The Putative Effect of Identity on Extremist Radicalization: A Systematic Review of Quantitative Studies

Figures & data

Figure 1. Flow chart of the study selection process.

Table 1. Overview of the main characteristics of the included articles and studies.

Authors	Study design	Identity			Extremism			Results								Significant covariates	Sample
Authors	Study design	Theoretical perspective	Concept	Operationalization (IV)	Ideology	Category	Operationalization (DV)	IV1	IV2	IV3	IV4	IV5	IV6	IV7	IV8	Significant covariates	Sample
Alcorta, Smits, Swedlund, and de Jong (2020)^45	Survey	Social & contextual	Ingroup identification Separated identity	Ethnicity or nationality Ethnicity versus nationality	None	Attitudes and beliefs Behavior	Support for political violence Self-reported use of political violence	+ +	0 0								2000 randomly selected residents of 27 African countries (Afrobarometer) 50% male Mean age 37 years
Bélanger et al. (2019)^46(1)	Survey	Social & contextual	Cultural identity	Alienation from (Canadian) culture	None	Attitudes and beliefs	Acceptance of violence for change of social system	+									470 Canadian Amazon Mechanical Turk recruits Mean age 32.7 years
Bélanger et al. (2019)^46(2)	Survey	Social & contextual	Cultural identity	Alienation from (Pakistani) culture Alienation from (Spanish) culture	None	Attitudes and beliefs	Acceptance of violence for change of social system	+	+								422 Pakistani (study 2a) and 233 Spaniards (study 2b) 60% and 29% male Mean age 20.7 and 34.2 years
Bélanger et al. (2019)^46(3)	Experiment	Social & contextual Needs & motives	Cultural identity Need to belong	Social alienation manipulation Alienation from (US-American) culture (as manipulation check)	None	Attitudes and beliefs	Acceptance of violence for change of social system	+(+)	+(+)							Moral justification (and interaction between both IVs)	319 American Amazon Mechanical Turk recruits 45% male Mean age 40.1 years
Berger (2016)^47	Survey	Social & contextual	Separated identity	Religion (Muslim) versus - Spanish nationality - German nationality - UK nationality - French nationality	Islamism	Attitudes and beliefs	Justification of suicide bombing Support for violent extremist group	0 +	0 0	0 0	0 0						1630 Muslims living in the UK, France, Germany, & Spain (Pew Research Center dataset, 2006)
Besta (2014)^48	Survey	Social & contextual	Identity fusion Ingroup identification	Fused with country Identified with country (Poland)	Nationalism	Intentions	Willingness to fight for in-group (country) Willingness to die for in-group (country)	+ +	0 +								367 Polish online recruits 66% male Mean age 33.4 years
Besta, Gómez, and Vázquez (2014)^49(1)	Survey	Social & contextual	Identity fusion Ingroup identification	Fused with country Identified with country (Poland)	Nationalism	Intentions	Willingness to fight for in-group (country)	+	+								365 university students in Poland 46% male Mean age 21.8 years
Besta, Gómez, and Vázquez (2014)^49(2)	Survey	Social & contextual	Identity fusion Ingroup identification	Fused with country Identified with country (Poland)	Religious	Intentions	Willingness to fight for in-group (religion)	+	+								138 university students and their family members in Poland 50% male Mean age 26.9 years
Besta, Szulc, and Ja’skiewicz (2015)^50	Survey	Social & contextual	Identity fusion Ingroup identification	Fused with country Identified with country (Poland)	None Left-wing Right-wing	Intentions Attitudes and beliefs	Willingness to fight for in-group (country) Acceptance of violent change of social system	+(+) (+)	+ 0							Left- and right-wing authoritarianism	24 football hooligans and 155 university students in Poland 92% and 52% male Mean age 22.9 and 19.6 years
Çelebi, Verkuyten, Köse, and Maliepaard (2014)^51	Survey	Social & contextual	Ingroup identification Common ingroup	Turkish identified with ethnicity Kurds identified with ethnicity Turkish identified with nationality Kurds identified with nationality	Ethnicity- based	Attitudes and beliefs	Justified inter-group violence (Turks/Kurds)	+	+	+	–						625 Turkish and Kurdish university students in Turkey 46% male Mean age 21.2 years
