Discussions of mass shootings and extremist violence may give the impression of an American public in an era of unique and unprecedented threat, but these phenomena are not recent. The following overview describes the historical antecedent events and perpetrators of personal and ideologically motivated mass violence. Over the past 140 years, the patterns and themes are recurrent, while cultural framing of individuals as predisposed to mass violence has changed over time [253].

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].

Many reviews of mass shooting events in the United States cite the 1966 incident perpetrated by Charles Whitman at the University of Texas at Austin (UTA) as the first such offense. This is true of the modern era, but the first true incident occurred 63 years earlier, on August 13, 1903 [11].

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).

In mass shooters with psychiatric diagnoses, perpetration is motivated by long-standing, pervasive feelings of extreme anger, persecution, violent revenge, and severe narcissism, and not by formal symptoms of the psychiatric disorder [27]. These abnormalities reflect character pathology traits or symptoms of personality disorders, which differ from serious mental illness [26].

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.

Individuals with paranoia are hypersensitive to perceived slights. Obsessed with revenge, they justify the revenge as "payback" for a perceived injustice. They often react disproportionately to perceived slights, and their "mistreatment" by others may not have been extreme or unusual. Eric Harris (one of the Columbine killers) left a diary describing a hatred of his bullying, persecutory peers; this was unsubstantiated after extensive interviewing of Columbine students [47]. The final writings of Virginia Tech shooter Seung-Hui Cho portrayed other students (whom he barely knew) as having "raped my soul" and having "crucified" him [46].

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].

Narcissistic injury occurs when the pathologic narcissist perceives a threat to their self-esteem that reveals to others their hidden, "true" defective self [44]. Narcissistic injury can provoke narcissistic rage, an ego preservation response that serves to restore a sense of safety and power by destroying a threat, to satisfy the need for revenge, and to right a wrong by inflicting pain on another [13,53]. When present with paranoid traits, the interaction can produce a severe reaction with excessive retaliation and disproportionate transfer of pain to perceived persecutors believed to be the only resolution [23,24].

Malignant narcissism, a syndrome with core components of pathologic narcissism, antisocial features, paranoid traits, and unconstrained aggression, may also be present. Malignant narcissism is psychoanalytically described as a level of personality fracture or disorganization—a disturbance of object relations—whereby a profoundly fragile sense of self is compensated by antisocial grandiosity ("I am above the rules") and preoccupation with mistreatment and disrespect by others [52,53].

ASD is differentiated from other disorders that may present with social-interaction abnormalities and restricted interests. Unlike schizoid personality disorder, persons with ASD often have a desire to make friends or have intimate relationships, but profound social-skills deficits make them unable to appropriately engage, empathize with, or respond to others. Unlike schizotypal personality disorder, social-interaction impairments in ASD are rooted in empathic and perspective-taking deficits [57].

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].

Since the 1990s, mass murders have not just increased, but have arguably taken on a different quality, especially mass shootings. With an American culture that promotes an influential value system centered on celebrity and fame, narcissism has been described as the classic American pathology. An upswing in the narcissistic values of American culture since the 1990s has also been documented [61,62,63].

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].

Social media is an important contributing factor to this disturbing finding, given the appeal of fame, or rather infamy, without achievement other than successfully killing innocents [66]. Performative violence, a related concept, describes the construction of identity or position through a violent act that, by demanding audience attention and compelling the audience to look intently at the perpetrator and his or her act, fulfills perpetrator needs for recognition and acknowledgement of their existence and uniqueness [67].

Contagion and copycat effects are related but distinct. Contagion is imitation of the violent act, an effect active over days or weeks. Copycat is identification with the actor, an effect that may remain active for months or even years. Copycats can aggregate over time to become a cultural script [50]. School shooters are more likely than other types of mass murderers to commit copycat violence for achieving notoriety [28].

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].

It is important to note that most video game players do not commit violence, and most mass shooters have no documented history of violent media consumption. It has been suggested that player motivations, frustration, and the social context of play may influence the possible risks associated with violent video games [75].

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.

Many persons who harbor profound grievances and violent revenge fantasies do not progress to violence and withdraw into an omnipotent fantasy of violent retribution [24]. Others become so enthralled by violent ideation and psychologically consumed by the compensatory relief it affords their fractured ego that they lose the desire or ability to pursue nonviolent means of resolution [78].

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].

The offender assesses the level of security and barriers that must be defeated to gain close physical proximity to the target in the breach stage. Without normal access to a targeted facility, the offender may breach by conducting a "dry run" penetration test, intruding into a facility where he or she does not have legitimate access to identify security countermeasures. Breaching can also involve the smuggling of weapons into a classroom or theater, and then waiting to attack, or dressing as a security guard or package delivery person for a non-forcible entry [78,89].

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].

Mass shooters often have behavior demonstrating a warrior mentality or psychologic desire to be a pseudocommando. These individuals identify with military or law enforcement weapons, uniforms, or paraphernalia, or with previous attackers. They may self-proclaim as agents to advance a cause or belief system [79].

