The Link Between Migration And Terrorism

The link between migration and terrorism

How do different migration forms impact international terrorist incidents?

Introduction

“Where were you during 9/11?”

This question often sparks vivid memories, with people recalling the exact time and place they learned of the terrorist attacks that forever altered the world. This question underscores the profound impact of 9/11 – not just on the United States, but globally – fueling a war on terror, and reshaping security, policies, and societal dynamics worldwide. The consequences were not limited to immediate violence; the attacks also intensified xenophobia and redefined how migration, security, and terrorism were discussed, particularly in Western countries.

More than two decades later, Europe, and Germany in particular, faced a resurgence of terrorist violence. Although less deadly than the attacks of 9/11, a series of primarily Islamist attacks in 2024 shocked the public. Three major attacks in Solingen, Magdeburg, and Mannheim resulted in ten fatalities and nearly 220 injuries, reigniting debates about the link between migration and terrorism. (Parlamentsnachrichten Deutscher Bundestag, 2024; SWR Aktuell, 2024; tagesschau, 2024). The perpetrators were migrants, two of whom were individuals who already should have been deported. These tragic events not only intensified public discourse but also shaped the agenda for the German Federal Elections in February 2025, with migration and terrorism emerging as central issues (Forschungsgruppe Wahlen, 2025; tagesschau, 2025).

In this complex context, migration is often viewed as a security risk in Germany, Europe, and beyond. While the fear of terrorism fuels these concerns, Western societies, including Germany, face pressing demographic challenges: aging populations and shrinking workforces. Migration is presented as a solution to address the labor shortage, with projections suggesting that without migration, Germany’s workforce could shrink by up to 10% by 2040 (Kubis & Schneider, 2024). In the European Union, labor migration accounts for over 20% of inflows, with asylum-seekers constituting only 15%. Similar trends are seen across member states of the Organization for Economic Co-Operation and Development (OECD), where family migration also plays a significant role (Eurostat, 2022).

Given these facts, the often populist call for reduced migration, especially as a response to security concerns, fails to account for the nuanced differences between different types of migration. While public debates and much of the existing literature tend to group all migrants together, distinguishing between humanitarian and non-humanitarian migration is crucial for an informed discussion. As this article will demonstrate, the lack of differentiation in these discussions has led to oversimplifications that overlook the distinct implications of different migration types on security.

What does the Literature say?

The scientific literature explores the relationship between migration and terrorism from various perspectives. Approximately ten key papers have attempted to answer the question of whether increased migration leads to a rise in terrorism, with the first notable contribution dating back to 2008. This indicates that the field is relatively new, largely driven by the 9/11 terrorist attacks in New York and Washington, DC. In 2022, Helbling and Meierrieks published a comprehensive review, summarizing essential contributions to the literature on the relationship between terrorism and migration. They emphasize the potential public policy implications of this ostensible relationship, such as stricter immigration laws under Trump in 2017 (Executive Order 13769 - Protecting the Nation From Foreign Terrorist Entry Into the United States) or electoral politics (e.g., German Federal Elections 2025).

Furthermore, Helbling and Meierrieks (2022) identify three large N-studies examining the overall effect of migration on terrorism without distinguishing between humanitarian and non-humanitarian migration. Bove and Böhmelt (2016) aim to answer the question of whether immigration is associated with domestic and transnational terrorism. Their findings suggest that migrants stemming from terrorist-prone states moving to another country are indeed an important vehicle through which terrorism does diffuse. However, they also conclude that, in general, migrant inflows themselves tend to lead to a reduction in terrorist attacks. Dreher et al. (2020) investigate how a country’s immigrant population – defined as the stock of people born abroad – affects the probability of a terrorist attack in the host country. They demonstrate that the probability of immigrants from a specific country of origin carrying out a terrorist attack in the host country increases as the number of foreigners from that country increases. However, they find that this “scale effect” does not differ from the impact that domestic populations have on domestic terror. Their study provides little evidence that terrorism is systematically imported from countries with large Muslim populations or from those with active terror networks. In another paper, Forrester et al. (2019) examine the relationship between immigration levels and terrorism. They could not find a relationship between immigration and terrorism in destination countries, regardless of whether it was measured by the number of attacks or the number of victims. Helbling and Meierrieks (2022) conclude that these findings “leave little evidence […] in favor of the hypothesis that immigration unconditionally promotes terrorism in receiving countries” (Helbling & Meierrieks, 2022, p. 981). Focusing on the conditional effects of immigration on terrorism, Bove and Böhmelt (2016) and Dreher et al. (2020) acknowledge that the composition of migration inflow plays a significant role in the migration-terrorism relationship. While gender does not appear to be a crucial factor, both studies highlight that the skill level of migrants alters the effects, with high-skilled immigration tending to reduce the risk of terrorism. This may be attributable to the side effects of human capital, since high-skilled individuals are typically over-represented in migration flows (Bove & Böhmelt, 2016).