Cherney & Murphy (2019)^52	Survey	Social & contextual	Common ingroup Separated identity	Country of residence (Australia) Religion (Muslim) versus nationality (Australian)	Islamism	Attitudes and beliefs	Belief that terrorists have valid grievances	–	0							Belief in violent jihad	800 Muslims living in Australia Mean age 34.9
Coid, Bhui, MacManus, Kallis, Bebbington, and Ullrich (2016)^53	Survey	Social & contextual	Cultural identity	Country of residence (Britain; inclusion) Country of heritage, religion or political beliefs versus majority group (British people)	Anti-British/ -Western	Intentions	Fight against British army	0	+(+)							Opposition to war in Afghanistan	3679 British males (representative) 100% male Mean age 25.9
Doosje, Loseman, and Van den Bos (2013)^54	Survey	Social & contextual Needs & motives	Ingroup identification Uncertainty	Religion (Muslim) Experienced uncertainty	Islamism None	Attitudes and beliefs Intentions	Attitudes towards Muslim violence by others Own violent intentions (general)	(+) (+)	(+) (+)							In-group superiority, perceived illegitimacy of authorities, symbolic threat	131 Muslim adolescents and young adults in the Netherlands 39% male Mean age 17.3
Doosje, van den Bos, Loseman, Feddes, and Mann (2012)^55	Survey	Social & contextual Needs & motives	Ingroup identification Uncertainty	Nationality (Dutch) Experienced uncertainty	Right-wing None	Attitudes and beliefs Intentions	Attitude toward right-wing violence by others Own violent intentions	(+) (+)	0 0							In-group superiority	1086 adolescents and young adults in the Netherlands 56% male Mean age 16.64
Gøzsche-Astrup (2019)^56(1)	Survey	Social & contextual Needs & motives	Ingroup identification Uncertainty	Political affiliation (0 = Democrat versus 1 = Republican) Felt uncertain about themselves, their place in world, and their future	None	Intentions	Political violent activism intentions	-(+)	+(- + - + +)							Openness to experience, neuroticism, extraversion, gender, and education	2317 young adults in the US (representative), among which 38% Democrats, 24% Republicans, 37% Independents 53% male 18-30 years
Gøzsche-Astrup (2019)^56(2)	Survey	Social & contextual Needs & motives	Ingroup identification Uncertainty	Political affiliation (0 = Democrat versus 1 = Republican) Felt uncertain about themselves, their place in world, and their future	None	Attitudes and beliefs	Support for political violence	(-)	+(- -)							Neuroticism and openness to experience	2489 eligible voters in the US 51% male At least 18 years
Kunst, Dovidio, and Thomsen (2019)^57(1)	Survey	Social & contextual	Identity fusion Ingroup identification	Fused with political figure Fused with political party members (Republicans) Identified with political party members (Republicans)	Right-wing	Intentions	Active and violent support for law in banning Muslim cultural organizations/ willingness to support violent ethnic persecution	+	0	0							205 white Republican Amazon Mechanical Turk recruits living in the US 43% male Mean age 42.5 years
Kunst, Dovidio, and Thomsen (2019)^57(2)	Survey	Social & contextual	Identity fusion Ingroup identification	Fused with political figure Fused with political party members (Republicans) Identified with political party members (Republicans)	Right-wing	Intentions	Active and violent support for law in banning Muslim cultural organizations/ willingness to support violent ethnic persecution	+	0	0							385 white Republican Qualtrics panels recruits living in the US (representative) 50% male Mean age 49.3 years
Kunst, Dovidio, and Thomsen (2019)^57(3)	Experiment	Social & contextual	Identity fusion	Fused with political figure Fused with political party members (Republicans)	Right-wing	Intentions	Willingness to violently challenge democratic presidential election outcome	+(+)	0							Experi-mental deprivation mani-pulation	301 white American Amazon Mechanical Turk recruits living in the US 40% male Mean age 42.7 years
