violence is planned and methodically prepared over time.

The perpetrator must have been Muslim.

The catastrophic attack on September 11, 2001, by violent Islamist extremists continues to shape public and law enforcement perception of Muslims as uniquely terrorism-prone. As discussed later in this course, Islamist extremist violence has become infrequent in the United States and other extremist subgroups present a higher level of threat.

The assailant must have been mentally ill.

In mass shootings that capture media attention, perpetrators are often depicted as schizophrenic, psychotic, or "psycho." Mental illness has long been used to explain why these rampages occur, in part because it rejects the idea that a sane person could do something so horrific. Mass violence is very rare by persons experiencing serious mental illness (as it is among those without mental illness).

The umbrella term "serious mental illness" refers to psychoses, schizophrenia (including paranoid type), bipolar disorder, and severe major depression. Active delusions and psychotic symptoms, such as command hallucinations, acutely elevate the risk of violent behavior, especially if substance abuse or cognitive impairments are present. Some persons with serious mental illness who are non-adherent to their medication have a higher risk of violence, either against others or self-directed (e.g., suicidal behavior) [31]. Overall, however, persons with serious mental illness and other psychiatric disorders are not more violent than individuals without psychiatric conditions. Importantly, mass shootings committed during episodes of serious mental illness are rare [9].

Mass shooters seldom use substances, probably to avoid impairing effects on planning, preparation, and maximizing the casualty rate. The exceptions were two cases in which therapeutic amounts of sedating drugs were ingested [37].

In contrast, other violence commonly involves substance use, especially alcohol. With intimate partner homicide, the victim, perpetrator, or both are often intoxicated [37]. Alcohol and drug use increase the risk of violent crime as much as seven-fold, even in persons without a history of mental illness [38]. This is especially concerning in states with laws that allow persons to bring loaded handguns into bars and nightclubs. A history of childhood abuse, binge drinking, and male sex are predictive factors for serious (but not mass) violence [19,39].

Limitations of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5-TR) have interfered with efforts to identify the psychopathology of mass shooters [40]. The DSM-5-TR uses a categorical diagnostic system, whereby personality and other psychiatric disorders are determined as present or absent, based on whether the number of diagnostic criteria meets the diagnostic threshold [41]. Dimensionality is a truer measure of personality pathology, because personality traits fall on a spectrum of trait dimensions that may be present in differing degrees. Destructive narcissistic or paranoid traits may be present in an individual, but when the number of symptom criteria are insufficient to meet DSM-5-TR diagnostic criteria, important dimensional aspects of the psychopathology are missed [24].

Extremes of character and temperamental traits do not fit easily into the categorical diagnostic system of the DSM-5-TR and can require more complicated formulations and assessments. The DSM has also contributed to checklist-style psychiatric examinations that may blur important diagnostic distinctions [41].

The DSM-5-TR classifies psychiatric disorders by symptom-based criteria and not by underlying cause. This modern DSM system increases diagnostic reliability, but some argue at the expense of validity. This is most relevant in pathologic personality traits; the dimensional aspects of mental structure and functioning and pathologic disturbances in cognition, ideation, fantasy, affect, psychologic defenses, object relating, moral functioning, and impulse control are better understood and evaluated using psychodynamic concepts [40].

Media and behavioral health specialists commonly (but usually erroneously) ascribe mass shootings and terrorist attacks to delusional, psychotic beliefs [42]. The DSM-5-TR classification of psychotic disorders invites interpretation of rigid but non-delusional beliefs as psychotic-spectrum conditions [41].

An extreme overvalued belief is a core concept in understanding ideologic violence and mass shootings. Extreme overvalued beliefs are rigidly held, non-delusional beliefs shared by one's subgroup. The belief is often defended, becoming more dominant, refined, and resistant to challenge over time. The individual develops an intense emotional commitment to the belief and may act violently in its service—justified by a sense of moral superiority [42].