Three large N-studies examine the relationship between “involuntary” migration (i.e., humanitarian migration) and terrorism. Choi and Salehyan (2013) and Milton et al. (2013) both identify a positive relationship between hosting refugees and terrorist activities in the hosting countries. These studies argue that poor conditions in refugee camps, as well as general mistreatment of refugees in host countries, contribute to higher levels of terrorist attacks. In contrast, Polo and Wucherpfennig (2022) argue that the rise in terrorist activities in refugee host countries is primarily due to increased attacks on refugees themselves – refugees as scapegoats – rather than refugees acting as “Trojan Horses” for terrorism. These findings are particularly apparent in developed countries.

The aforementioned studies either focus on refugee migration or keep the term “migration” generic and do not specify whether they focus on humanitarian or non-humanitarian migration. Helbling and Meierrieks (2022) highlight three particularities that occur in refugee migration that could influence terrorism – features not found in other forms of migration. First, refugee migrants are less likely to self-select into migration. The opposite accounts for the groups of labor migrants, in which educated migrants are often overrepresented (Grogger & Hanson, 2011). This effect may also lead to refugees struggling more frequently with economic and social integration, which can increase their vulnerability to politico-economic hardship and discrimination (Helbling & Meierrieks, 2022). Second, refugees, by definition, typically come from countries affected by conflict and violence more often than other migrants (Echevarria & Gardeazabal, 2016). Third, refugees typically encounter less favorable living conditions in their host countries, which hinders their integration compared to non-humanitarian migrants (Walther et al., 2020).

Different Forms of Migration

Migration is a global phenomenon. The International Organization for Migration (IOM) estimates that, in 2020, over 280 million people migrated internationally, with an additional 60 million people being displaced internally (International Organization for Migration, 2024). While these numbers seem significant, migrants represent only about four percent of the global population. Nevertheless, the estimated number of international migrants has steadily increased over the past five decades, tripling since 1970.

It is important to mention that most examinations of the difference between migration and terrorism rely on official data on “regular” migrants (i.e., those who migrate through formal channels). Irregular (i.e., undocumented) migrants are excluded from the scope as they do not appear on official statistical data. Although this is a large caveat, there is currently no other way around.

In a recently published editorial, Scholten (2022) provides an introduction to migration studies, offering explanations of fundamental facts and trends in international migration. In chapter six, Talleraas (2022) gives a simple yet comprehensive definition of migration and migrants, describing them as “people who move or have moved” (Talleraas, 2022, p. 112). Nevertheless, she emphasizes that some people are labeled and governed as migrants while others self-identify as such.

Building on Scholten's (2022) work, I have developed a framework to categorize different forms of migration. As shown in Figure 1, migration can be broadly divided into two categories: international and internal migration. International migrants are those who cross an international border during their journey, while internal migrants remain within a single country and do not cross state borders. Both categories can be further divided into forced and voluntary migration. This distinction is based on the drivers of migration and is not as rigid as the international-internal categorization. In practice, migration often involves “mixed flows” of forced and voluntary migrants as the drivers and experiences inherent in any migration journey exist along a spectrum rather than as a dichotomy (Erdal & Oeppen, 2018).