Kunst, Dovidio, and Thomsen (2019)^57(4)	Longi-tudinal survey	Social & contextual	Identity fusion	Fused with political figure Fused with political party members (Republicans)	Right-wing	Attitudes and beliefs	Support for violent ethnic persecution	+	0								194 Republican Amazon Mechanical Turk recruits living in the US 40% male Mean age 40.4 years
Kunst, Dovidio, and Thomsen (2019)^57(5)	Survey	Social & contextual	Identity fusion Ingroup identification	Fused with political figure Fused with political party members (Republicans) Identified with political figure Identified with political party members (Republicans)	Right-wing	Attitudes and beliefs	Support for violent ethnic persecution	+	0	0	0						176 Republican Amazon Mechanical Turk recruits living in the US 41% male Mean age 43.2 years
Kunst, Dovidio, and Thomsen (2019)^57(6)	Survey	Social & contextual	Identity fusion	Fused with political figure Fused with fellow political supporters	Right-wing	Attitudes and beliefs	Support for violent ethnic persecution	+	0								171 Republican Amazon Mechanical Turk recruits living in the US 50% male Mean age 43.2 years
Kunst, Dovidio, and Thomsen (2019)^57(7)	Survey	Social & contextual	Identity fusion	Fused with political figure Fused with fellow political supporters Fused with political party members (Republicans)	Right-wing	Intentions Attitudes and beliefs	Willingness to defeat US border from immigrant caravan Acceptance of mail bombing as a means to intimidate Trump’s opponents	+ 0	0 0	0 0							176 Republican Amazon Mechanical Turk recruits living in the US 47% male Mean age 45.1 years
Levin, Henry, Pratto, and Sidanius (2003)^58	Survey	Social & contextual	Ingroup identification	Ethnicity (Arab) Nationality (Lebanese)	Islamism	Attitudes and beliefs	Support for terrorist organizations Support for World Trade Center attack	+ +	0 0							Social dominance orientation	51 Muslim and 58 Christian Lebanese 50% male Mean age 20.6 years
Lobato, Moya, Moyano, and Trujillo (2003)^59	Survey	Social & contextual	Cultural identity	Western-Christian culture Arab-Muslim culture (both inclusion)	None	Intentions	Radicalism intentions (for in-group)	(+)	0							Perceived ingroup oppression	98 Muslims and 167 non-Muslims living in Spain 42% male Mean age 28.0 years
Lyons-Padilla, Gelfand, Mirahmadi, Farooq, and van Egmond (2015)^60	Survey	Social & contextual Needs & motives	Cultural identity Need for significance	Marginalization Separation Assimilation Integration Experienced loss of significance	Islamism	Attitudes and beliefs	Support for violent Jihad	(+)	+	0	(-)	+(+)				Experiences of significance loss and discrimination	198 first (92) and second (105) generation US-Muslims 39% male Mean age 27.4 years
Moreira, Rique Neto, Sabucedo, and Camino (2018)^61	Survey	Social & contextual Needs & motives	Ingroup identification Civic identity Need for significance	Political student group Being an activist Perception of life purpose (reversed)	Egalitarian	Intentions	Radicalism intentions (Brazil) Radicalism intentions (Spain)	(+) (+)	(+) (+)	(+) 0						Post-conventional moral reasoning and egalitarian views (versus conventional moral reasoning and conservative views)	251 Brazilian and 201 Spanish university students 26% male Mean age 21.3 years
Moskalenko & McCauley (2009)^62(1)	Survey	Social & contextual	Ingroup identification	Country Ethnicity Religion College Family	None	Intentions	Extremism intentions (for most important social group)	0	+	0	0	0					140 university students in the US 13% male Mean age 19.6 years
Moskalenko & McCauley (2009)^62(2)	Survey	Social & contextual	Ingroup identification	Country Ethnicity Religion College Family Political party	Nationalism	Intentions	Extremism intentions for country Extremism intentions for political party	0 0	0 0	0 0	0 0	0 0	+ +				146 university students in Ukraine 26% male Mean age 17.5 years
Obaidi, Bergh, Akrami, and Anjum (2019)^63(1)	Survey	Social & contextual	Ingroup identification	Religion (Muslims)	Islamism	Intentions	Violent behavioral intentions to support Islamism	+(+)								Being Western-born Muslim	59 Muslims from various community centers and mosques in Denmark 61% male 18-74 years