As discussed, psychiatric disorders alone do not cause individuals to commit mass shootings. But psychiatric symptoms may exacerbate other problems, making it more difficult to deal with family, work or school problems, peer relationships, or personal crises [43]. Mass shooters may report their acts of violence were precipitated by anger over blocked goal achievement (e.g., being expelled from school or fired from work) or negative social interactions (e.g., peer bullying, rejection, humiliation) [43]. The disproportionality and perceived basis of their rage and vengeance is not adequately explained by psychologic conditions (e.g., depression, psychosis, antisocial personality) or social experiences (e.g., being bullied) [44,45].

Instead, this requires contribution from other conditions. With narcissism, psychopathy, or paranoia present, one's perspective and interpretation of the world readily distorts, which promotes irrational and exaggerated perceptions of one's victimization and persecution, ultimately leading to the targeting of those perceived to represent their persecutors [25,43]. The interaction of paranoid ideation and narcissistic pathology captures the psychopathology of mass shooters.

Narcissism is a dimensional personality trait that, in more pronounced cases, involves an inflated and grandiose regard of self, extremely low regard of others, and inability to experience empathy, concern, or compassion for others' suffering [51]. With a grandiose and unstable sense of self, hypersensitivity to ego threats results in retaliatory aggression and violence to perceived social rejection and insult [52]. In pathologic narcissism, destructive rage is an externalized defense reaction against intolerable feelings of shame or powerlessness and aversive self-awareness of defect [23].

ASD alone does not increase the risk for mass violence; this requires the presence of additional factors that interact with ASD features, such as deficits in social cognition and empathy, emotion-regulation deficits, and intense restricted interests [57,58]. History of childhood neglect or abuse correlate with later criminal behavior. Comorbid psychopathy with ASD is rare but potentially very serious and a significant violence risk and threat assessment issue. At first assessment, it may be difficult to distinguish between the two because lack of empathy is characteristic of both disorders, but the underlying reasons differ [54,57,59,60]. An increased intensity of preoccupations with disturbing or violent content is a possible warning sign [43].

Research suggesting ASD may be over-represented in mass shooters was investigated using the Mother Jones database of mass shootings (as noted, defined as at least four deaths in a single event). Evidence of ASD was evaluated in 75 cases, and 8% of perpetrators had a pre-event ASD diagnosis; this increased to 9% after adding Elliot Rodger, the Santa Barbara mass shooter [43]. An additional 21% of the cases had ASD traits or symptoms [43].

The idea of a "cultural script" has also been examined. A "cultural script" describes a schema, or a prescription, for behavior. Media and sociocultural factors have propagated a "script" of mass shootings that points to armed attack as a model for problem-solving—a "masculine" solution to lessen an inferior social position, especially for altering the shooter's reputation from a socially marginalized loser to a notorious antihero [27,50]. Media attention to mass murder may perversely glamorize the act in the eyes of subsequent perpetrators; the instant notoriety feeds narcissistic pathology [9,65].

The "Columbine effect" describes this cultural script of aggregated copycats; mass shootings are ritualized and self-referential, with perpetrators identifying with past shooters. This expands beyond the Columbine legacy, with mass shooters citing many others before them. The Internet has propagated this script by increasing the ease by which perpetrators can study and idolize previous mass killers [27].

Suicide reduction has long relied on suicide prediction using risk factors to place patient suicide potential as low, moderate, or high. However, this approach fails to consider the fluidity of proximal factors that drive acute suicide behaviors. Today, experts believe suicide cannot be predicted but can be prevented, and this paradigm shift has transformed suicide prevention efforts [76].

Predicting future violence is likewise difficult. Predictive methods of assessing violent antisocial behavior rely on risk assessment, whereby risk factors are measured and used to statistically predict future violence. To examine the value of risk assessment, 409 patients detained for violent criminal behavior were evaluated and followed 12 months after discharge to the community. Risk assessment had little value in predicting future violence and could not identify essential risk factors that should be targeted to prevent violence [77].