Forced internal migrants are classified as Internally Displaced People (IDPs). IDPs are considered part of the group of humanitarian migrants; however, since they do not cross international borders, they represent a distinct group within the migration framework. In the context of international migration, forced migration includes refugees, asylum seekers, and individuals under subsidiary protection. These forms of migration can also be described as humanitarian migration. In contrast, non-humanitarian, or voluntary, migration includes labor migration, family migration, student or educational mobility/migration, and lifestyle migration (Scholten, 2022).

Figure 1 - Self-Developed Framework for Migration based on Scholten (2022)

3.1 Differences in Humanitarian and Non-Humanitarian Migration

Humanitarian migration is loosely defined as “the movement of people who feel some-how forced to move” (Scholten, 2022, p. 151). This category is not highly selective, encompassing a variety of motivations for an individual to migrate. Research on humanitarian migration is historically grounded in the events of World War I and World War II. These wars significantly contributed to the establishment of International Humanitarian Law as it exists today. Nevertheless, the historical context also introduces a bias in contemporary analysis, as much of the literature on humanitarian migration tends to focus on the European asylum system and governance models in other “Western” receiving countries. In contrast, over 80% of the global refugees are hosted by developing countries (Talleraas et al., 2022). Humanitarian migration is characterized by the well-defined drivers of migration, and the legal status of humanitarian migrants is clearly outlined in international law. The 1951 Refugee Convention remains the primary source of these legal guidelines.

Non-humanitarian migration includes voluntary forms of migration that differ fundamentally from forced migration. There is no central governing document, like the 1951 Refugee Convention, for these movements. Instead, voluntary migration is regulated by regional, sub-regional, dyadic, or national contracts.

It is important to notice that there is often a blurred line between labor migration and other forms of migration. While refugees or family migrants may initially move for various reasons, they may later stay in a country for employment or income purposes. A current paradox exists in the distinction between various forms of migration, which are legally categorized based on factors like visa conditions. At the same time, public discourse and even academic research often treat migrants as a homogeneous group. This blurring of lines presents both opportunities and challenges. While humanitarian migrants could help address the labor shortages in Europe, the legal frameworks, often referred to as migration regimes, fail to adequately categorize them, thereby hindering the development of effective migration policies.

How Migrants become Terrorists

Several pathways have been identified in the literature through which migrants might become involved in terrorism. One prominent theory is the “Trojan Horse” theory, which is not typically supported in academic circles but remains a popular concern in public discourse. This theory suggests that terrorist operatives might infiltrate countries disguised as migrants, exploiting the migration process to enter a country undetected. Radicalization would occur outside the target country, and migrants serve as vehicles for terrorism. However, studies such as that by Polo and Wucherpfennig (2022) show little empirical support for this theory. The asylum process, which generally lacks the pre-entry screening required for other types of migration, could make it easier for individuals to enter countries and potentially carry out terrorist activities during lengthy asylum verification processes. While this may seem to support the Trojan Horse argument, more effective security measures in OECD countries have reduced this risk (Özbek, 2018).

Another pathway to terrorism is through domestic radicalization, as seen in the “homegrown terrorist theory.” This theory posits that migrants who have been in a host country for some time may become radicalized domestically, rather than as a result of their initial migration. Research also highlights how migrants are recruited and persuaded by terrorist organizations. Yayla (2021) outlines an eight-step recruitment process that terrorist groups use, including the identification of potential recruits, establishing contact, building rapport, and ultimately isolating the recruit from their prior social environments. This process is particularly effective when a recruit’s social and economic circumstances make them vulnerable.

Labor migrants and family migrants, who generally experience more stable environments due to employment and family integration, are less likely to become radicalized compared to refugees and asylum seekers, who often face social isolation. This difference in vulnerability is an important factor in understanding how humanitarian migrants may be more susceptible to radicalization and recruitment by terrorist organizations compared to non-humanitarian migrants.

Theories related to individual vulnerability to radicalization emphasize factors such as age, social identity, and education. Younger individuals, particularly those without stable familial or social ties, may be more easily recruited due to identity struggles and lower self-esteem. Educational attainment also plays a role; higher levels of education tend to be associated with lower rates of radicalization, although individuals with specialized skills (e.g., in STEM fields) may be targeted for specific roles within terrorist organizations.