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].

Would-be offenders frequently express threats or intentions to others verbally or in writing, posting a manifesto or on online fora. In most school shootings, at least one person knew about the killers' intentions [28,40,92,93].

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].

While proximal warning behaviors are signs of growing or imminent threat of targeted violence, distal characteristics are long-term psychodynamic and psychosocial factors that may be necessary but not sufficient for targeted violence [66]. The most frequently identified distal characteristics in the TRAP-18 validation study were framed by an ideology (100%), changes in thinking and emotion (88%), failure of sex-pair bonding (84%), and personal grievance and moral outrage (78%) [81].

Moral outrage can develop via vicarious identification with a victimized group when the offender has not personally experienced the victimized suffering. An example of this type of thinking is evidenced by Timothy McVeigh, the Oklahoma City bomber. He displayed superior intelligence, hypervigilant narcissist characteristics, and "ultimate warrior" identification. He was humiliated by rejection from the Special Forces (i.e., the grievance). He was also abandoned by his mother and distrusted women, with a sexualized interest in weapons. McVeigh saw himself as the first hero of a second American revolution. His research, planning, and preparation began following moral outrage over the Branch Davidian compound assault by the FBI and the ATF.

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.

Radicalization is a process that intends to transform thinking, belief, and perception from socially normative to extremist, but this term frequently conflates extremism, radicalism, and terrorism. Radicalism describes intent to overthrow a status quo, not necessarily using illegal or violent means. Extremism refers to deviation from a norm. Radicalism and extremism are not societal threats unless connected to violence or inciting hatred; neither automatically leads to violence, and almost all of those with radical extreme ideas never act on them [101,102,104].

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].

Terrorism is not defined by lethality, and violence includes property destruction. For example, terrorist acts by far-left animal-rights and environmentalist extremists in the 1990s and 2000s targeted property and not people. Horrific mass violence is not terrorism when ideologic goals or motives are absent [111,112].

The evolution of terrorism in the United States began in the 1880s with the anarchist wave, which lasted roughly 40 years, followed in the 1920s by an anti-colonial wave, which lasted to the 1960s, then a new left wave, which in turn faded as the religious wave formed [113].

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].

Anti-abortion extremists not motivated by traditional far-right issues (e.g., anti-government, race superiority) are single-issue extremists. Between 1973 (when abortion was nationally legalized) and 2007, more than 200 abortion clinics were bombed or set on fire and more than 4,000 acts of violence were perpetrated (including homicide) or threatened against abortion providers or clinic workers [89,120].

The sprawling alt-right universe envelops neo-Nazis, White supremacists, male supremacists, misogynists, conspiracy theorists, techno-libertarians, White nationalists, anarcho-capitalists, and Dark Enlightenment adherents through a loosely affiliated aggregation of blogs, fora, podcasts, Twitter/Gab, and YouTube personalities united by a hatred of feminism, multiculturalism, and liberalism, and the belief that "political correctness" threatens individual liberty [97,125,127].

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.

Mass violence is followed by questions of whether the act was terrorism. Public perception of terrorist acts and actors has far-reaching consequences that influence governmental and mental health policy and how citizens treat each other. In essence, media reporting shapes this perception [139].

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].

Extremist violence, as with all forms of targeted violence, cannot be disrupted using prediction. Realizing terrorist acts are too difficult to predict, the focus turned to radicalization as a proxy for pre-empting terrorism, because radicalized individuals are substantially greater in number and easier to detect than individuals who commit extremist violence [101].

Anger, contempt, and disgust (ANCODI) are moral emotions associated with violations of ethics, morality, and divinity. Disgust is also an evolved defense to ward off contaminants and purge the environment of toxins [90]. A highly relevant body of research demonstrates how ANCODI emotions can combine to drive ideologically motivated intergroup violence [153,154,155].

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].

Anger, contempt, and disgust compressed together become dangerous in the processes of dehumanization and extremist violence across all languages and cultures. Monitoring communications for expression of ANCODI emotions directed at out-groups may provide an early-warning mechanism of impending violence [153,155]. The same is true of the language used by individuals encountered in a health or mental health setting.

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].

Regardless of where it falls on the political spectrum, the task of extremist ideology is to advance radicalization by identifying an entity to blame for the humiliation, justifying aggression against the entity on moral grounds, and indoctrinating the individual into simplistic thinking that sees the world in black and white. This narrative greatly appeals to those striving for significance [151].

The network makes a violence-justifying narrative cognitively accessible; their support of the narrative validates it and proves its soundness. The network may convince the individual that, under present circumstances, violence is an acceptable and legitimate means. Violence becomes perceived as less extreme and more normative, making it easier to deviate from broad societal norms without the burden of guilt [149,161].

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.

Analysis of historical data from PIRUS identified four correlates of extremist violence [164]:

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 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

Severe grievance is a common starting point among mass shooters and violent extremists. Both offender groups share pathologic narcissism, whereby sensitivity to shame and humiliation is activated by actual or perceived loss and public exposure of self as deficient. This, in turn, fuels the development of grievance against the humiliating entity. The path to violence diverges, but finally converges against a persecutory entity and past humiliation is undone through contempt, devaluation, and violence [90].