Thus, predictive methods fail to identify future violence in mass shooters because predictor risk variables (e.g., criminal history, psychiatric diagnosis, drug history) are static factors that are causally and temporally unrelated to violence [77]. Standard prediction and profiling methods cannot identify individuals posing a high, increasing, or imminent threat. Profiling is helpful in identifying perpetrators who have already acted violently, such as serial murderers, but is not useful with future mass shooters [66,78].

One approach, based on the concept that targeted violence is distinct from affective violence, is already showing promise in interrupting mass shooters before they act [69,78,79,80,81,82]. Targeted violence (also referred to as instrumental or predatory violence) is methodically planned against individuals, groups, or locations. Affective violence (also referred to as impulsive or emotional violence) is emotionally charged, impulsive, and reactive and typifies intimate partner violence (IPV). While affective violence is the most common subtype of violence, it does not accurately describe mass shootings. Mass shootings are considered an example of targeted violence, the endpoint of an understandable process of thinking and behavior that is neither spontaneous nor impulsive. Potential offenders on a pathway to targeted violence can be identified and prevented, but not usually predicted.

The first stage of the Pathway to Violence Model is a perceived injustice, threat, or loss of a highly personal significance. In this context, grievance refers to the cause of the offender's distress or resentment—a perception of having been wronged or treated unfairly or inappropriately. More than a feeling of anger, grievance can result in a desire, even a sense of mission, to right the wrong and achieve a measure of deserved justice. Grievance is more than a feeling of discontent or a short-lived, even explosive, expression of anger or frustration; it is a conclusion reached about the reason for the offender's suffering. A grievance is external to the offender, and by externalizing blame, the offender creates a target for retribution. The grievance becomes an organizing principle as the offender seeks to address the unjust treatment causing the anguish [87].

The grievance is exacerbated by a robust narcissism laced with an inflated sense of entitlement, privilege, or ability that, when perceived as unrecognized or insulted by others, results in an intolerable state, whereby the only compensatory relief to their sense of humiliation comes from rage and violent fantasy (i.e., ideation) [78]. However, few who are aggrieved progress to committing targeted violence.

Eventually, fantasy may escalate beyond ideation into action; research and planning bridges the gap between idea and action. During this stage, the offender takes concrete steps toward an attack and dedicates effort and energy toward the goal, which can include selecting and gathering information on the target, stalking the target, photographing targeted areas (e.g., classrooms, hallways, theaters), and charting out areas for explosive devices. Other behaviors can include Internet searches and conversing with like­minded others on social media or online [26,78].

Fixation is defined as an extreme preoccupation with another person, activity, or idea, often involving a grievance and a personal cause. With increasingly pathologic preoccupation comes social and occupational deterioration. Fixation is observed by increasing perseveration on persons or cause; increasingly strident opinion, negative characterization of the object of fixation, and angry emotional undertone; and impact on family or associates of the object of fixation, if present and aware [50].

Novel aggression is an act of violence that appears unrelated to any pathway behavior and is committed for the first time. This behavior tests the perpetrator's ability to become violent and can be thought of as experimental aggression [37].

Some perpetrators will make an unambiguously stated or written threat to either a target or to law enforcement expressing intent to commit violence. For decades, law enforcement academies taught that explicit threats were a precursor to violence [66]. This is valid in the context of a current or past sexual intimate; in these cases, directly communicated threats indicate heightened risk of violence against the target, referred to as the "intimacy effect." However, in targeted violence, this is disproven, and directly communicated threats are rare.

The validity of TRAP-18 was examined in 111 violent Islamist, right-wing, and single-issue extremist cases in 1990–2014 [81]. Researchers found that 70% demonstrated at least half of the 18 TRAP variables and more than 77% showed all four warning behaviors (i.e., pathway, fixation, identification, and leakage). Leakage (85%) was the most frequent proximal warning behavior. Less frequent proximal warning behaviors were directly communicated threat (22%), novel aggression (17%), and energy burst (8%). Few differences were observed among extremist ideology groups. The authors concluded the TRAP-18 appeared useful across the spectrum of ideologies that drive targeted violence [81].

