We analyze the determinants of left-wing and nationalist-separatist terrorism for 18 Western European countries for the 1970–2007 period. Focusing on the influence of the Cold War, we find that it predicts left-wing but not nationalist-separatist terrorism, suggesting that there is indeed some heterogeneity in the causes of terrorism. However, we also find that a number of factors determine both kinds of terrorism, indicating that there were differences but also commonalities in the causes of left-wing and nationalist-separatist terrorism in Western Europe during our observation period.
In recent years a plethora of cross-country studies have tried to unveil the causes of terrorism (for a survey of the evidence, see Krieger and Meierrieks 2011). Usually, these studies implicitly assume that a single empirical and behavioral model can explain terrorist activity, irrespective of differences in the underlying ideological motivations of distinct terrorist organizations.
A small body of literature, however, suggests that terrorism is not a uniform but heterogeneous phenomenon. For instance, Kis-Katos, Liebert, and Schulze (2014, 116) argue that “terror of different ideological types should have different determinants as the terror groups cater to different grievances, engage in compromise to different degrees and have different organizational constraints.” In other words, the ideological background of a terrorist group may matter, where groups are motivated by distinct (heterogeneous) grievances (i.e. determinants) associated with the respective group’s ideology. For instance, such reasoning would suggest ethnic discrimination may matter to the genesis of nationalist-separatist terrorism but not the emergence of left-wing terrorism. Indeed, e.g. Robison, Crenshaw, and Jenkins (2006) and Kis-Katos, Liebert, and Schulze (2014) find that there are differences – but also commonalities – in the determinants of left-wing, right-wing, ethnic and Islamist terrorism.
In this contribution we account for the aforementioned (potential) heterogeneity in the determinants of terrorism (that may be a consequence of the use of terrorism by groups with heterogeneous ideological backgrounds) by analyzing the determinants of the two most prominent types of terrorism in 17 Western European countries, left-wing and nationalist-separatist terrorism.  For this research note we focus on one particular determinant, the Cold War, and its relationship with left-wing and nationalist-separatist terrorism.
With respect to the role of the Cold War in terrorism, there is ample evidence that – in addition to country-specific factors – the dynamics of the international political system and especially of the Cold War have mattered to the genesis of terrorism. For instance, Sobek and Braithwaite (2005) find that US dominance in the international political system explains anti-American terrorism, while O’Brien (1996) and Volgy, Imwalle, and Corntassel (1997) find that fluctuations in the international political system (e.g. conflict between the Cold War superpowers) can explain the patterns of terrorism. Here, terrorism may be used by states as a foreign policy tool, e.g. to challenge US dominance (O’Brien 1996; Sobek and Braithwaite 2005).
In Western Europe terrorist groups received political and material (i.e. financial and logistical) support from the Soviet Bloc during the Cold War era, so as to use these groups in proxy wars against the US-dominated Western bloc (O’Brien 1996). We expect left-wing terrorism to be especially responsive to international developments associated with the Cold War, given that left-wing groups shared politico-ideological goals with the Soviet Bloc; while it is true that ethnic terrorist groups also received some support from the Soviet Bloc during the Cold War era, the relationship between nationalist-separatist terrorism and the Cold War is not expected to be especially strong. Consequently, the end of the Cold War is expected to have reduced especially revolutionary left-wing terrorism. First, the end of financial and military support by the Soviet Union reduced the clout of this kind of terrorism. Second, the end of the Soviet Bloc also meant an undermining of the ideological foundations of left-wing terrorism, which can be expected to have strongly discouraged the attractiveness of this kind of terrorism (Shughart 2006). Below, we test our hypothesis of a negative (non-existent) effect of the end of the Cold War on left-wing terrorism (nationalist-separatist) in Western Europe. 
2 Data and methods
2.1 Dependent variables
We use the Global Terrorism Database (GTD) of the National Consortium for the Study of Terrorism and Responses to Terrorism (START 2015) to create our three dependent variables, (i) the number of domestic left-wing terrorist attacks per country-year, (ii) the number of nationalist-separatist terrorist attacks per country-year and (iii) the total number of terrorist attacks per country-year (where we “pool” terrorism by left-wing, ethnic, but also right-wing groups as well as international and unknown actors).