Economic factors, including unemployment and poverty, are key drivers of radicalization. Refugees often experience higher levels of economic deprivation compared to non-humanitarian migrants, making them more vulnerable to recruitment by terrorist groups. Additionally, discrimination, lack of political representation, and social exclusion exacerbate feelings of alienation, increasing the likelihood of radicalization. Humanitarian migrants, by definition, often come from conflict zones and are exposed to higher levels of political violence, which further contributes to their vulnerability to radicalization.

Overall, the literature outlines how various factors, both individual and external, contribute to the likelihood of migrants becoming involved in terrorism. Humanitarian migrants, due to social isolation, economic hardship, and political exclusion, face higher risks of radicalization compared to non-humanitarian migrants, who typically enjoy more stable social and economic conditions. Understanding these pathways is crucial for addressing the underlying causes of terrorism and improving migration and integration policies.

Empirical Evidence

To date, there is no empirical evidence regarding the question of whether there is a difference in effects between humanitarian and non-humanitarian migration on international terrorism, particularly in 38 OECD countries. I conducted an OLS and Fixed Effects regression to test the hypothesis that there is a difference in effects between humanitarian and non-humanitarian migration, with the former having a more positive effect than the latter. Fi. To do so, I used data from the Global Terrorism Dataset (GTD). I regressed a count variable for terrorist incidents in OECD countries between 1970 and 2020 on the numbers of arriving humanitarian and non-humanitarian migrants to OECD countries in these years. The data is available at the OECD’s International Migration Database. Furthermore, by matching the home countries of these organizations with the countries of origin of humanitarian and non-humanitarian migrants, I classify the migrants into two groups: those from countries with transnational terrorist organizations (TNT) and from countries without TNT. I replicate this approach, as outlined in Polo and Wucherpfennig (2022, p. 39). The table below presents the total number of migrants in the dataset, categorized by humanitarian and non-humanitarian migrants, as well as by migrants from TNT (transnational terrorism) countries and non-TNT countries (percentage of total in parentheses).

Table 1 – Migration Data

Non-Humanitarian

Migrants

Humanitarian

Migrants

Total

TNT Migrants

A)

C)

15,882,099

12,770,310

(10.17%)

3,111,789

(2.48%)

Non-TNT

Migrants

B)

D)

109,648,686

98,019,226

(78.08%)

11,629,460

(9.26)

Total

110,789,536

14,741,249

125,530,785

The results show that humanitarian migrations would need to increase by a factor of 21 for a 1% increase in terrorist activities. Having a look at a concrete example, according to the GTD, Germany experienced 3 (three) international terror attacks in 2018 while hosting 67,860 refugees from countries with international terrorist organizations. According to this (limited) model, to cause the number of terrorist attacks to increase from 3 (three) to 4 (four), i.e., one additional attack, the number of humanitarian migrants would need to increase to approximately 30.9 million. This is an increase of about 45.5 times the initial number of migrants.

Figure 2: Model Coefficients with Estimates and Statistical Significance

In more technical terms, a 1% increase in non-humanitarian migrants is associated with a 0.012% decrease in international terrorist attacks in OECD countries, all else equal, although this effect is not statistically significant. In contrast, the estimate for humanitarian migration suggests that a 1% increase in humanitarian migrants is linked to a 0.047% increase in international terrorist attacks, statistically significant at the 5% level. Nonetheless, other models confirm the positive trend for humanitarian migration and the negative trend for non-humanitarian migration.

Conclusion & Outlook

In conclusion, the relationship between migration and terrorism is complex and multifaceted. This article has emphasized the critical distinction between humanitarian and non-humanitarian migration, highlighting that humanitarian migrants, due to their socio-economic vulnerabilities, are more susceptible to radicalization compared to labor or family migrants. While concerns about terrorism often dominate public discourse, the empirical evidence presented here shows that the effect of migration on terrorist activities is more nuanced. A significant increase in humanitarian migration would be required to cause a noticeable rise in terrorism, with even a large influx only resulting in small changes in terrorist incidents.

The findings underscore the importance of differentiating between migration types when discussing migration's impact on security. While the correlation between humanitarian migration and increased terrorism is evident, it is not deterministic. Further research, especially empirical studies, is needed for understanding the complexities of this relationship and shaping effective migration and security policies.

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