Rejection by an extremist group the actor wants to join, due to either lifelong interpersonal problems or beliefs seen as too extreme by others in the group, is a distal characteristic of violent extremists. The rejection further isolates and may harden the belief system and violent intent. In one study, all 10 violent extremists (i.e., Timothy McVeigh, Joseph Franklin, John Salvi, Eric Rudolph, Buford Furrow, Ted Kaczynski, Benjamin Smith, Paul Hill, Michael Griffin, and Terry Nichols) attempted to affiliate with an extremist group, but their rejection led to further hardening of radical position and violent intent [66]. In the specific case of Paul Hill, he was a minister of a Presbyterian Church in Florida, but was excommunicated for his radicalization in the anti-abortion extremist movement. Three years after his excommunication, Hill shot and killed Paul Britton, MD, and his bodyguard James Barrett.

Over time, the thoughts of mass shooters and their expression become more strident, simplistic, and absolute. Prior to a violent attack, argument, persuasion, and critical thinking ceases, and dogmatic preaching and imposition of one's beliefs on others begins. Beliefs become more rigid, simplistic, and absolute; a "moral authority" is embraced. Violence is cloaked in self-righteousness and the pretense of superior belief.

The failure to form a sexually intimate relationship from puberty until the violent offense and death or incarceration is a common characteristic [81]. Incels (involuntary celibate men) are individuals who, having failed to find women they can talk or coerce into sex, radicalize their anger into calls of violence [97]. More than believing they are entitled to sex but unable to find a willing partner, their hatred of women stems from believing women are (or should feel) required to give them sex but purposefully withhold it. This distinction is crucial to understanding the disproportionality of rage against women [98]. Several mass shooters/murderers since 2015 have been identified as incels, including Elliot Rodger, Alek Minassian, Chris Harper-Mercer, and Scott Beierle.

Mass violence may be committed for personal or ideologic motive, but many former distinctions between the two have dissolved. The Warning Behaviors Model, initially applied to ideologic terrorism, was later found similarly reliable and valid with non-ideologic targeted mass violence, and mass shootings fueled by personal or ideologic motive often appear identical. The paths to targeted violence of both offender types largely overlap, and both originate from grievance and alienation. Extremist violence purported to advance an ideology is frequently grievance-driven violence cloaked in ideology.

Essential distinctions are extremist ideology versus behavior and movement from non-violence to violence [101,105]. "Violent ideology" and "violent extremist beliefs" are misnomers. Most individuals who harbor extreme beliefs/extremist ideologies do not commit violence to advance the belief or ideology [101]. Individual factors, not ideology, largely influence extremist violence (as will be discussed later in this course).

Terrorism is defined by the Central Intelligence Agency (CIA) and U.S. State Department as premeditated, politically motivated violence against noncombatant targets by non-state actors, usually intended to influence an audience. Counterterrorism experts consider this definition accurate, in contrast to the description used by other U.S. governmental agencies of "coercion through fear or intimidation" [109].

The religious wave of transnational Islamism emerged in the 1980s and can be divided into four sub-waves [113,117]. The initial sub-wave propagated beliefs of an international oppression of Muslims, which drew religiously inspired fighters to join the Mujahideen in the Afghanistan conflict against the Soviet Union. This sub-wave included Osama bin Laden and other original al-Qaeda members. The second sub-wave involved the Bosnia, Chechnya, and Kashmir conflicts and the 9/11 attacks. These violent Salafi extremists were generally middle class and educated; hardened criminals were nearly absent. The third sub-wave emerged in the wake of the Iraq War as "homegrown" rather than international extremists. The fourth sub-wave emerged in 2010–2014 with ISIS leaders and members substantially lower in education and higher in criminal histories than prior sub-waves, and with sole actors in the United States inspired by violent Salafi extremist leaders.