Obaidi et al. (2019)^63(2)	Survey	Social & contextual	Ingroup identification	Religion (Muslims)	Islamism	Intentions	Violent behavioral intentions to support Islamism	+(+)								Being Western-born Muslim	232 Muslims from 32 Islam-related Facebook sites in Denmark 42% male 16-71 years
Obaidi et al. (2019)^63(3)	Survey	Social & contextual	Ingroup identification	Religion (Muslims)	Islamism	Intentions	Violent behavioral intentions to support Islamism	+(+)								Being Western-born Muslim	259 Muslims from 32 Islam-related Facebook sites in Denmark 32% male 18-34 years
Obaidi et al. (2019)^63(4)	Survey	Social & contextual	Ingroup identification	Religion (Muslims)	Islamism	Intentions	Violent behavioral intentions to support Islamism	+(+)								Being Western-born Muslim	243 snow-ball sampled Afghan Danes, of which 48% were born in Denmark and 52% outside Denmark 53% male 18-65 years
Obaidi et al. (2019)^63(5)	Survey	Social & contextual	Ingroup identification	Religion (Muslims)	Islamism	Intentions	Violent behavioral intentions to support Islamism	+(+)								Being Western-born Muslim	104 Muslims from 15 Islam-related Facebook sites in Belgium 46% male 18-45 years
Obaidi et al. (2019)^63(6)	Survey	Social & contextual	Ingroup identification	Religion (Muslims)	Islamism	Intentions	Violent behavioral intentions to support Islamism	+(+)								Being Western-born Muslim	366 Pakistani Muslims living in one of nine Western countries, of which 49% were born in the West and 51% were born in Pakistan 51% male 15-45 years
Obaidi et al. (2019)^63(7)	Survey	Social & contextual	Ingroup identification	Ethnicity (Tamil)	Islamism	Intentions	Violent behavioral intentions to support Islamism	0									60 non-Muslim Sri Lankan Tamils living in Denmark, of which 53% were born in and 47% outside Denmark 55% male 18-45 years
Obaidi, Bergh, Sidanius, and Thomsen (2018)^64(1)	Survey	Social & contextual	Ingroup identification	Religion (Western-born Muslims) Religion (Muslims born in country targeted by West)	Islamism	Intentions	Violent behavioral intentions to support Islamism	0(+)	0							Group-based anger	491 Muslims recruited from 32 Islam-related Facebook sites in Denmark, of which 185 were born in Denmark, and 225 in conflict zones 33% male 16-74 years
Obaidi et al. (2018)^64(2)	Survey	Social & contextual	Ingroup identification	Religion (Western-born Muslims) Religion (Muslims born in conflict zone)	Islamism	Intentions	Violent behavioral intentions to support Islamism	+	+								243 snow-ball sampled Afghan Danes, of which 48% were born in and 52% outside Denmark 53% male 18-65 years
Obaidi, Kunst, Kteily, Thomsen, and Sidanius (2018)^65(1)	Survey	Social & contextual	Ingroup identification	Religion (Muslims)	Anti-Muslim extremism	Intentions	Willingness to participate in Muslim persecution	(+)								Symbolic and realistic threat	205 Non-Muslim Amazon Mechanical Turk recruits living in the US 55% male Mean age 34.8 years
Obaidi et al. (2018)^65(2)	Survey	Social & contextual	Ingroup identification	Religion (Muslims)	Anti-Western extremism	Attitudes and beliefs	Support for anti-Western violence	(+)								Symbolic threat	151 Muslims recruited from 10 Swedish Islam-related Facebook sites 43% male 18-34 years
Obaidi et al. (2018)^65(3)	Survey	Social & contextual	Ingroup identification	Religion (Muslims)	Islamism	Intentions	Violent behavioral intentions in defense of Muslims	+(+)								Symbolic threat	247 snow-ball sampled Turkish respondents 30% 18-30
Obaidi et al. (2018)^65(4)	Survey	Social & contextual	Ingroup identification	Religion (Muslims)	Islamism/ right-wing	Intentions	Violent behavioral intentions in defense of Muslims/ Westerners	(+)								Symbolic and realistic threat	142 Muslim Danes, 112 Non-Muslim Danes, 155% Muslims Afghans 49% male Mean age 26.1 years
Pauwels & Heylen (2020)^66	Survey	Social & contextual	Ingroup identification	Region (Flemish)	Nationalism	Behavior	Self-reported political violence	0(+)								Ethnocentrism	723 Flemish adolescents and young adults 36% male 18-25 years