Efforts to counter extremist narratives and recruitment efforts should address perceptions of community victimization by challenging myths or misperceptions. Legitimate grievances should be acknowledged, with a focus on alternatives to address these grievances. Those countering violent extremism should be aware that cognitive biases make members less responsive to the disconfirming evidence central to counter-narratives.

Successful programs to counter violent extremism address underlying psychologic and emotional vulnerabilities that make individuals open to extremist narratives. These may result from traumatic experiences and losses, or personal and community marginalization. Programs that emphasize the acquisition of job-relevant skills may be effective for promoting sustained employment of at-risk individuals.

The I-3 Model, a general framework to understand aggression, identifies three factors that influence the likelihood and intensity of aggressive behavioral response: instigation, impellance, and inhibition [167].

Impellance encompasses the situational or dispositional qualities that influence how strongly the instigator fosters a proclivity to aggress. Factors that increase impellance strength include trait aggressiveness, Dark Tetrad personality traits (i.e., Machiavellianism, narcissism, psychopathy, and sadism), trait anger, hostile rumination, and presence of a weapon.

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].

Some are quick to link Islam or mental illness to the actions of mass shooters, but the strong association with domestic violence/IPV goes largely unaddressed [174]. Domestic violence mass murders comprise more than 50% of all mass murders. Everytown for Gun Safety (Everytown) is a non-profit organization involved in research, education, and policy related to gun violence prevention. Because domestic mass shootings are often excluded from public mass shooting databases, Everytown examined the prevalence of mass shootings (defined as at least four people killed with a firearm, shooter excluded) during 2009–2020 [175,176]. They found that in 240 mass shooting incidents, 1,363 people were killed and 947 were injured. Fifty-three percent of mass shootings were domestic violence-related, during which 632 intimate partners or other family members were killed and 106 were wounded. Sixty-one percent of incidents took place entirely in private residences, and another 9% occurred in the home and in public. During this period, 260 children and teens were killed with a firearm in a domestic mass shooting incident [175,176]. In total, 33% of shooters were not legally allowed to possess a firearm due to such restrictions as previous domestic violence charges and restraining/protective orders.

Of female victims of homicide, 90% are killed by a person they know, and half of these offenders are current or former intimate partners [180]. In contrast, a 2017 analysis places the annual risk of being killed by a stranger with severe psychoses at 1 in 14 million [32]. In mass shooting incidents, 32% ended when the perpetrators killed themselves [175]. Most suicides that follow homicide occur in the context of IPV; the perpetrators are motivated by dependency on, and/or desire to be reunited with, the victim(s) [181].

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]:

Gun safety counseling is a key component in preventing firearm injury and deaths, including IPV and mass shootings, but healthcare professionals have a longstanding reluctance in addressing gun risks in their patients. Efforts by the American College of Physicians, the American Academy of Pediatrics, the National Association of Social Workers, the APA, and many other health and mental health organizations are helping to overcome this resistance [211,212]. Asking patients about firearms, counseling them on safe firearm behaviors, and taking further steps with high-risk patients are some critically important actions to help prevent gun violence and accidents.

Many barriers exist. Perhaps the most important is an unfamiliarity with firearms themselves, with the benefits and risks of firearm ownership, and with what to say during firearm safety counseling and how to say it. Some may worry that asking questions that seem intrusive may invite discord or damage the patient relationship. They may feel uncomfortable asking about firearms, even when they are well-informed, or worry that patients will not be truthful. Some may believe that firearm counseling is outside their scope of practice or infringes on patients' Second Amendment rights [213].

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.

For patients with acute risk of gun violence and/or whose information or behaviors suggests suicidal or homicidal ideation or intent, immediately determine access to lethal means and promptly reduce access, with patient cooperation if possible (i.e., lethal means counseling). Temporarily relinquishing guns may be needed; use a gun violence restraining order or red flag law, family members, gun shops, or law enforcement (as allowed by state laws). Disclose to others who can reduce risk (e.g., family, law enforcement, psychiatric services). Hospitalize when necessary; bed availability should be long enough to significantly reduce suicide/homicide risk. Those with prescriptive authority should avoid prescribing disinhibiting medication, such as benzodiazepines [205,213,222].

Patient disclosures to mental health professionals are typically protected by federal and state laws covering doctor/patient privilege and by practitioners' ethics rules governing confidentiality. Duty to warn is the exception, summarized to mean that privacy and privilege end where danger to the public begins [26]. This includes potential mass violence.

Tarasoff law states that therapists, clinicians, and other mental health counselors have the duty to protect third parties from harm. As a result of this legislation, clinicians have a duty to protect the third party by warning the targeted victim or others who can then warn the intended victim, notifying law enforcement, and implementing other steps to protect the potential victim [228]. These laws are state-specific and the professions affected vary.

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].