In each successive sub-wave, the "religiosity" of participants noticeably declined from the preceding sub-wave. Anti-terrorist experts described this pattern as an "extremist social trend," with individuals radicalized to violence by extremist interpretation of Islam replaced by what are best described as "Islamized radicals." In the fourth wave, 90% were motivated for personal reasons, including looking for a fight, adventure, or revenge against perceived rejection. Religion was not the primary driver of this movement [118,119]. Corroboration came from recent interviews of former al-Qaeda members, describing being attracted to terrorism motivated primarily by a pre-existing anger and alienation related to childhood abuse or trauma, lack of integration and assimilation, and/or socioeconomic grievances. Foreign policy grievances were described as a channel for releasing deeply held tensions, instead of a primary motive [113].

The alt-right movement is largely traced to 2012–2014, with the killing of Black teenager Trayvon Martin and the "Gamergate" harassment campaign that targeted female game developers and journalists for entering the male-dominated space. Using 4chan and other platforms to organize, the targets were "doxxed" (i.e., had their personal information published online) and systematically threatened with rape and death by anonymous abusers. Gamergate was formative in the development of the alt-right; young men from right-wing online spaces came together in a shared campaign against liberal "politically correct" culture [126,127]. Male supremacy was fundamental to the formation of the racist alt-right [97]. Alt-right, White supremacist, and male supremacist circles tightly overlap to reinforce shared narratives of dispossessed, oppressed White men, blamed on minorities, women, and immigrants [97]. Gamergate crystalized the "manosphere" of misogynist websites that encourage harassment of women and launched the incel movement.

The radicalization pathways and outcomes of far-right and Islamist extremists are markedly similar, the issues leading to a path highly overlap, and both should be regarded as similarly problematic [103,136]. The following case suggests how similar factors may influence radicalization to either extremism.

Some terrorist attacks are sensationalized and extensively covered, but most receive little to no media attention [140]. A terrorist attack receives less coverage when framed as a crime, while crime reports of incidents committed by Muslims are more likely to be labeled as terrorism [141]. Events are considered more newsworthy if they can be typified as reflecting current beliefs and social structures and can be scripted in ways that reinforce stereotypes. Media framing of terrorism as a specifically Muslim problem is the dominant narrative [142].

Media coverage increases when terrorist perpetrators are members of an out-group, or "others." Social identity research highlights in-group and out-group dynamics, whereby people perceived as "others" are portrayed and perceived more negatively. The biased portrayal of Muslims and Arabs as "others" in entertainment and news media may explain why people implicitly connect terrorism and Islam, Muslims as threats to national security, and an incident as "terrorism" when the perpetrator is Muslim [135,143,144,145]. The substantially greater media attention to extremist attacks by Muslims reinforces the cultural narrative of who should be feared. Framing this type of event as more prevalent helps explain why 36% of Americans are very or somewhat fearful that they or someone they know will be a victim of terrorism and implicitly link terrorism and Islam [145,146].

Research on aggression has focused on anger, but disgust transforms aggression into hostility and anger into hatred. Directed at a despised out-group, anger motivates action, contempt motivates devaluation, and disgust motivates dehumanization and elimination. Thus, the ANCODI emotions work in a sequence (or pathway) that starts with a perceived injustice and evolves to elimination [155].

As discussed, radicalization is a gradual process that intends to change the beliefs, feelings, and behaviors of individuals with the objective of aligning them against the core values of societies they inhabit and preparing them for intergroup conflict against an out-group that must be fought [117]. Social factors influence this process and the progression from extremist beliefs (non-violence) to extremist violence [151]. The radicalization process may be linear or nonlinear, but it starts with social or political grievances and perceived injustices, a subsequent identity crisis, and the search for significance, identity, or purpose that follows [105].

Pathways that combine loss of significance and other individual-level vulnerabilities with perceptions of community victimization are particularly important for explaining shifts from nonviolent to violent extremism. Personal vulnerabilities can fuel identity-seeking behaviors in individuals who then find direction and meaning in extremist narratives. Individual-level factors interact with social identity dynamics, and individuals are persuaded that their personal deficits largely result from their membership of a collectively victimized or threatened community.