Pfundmair, Aßmann, Kiver, Penzkofer, Scheuermeyer, Sust, and Schmidt (2022)^67(1)	Mixed- method (case files and survey)	Social & contextual Needs & motives	Ingroup identification Dual identity Need for self-esteem Need for significance Need to belong	(coded) two categories for need to belong: need for belonging (IV5) level of exclusion (IV6)	Islamism	Attitudes Intentions Behavior (combined)	Being radical versus non-radical (based on attitudes, intentions, and behaviors)	0	0	0	0	0	+				56 radical vs. 19 non-radical individuals currently under observation by a state office of criminal police (from 18 countries of origin) Germany 80% male Mean age 22 years
Pfundmair, Aßmann, Kiver, Penzkofer, Scheuermeyer, Sust, and Schmidt (2022)^67(2)	Mixed- method (news articles)	Social & contextual Needs & motives	Ingroup identification Dual identity Need for self-esteem Need for significance Need to belong	(coded) two categories for need to belong: need for belonging (IV5) level of exclusion (IV6)	Islamism	Behavior	Being an departer versus attacker	0	+	0	0	–	0				86 radicals (56 attackers versus 27 departers) reported about in newspapers for planning or committing a terrorist attack, returning from Syria Germany 98% male Mean age 21 years
Phalet, Baysu, and Verkuyten (2010)^68(1)	Experiment	Social & contextual	Ingroup identification	Religion (Islam) Ethnicity (Moroccan)	Nationalism Islamism Human rights	Intentions	Non-normative action intention (depending on the priming condition either in defense of Islam, in support of homeland/ethnic group, or in defense of human rights)	+(-)	0							Identity salience (through manipulation)	521 Moroccan-Dutch self-identified Muslims 58% male Mean age 32 years
Phalet, Baysu, and Verkuyten (2010)^68(2)	Experiment	Social & contextual	Ingroup identification	Religion (Islam) Ethnicity (Turkish)	Nationalism Islamism Human rights	Intentions	Non-normative action intention (depending on the priming condition either in defense of Islam, in support of homeland/ethnic group, or in defense of human rights)	0(+)	0(+)							Identity salience (through manipulation)	625 Turkish-Dutch self-identified Muslims 58% male Mean age 32 years
Rip, Vallerand, and Lafrenière (2012)^69(1)	Survey	Social & contextual	Identity fusion Civic identity	Fused with political cause Harmonious identification with political cause Obsessive identification with political cause (Quebec sovereignty)	Nationalism	Attitudes and beliefs	Support for radical and aggressive political activism for Quebec Sovereignty	+(+)	0	+						Obsessive identification	114 Quebec nationalists or Quebec Sovereignty activists in Canada 75% males Mean age 33 years
Rip, Vallerand, and Lafrenière (2012)^69(2)	Experiment	Social & contextual	Civic identity	Harmonious identification with religious cause, Obsessive identification with religious cause (Islam)	Islamism	Attitudes and beliefs	Support for religious extremism and violence	0	(+)							Experience of hatred and identity threat manipulation	111 devout Muslims recruited at moderate mosques and prayer rooms in Canada 75% males Mean age 35 years
Rousseau, Miconi, Frounfelker, Hassan, and Oulhote (2020)^70	Survey	Social & contextual Needs & motives	Identity fusion Need for (collective) self-esteem	Fused with social group Perception that ingroup is prestigious (reversed)	None	Attitudes and beliefs	Sympathy for violent radicalization	+	0								1556 university students in Canada 33% male 16-25 years
Rousseau, Oulhote, Lecompte, Mekki-Berrada, Hassan, and El Hage (2021)^71	Survey	Social & contextual Needs & motives	Ingroup identification Identity fusion Need for (collective) self-esteem	Nationality, religion or ethnic group Fused personal and social identity Experience self-esteem through ingroup (reversed) Belief that others perceive ingroup as prestigious (reversed) All three combined (reversed)	None	Attitudes and beliefs Intentions	Sympathy for violent radicalization Radicalism intention	0 0	+ +	0 0	+(+) +(+)	0 0				Perceived discrimination	1190 Quebec university students 30% male 16-25 years
Saguy, Sobol-Sarag, Halabi, Stroebe, Bruneau, and Hasan-Aslih (2020)^72	Survey	Social & contextual	Ingroup identification Common ingroup Civic identity	Ethnic ingroup minority (Arab-Druze Israelis) Nationality (Israel) Loss of shared identity (with Israel) Ethnic outgroup minority (Palestine; i.e. politicized identity)	Anti-Zionism	Intentions	Intentions to partake in violent forms of collective action in protest of the nationality bill	0	0	+(+)	+					Young age/recently completed service in Israeli army	154 Arab-Druze Israelis 29% male Mean age 28.7 years
Sheikh, Gómez, and Atran (2016)^73(1)	Survey	Social & contextual	Identity fusion	Group of friends	Islamism	Intentions Attitudes and beliefs	Willingness to make costly sacrifices for sharia Support for militant jihad	(+) (+)								Perception that sharia is sacred value	260 Moroccan divided over two age groups (18-25 and 35-50) approached in public places 50% male Mean age 22 and 42 years
Sheikh, Gómez, and Atran (2016)^73(2)	Experiment	Social & contextual	Identity fusion	Group of friends	Democracy	Intentions	Willingness to make costly sacrifices for democracy	(+)								Perception that democracy is sacred value and having received reminder of Sharia law (perceived threat)	644 online-recruited Spanish citizens 37% male Mean age 35 years
Simon & Grabow (2010)^74	Survey	Social & contextual	Common ingroup Ingroup identification Integrated/ dual identity Separated identity	Country of residence (Germany) Country of heritage (USSR) Both countries Only one country Religion	None	Attitudes and beliefs	Support for political violence	0	0	0	0	0					152 immigrants to Germany form the former Soviet Union 27% male Mean age 46 years
Simon & Ruhs (2008)^75	Longi- tudinal survey	Social & contextual	Common ingroup Ingroup identification Integrated/ dual identity Separated identity	Country of residence (Germany) Country of heritage (Turkey) Both countries Only one country Religion	Islamism	Attitudes and beliefs	Support for political violence (time 1) Support for political violence (time 2)	0 -	0 0	0 0	0 0	0 0					66 Snowball-sampled Turkish Germans (at time 2) 67% male Mean age 35 years
Simon, Reichert, and Grabow (2013)^76	Longi-tudinal survey	Social & contextual	Common ingroup Ingroup identification Integrated/ dual identity Separated identity	Country of residence (Germany) Country of heritage (Turkey and Russia) Both countries Only one country Religion	None	Attitudes and beliefs	Sympathy for radical action (Turkish) Sympathy for radical action (Russians)	0 0	0 (-)	(+) (+)	0 (+)	(-) 0				Identity income-patibility	180 Turkish and 161 Russian migrant university students in Germany 33% male Mean age 24 years
Skrobanek (2004)^77	Survey	Social & contextual	Ingroup identification Common ingroup	Region (Eastern-Germany) Country (Germany) Continent (Europe)	Right-wing extremism	Attitudes and beliefs Behavior	Acceptance of violence for right-wing political goals Self-reported violent behavior toward immigrants	0 0	+ (+)	- (-)						Aversion toward immigrants and general acceptance of violence	1457 adolescents from former Eastern-Germany 44% male Mean age 17.8 years
Swann, Gómez, Seyle, Morales, and Huici (2009) (1)^78	Survey	Social & contextual	Identity fusion Ingroup identification	Fused with nationality Identified with nationality (Spain)	Nationalism	Intentions	Perceived willingness to fight for the in-group (country) (self vs others) Perceived willingness to die for the in-group (country) (self vs others)	+ +	0 0								177 university students in Spain 43% male Mean age 33.0 years
Swann, Gómez, Seyle, Morales, and Huici (2009)^78(2)	Survey	Social & contextual	Identity fusion Ingroup identification	Fused with nationality Identified with nationality (Spain)	Nationalism	Intentions	Willingness to fight for the in-group (country) Willingness to die for the in-group (country)	+ +	0 0								151 university students in Spain 17% male Mean age 32.3 years
Swann, Gómez, Seyle, Morales, and Huici (2009)^78(3)	Experiment	Social & contextual	Identity fusion Ingroup identification	Fused with nationality Identified with nationality (Spain)	Nationalism	Intentions	Willingness to fight for the in-group (country) Willingness to die for the in-group (country)	+(+) +(+)	+ +							Identity externally challenged (through mani-pulation)	602 university students in Spain 14% male Mean age 31.2 years