In the third stage, an "enabler" is identified—someone providing inspiration for terrorism (nearly all are indirect). The most frequent enablers identified were:

Islamists

White Supremacists and Anti-Government Extremists

In the I-3 model of aggression, the presence of a weapon increases the proclivity for aggressive response to provocation [167]. This "weapons effect" was first described more than 50 years ago following observations that the mere presence of a weapon increased aggression, especially in angered individuals. In response to a specific situation, whether a person behaves aggressively is greatly influenced by how they interpret, or appraise, the situation [171].

Before the incident, 56% of mass shooters showed "red flag" warning signs for dangerous gun behaviors indicating they posed a danger to themselves or others, including [175,176]:

Fully grasping and appreciating the perspectives, beliefs, and values of gun culture members is vital for providers who are not part of the culture (Appendix: Understanding Gun Culture). Now considered culture blindness, this may lead to failures in engaging the patient, understanding their interests, and communicating useful information to them or their family [32,216,217]. Effective work with gun owners is considered a cross-cultural issue that requires the integration of gun violence evidence with the culture and interests of gun owners [32,218].

Patient-centered care, a guiding principle in many disciplines, requires cultural competence for patient populations diverse by ethnic heritage, religious beliefs, sexual orientation, and other factors. Cultural competence includes respect for cultural variation, awareness of diverse beliefs and practices, interest in learning about other cultures and skills that enhance cross-cultural communication, and acknowledgment that practicing cultural competence enhances the delivery of quality health care [218]. Healthcare providers should view gun ownership as linked to membership of a subculture, with cultural competence for gun safety counseling requiring the recognition of multiple gun owner subpopulations with differing perspectives and motivations [218].

Patients are more open to firearm safety counseling that is tailored to their context, focused on well-being and safety, and involves the family in discussions. The following section provides suggestions on how to approach gun discussions [211,213,218,220].

Individualize and provide health context for questions. Explain the context for asking about guns when routinely assessing gun safety, such as part of routine household hazard screening for parents of toddlers and risk behaviors for teens. With counseling, use different educational messages for parents of young children, family members of patients with cognitive impairment, and suicidality. Acknowledge local cultural norms.

Avoid accusatory questions. If a patient is struggling with suicidal thoughts, instead of asking "Do you have a gun?", consider "Some of my patients have guns at home, and some gun owners with suicidal thoughts choose to make their guns less accessible. Are you interested in talking about that?"

Start with open-ended questions. To avoid sounding judgmental, instead of starting with, "Is your gun safely secured?", ask "Do you have any concerns about the accessibility of your gun?"

Avoid being overly prescriptive. Meet patients where they are. When risk is present, instead of prescribing one specific solution, consider brainstorming. Removing the gun may be objectively optimal but when resisted by the patient, turn to making the gun less accessible by discussing various options (e.g., surrendering the gun, disposing of ammunition, storing the gun outside the home). This is consistent with the principles of shared decision-making.

Mental health interventions to prevent mass shootings are based on the supposition that psychiatric evaluations can predict and thus prevent mass shootings. Such proposals are the logical conclusion of ascribing blame to untreated serious mental illness [9,229]. However, most mass murderers do not have identifiable serious mental illness; most have maladaptive personality configurations. As such, gun access, not serious mental illness, determines most gun homicides [230].

The framing of mass violence as a serious mental illness problem persists, despite the statistically improbable odds of dying from gunshot by a stranger with psychotic illness [32]. The behavior and motives of mass shooters should be distinguished from psychiatric diagnoses [9].

Mass shooters are typified by long-standing, pervasive anger, persecution, violent revenge, and egotism—psychopathology for which the mental health field has no immediate, quick-acting "treatment." Mental health professionals can help troubled individuals willing to engage in psychotherapy, medication therapy, and/or substance abuse counseling, but the persecutory narcissistic pathology of mass shooters subverts such willingness, and they usually shun mental health treatment [9].