Swann, Gómez, Seyle, Morales, and Huici (2009)^78(4)	Experiment	Social & contextual	Identity fusion Ingroup identification	Fused with nationality Identified with nationality (Spain)	Nationalism	Intentions	Willingness to fight for the in-group (country) Willingness to die for the in-group (country)	+(+) +(+)	0 0							Identity externally challenged (through mani-pulation)	326 university students in Spain 15% male Mean age 31.1 years
Swann, Gómez, Seyle, Morales, and Huici (2009)^78(5)	Experiment	Social & contextual	Identity fusion Ingroup identification	Fused with nationality Identified with nationality (Spain)	Nationalism	Intentions	Willingness to die for the in-group (country)	+(+)	0							Personal identity salience (through manipulation)	421 adolescents in Spain 12% male Mean age 15.8 years
Tausch, Spears, and Christ (2009)^79	Survey	Social & contextual	Ingroup identification Common ingroup	Religion (Muslim) Nationality (British)	Islamism	Attitudes and beliefs	Attitudes relating to the 7/7 London bombings	0	–								1000 British Muslims 50% male 18-65+ years
Van Bergen, Ersanilli, Pels, and De Ruyter (2016)^80	Survey	Social & contextual	Ingroup identification Common ingroup	Ethnicity (Turkish) Mainstream society (Netherlands)	None	Attitudes and beliefs Intentions	Attitude toward violent in-group defense by others Willingness to use violence in defense of the in-group	+ +	0 -							Parental ethnic socialization and ingroup superiority	133 Turkish Dutch adolescents 48% male Mean age 15.6 years
Van Bergen, Feddes, Doosje, and Pels (2015)^81	Survey	Social & contextual	Ingroup identification Common ingroup	Ethnicity (Moroccan) Mainstream society (Netherlands)	None	Attitudes and beliefs Intentions	Attitude toward violent in-group defense by others Willingness to use violence in defense of the in-group	(+) +(+)	(-) (-)							Ingroup superiority	166 Turkish and 232 Moroccan Dutch adolescents 47% male Mean age 15.8 years
Yustiasia, Shadiqi, Milla, and Muluk (2020)^82	Survey	Social & contextual	Civic identity	Activist groups (Jihadi)	Islamism	Attitudes and beliefs	Support for Islamist terrorism	+								Intergroup contact, perceived threat, perceived injustice, and group efficacy	66 terrorism detainees in Indonesia 100% male Mean age 34.8 years
Zimgrod, Rentfrow, and Robbins (2019)^83	Survey	Social & contextual	Identity fusion	Nationality (Turkish)	Nationalism	Intentions	Willingness to die for the in-group (country)	+								Cognitive flexibility	743 Amazon Mechanical Turk recruits living in the US 45% male Mean age 35.5 years
Isenhardt, Kamenowski, Manzoni, Haymoz, Jacot, and Baier (2021)^84	Survey	Personal & develop-mental	Identity diffusion	Experienced a diffused identity	Right-wing Left-wing Islamism	Attitudes and beliefs	Approval of right-wing extremism, including violence Approval of left-wing extremism, including violence Approval of Islamist extremism, including violence	+ + +								Parental inconsistency, parental monitoring (reversed), critical life events, academic performance (reversed)	8317 high school students from 123 schools in Switzerland 49,7% male 83,7% 16-19 years old
Milla, Hudiyana, and Arifin (2020)^85	Mixed-method /inter-ventional	Personal & develop-mental	Alternative (versus counterfinale) identities	Single Jihadist identity versus multiple alternative identities that trump Islamic identity	Islamism	Attitudes and beliefs	Support for jihad qital	–									89 terrorist detainees in Indonesia who participated in deradicalization programs, incl. 8 “leaders,” 58 “members,” and 20 “supporters” 99% male Mean age 34.9 years
Campelo et al. (2018)^86	Prospective obser-vational and inter-ventional study	Needs & motives	Motivational dimensions	Violence and megalomania Depression and abuse Responsibility and guilt Loneliness and poor insight Violence and uncertainty Issue with sexuality Loneliness and sensitivity	Islamism	Behavior	Degree of radicalization after arrest (varying from deradicalized to reached the IS or deceased)	+	0	0	0	0	0	0	0		150 terrorist suspects (67% Muslim converts) in France 33% male Mean age 19.8 years at time 1 and 21.8 years at time 2
Groppi (2017)^87	Survey	Needs & motives	Need to belong	Experienced uncertainty for wish to belong to Italian culture	Islamism	Attitudes and beliefs	Justification of violence in defense of Islam Perceived duty to punish offenders of Islam Support of Islamist terrorist organizations	0 0 +									440 Muslims from 15 Italian cities with Jihadist history and rel. big Muslim population 69% male Age range 16-60+ years
Macdougall, Van der Veen, Feddes, Nickolson, and Doosje (2018)^88	Experiment	Needs & motives	Uncertainty Need to belong Need for significance	Emotional uncertainty Need to belong trait Need for status motive Need for existential meaning motive	None	Attitudes and beliefs	Preference for violent organizations	0	-(-)	+(+)	0(+)					Combined riskiness factor (i.e. perceived individual & collective rel. deprivation, procedural justice, symbolic & realistic threat, participation in decision-making)	183 Dutch residents (representative) 53% male Mean age 27.2 years
Pfundmair and Wetherell (2019)^89(1)	Experiment	Needs & motives	Need to belong	Trait measure Being ostracized (versus socially included in manipulation) Interaction between both	None	Intentions	Willingness to fight and die for the in-group (that participants belong to, excl. close friends & family)	0	0	0							96 Amazon Mechanical Turk recruits living in the US 44% male Mean age 34.9 years
Pfundmair and Wetherell (2019)^89(2)	Experiment	Needs & motives	Need to belong	Trait measure Being ostracized (versus socially included in manipulation) Interaction	None	Intentions	Willingness to fight and die for the in-group (that participants belong to, excl. close friends & family)	0	0	(+)						Perceived moral correctness of ingroup	81 university students in Germany 30% male Mean age 23.6 years

Note. Statistically significant positive (i.e. increasing) associations are indicated with a “+”, statistically significant negative (i.e. decreasing) associations are indicated with a “-”. Effects in parentheses “()” indicate indirect or interaction effects with “significant covariates”.

Table 2. Overview of all identity concepts examined in the reviewed studies and the amount of statistically significant positive (i.e. increasing, “+”), statistically significant negative (i.e. decreasing, “-”), and statistically non-sigificant (“0”) relations each was reported to have with violent extremist attitudes and beliefs, intentions, and behaviors. Effects in parentheses “()” indicate indirect or interaction effects with significant covariates (see Table 1).

	Violent extremism
	Attitudes and beliefs			Intentions			Behaviors			Total
Identity concept	+	–	0	+	–	0	+	–	0	+	-	0
Social and contextual identity concepts (n = 68)
Ingroup identification (n = 33)	7(4)	(2)	15	11(5)	1	28	1	.	2	19(9)	1(2)	45
Identity fusion (n = 23)	5(3)	.	6	20(9)	.	5	.	.	.	25(12)	.	11
Civic identity (n = 5)	2(1)	.	2	1(2)	.		.	.	.	3(3)	.	2
Common-ingroup identification (n = 10)	1	5(1)	5	0	2(2)	1	.	(1)	.	1	7(3)	6
Managing multiple social identities (n = 9)
Dual identity	(2)	.	4	.	.	.	1	.	.	1(2)	.	4
Separated identity	(1)	.	10	.	.	.	.	.	.	(1)	.	10
Cultural identity (n = 6)
Alienation	4(1)	.	.	.	.	.	.	.	.	4(1)	.	.
Inclusion	.		.	(1)	.	2	.	.	.	(1)	.	2
Integration	.	(1)	.	.	.	.	.	.	.	(1)	.	.
Assimilation	.	.	1	.	.	.	.	.	.	.	.	1
Separation	1	.	.	1(1)	.	.	.	.	.	2(1)	.	.
Marginalization	(1)	.	.	.	.	.	.	.	.	(1)	.	.
Personal and developmental identity concepts (n = 2)
Identity diffusion (n = 1)	3	.	.	.	.	.	.	.	.	3	.	.
Alternative identity (n = 1)	.	1	.	.	.	.	.	.	.	.	1	.
Identity needs and motives (n = 16)
Uncertainty (n = 6)	1(1)	(2)	2	2(3)	(2)	1	.	.	1	3(4)	(4)	4
Need for significance (n = 6)	2(3)	.	1	1(1)	.	1	.	.	2	3(4)	.	4
Need to belong (n = 7)	3(1)	1(1)	4	2(1)	.	5	.	1	3	5(2)	2(1)	10
Need for self-esteem (n = 4)	1(1)	.	4	1(1)	.	2	.	.	1	2(2)	.	7

Note. The number of effects depends on the amount of identity concepts and outcome measures examined per study.