In order to be considered domestic, a terrorist attack has to be carried out by a group operating in its natural territory, i.e. in the very country whose politico-economic system (for left-wing terrorism) or territorial integrity (for separatist terrorism) the group challenges; the nationality of the victims of an attack does not matter.  A terrorist incident is classified as left-wing when it is carried out by anarchist, communist/socialist, anti-globalization or other leftist groups. Prominent examples of such groups are the German RAF, the French Action Directe and the Italian Informal Anarchist Federation. By contrast, an attack is classified as nationalist-separatist when the attacking terrorist group primarily strives for territorial change. This includes groups with predominantly separatist agendas such as the British PIRA and the Spanish Euskadi Ta Askatasuna. We also include armed groups that oppose these very territorial ambitions. For instance, we also coded terrorist actions by the British Ulster Freedom Fighters and the Spanish Grupos Antiterroristas de Liberación as nationalist-separatist terrorism.
For our observation period (1970–2007) the GTD reports 14,404 terrorist incidents by known and unknown terrorist perpetrators in Western Europe. A total of 1670 (11.6%) of these attacks were domestic left-wing terrorist incidents, while 7180 (49.8%) attacks were carried out by groups with separatist or nationalist agendas. This means that revolutionary and ethnic terrorism accounted for over 60% of all attacks during our observation period. Figure 1 illustrates the dynamics of the number of attacks per type. The number of nationalist-separatist attacks was constantly high during the 1970s and 1980s. The absolute number of attacks of left-wing terrorists was always much smaller and decreased further after the end of the 1970s. Given the visible reduction in terrorism after 1990, it seems possible – consistent with our main hypothesis – that the end of the Cold War indeed mattered to the dynamics of domestic terrorism in Western Europe.
2.2 Main explanatory variable
In this research note we are especially interested in the role of the Cold War in predicting ideology terrorism in Western Europe. We expect this variable to determine left-wing but not nationalist-separatist terrorism, thus suggesting that there is heterogeneity in the causes of terrorism considering this variable at least for Western Europe. In our analysis the Cold War is indicated by a dummy variable taking on the value 1 for the 1970–1991 period and 0 thereafter.
The choice for the controls is, on the one hand, guided by the literature review of Krieger and Meierrieks (2011). On the other hand, we follow earlier contributions on the causes of terrorism in Western Europe (Caruso and Schneider 2011; Sanchez-Cuenca 2009). The summary statistics, operationalization and data sources of all controls are reported in Table 1.
|Cold War era||684||0.58||0.49||0||1|
|Population size||684||9.10||1.54||5.32||11.32||Total population size, logged|
|GDP per capita||684||10.02||0.34||8.94||11.26||Real per capita income, logged|
|Ethnic fractionalization||684||0.33||0.25||0.02||0.87||Index reflecting the probability that two randomly chosen individuals from a country belong to different ethnic groups|
|Right-leaning government||666||0.50||0.50||0||1||Dummy variable (1=leader of government is liberal or conservative)|
|Unemployment||680||6.13||4.30||0.02||24.17||Unemployment rate as a percentage of civilian labor force|
|Growth of tertiary education||684||0.02||0.06||–0.05||1.45||Growth rate of post-secondary education|
Sources: Data on population size and income from the PENN World Tables (http://tinyurl.com/hkqvgn6); data on ethnic fractionalization from http://tinyurl.com/zxsz93v; data on incumbency and unemployment from the Comparative Political Data Set (http://tinyurl.com/z5hd967); data on education from the Cross-National Time-Series Data Archive (http://tinyurl.com/gux4ncm).
In detail, we control for population size, expecting it to positively predict terrorism due to scale effects. Economic factors (per capita income, unemployment) are also considered, where we expect better economic conditions to make terrorism less likely by increasing terrorism’s opportunity costs. We also expect ethnic fractionalization to predict terrorism, where this variable ought to matter especially to nationalist-separatist terrorism. For instance, fractionalization may coincide with discrimination, providing incentives for minorities to resort to terrorism. We also consider the effect of the growth of tertiary education on terrorism. Sanchez-Cuenca (2009) argues that the expansion of higher education may fuel terrorism, e.g. when such expansion leads to strong competition on the labor market. We also control for a political variable, the incumbency of a right-leaning (i.e. liberal, conservative or center-right) government. Given that such governments tend to be the enemies of both left-wing and nationalist-separatist terrorism, we expect terrorism to intensify when such governments are in power. Finally, a lag of the dependent variable is included in some specifications to account for potential contagion effects and path-dependency of terrorism (Krieger and Meierrieks 2011).
The choice of our econometric method is due to two factors. First, our dependent variables are count variables which only assumes discrete, non-negative values. Their variances are larger than their respective means (overdisperion), as shown in Table 1. Second, given that we use panel data, variables of interest are likely correlated over time. To adequately account for this data structure, we run a series of generalized estimation equation models for negative-binomially distributed (panel) count data, where we control for an AR(1) term to factor in temporal correlation (Zorn 2001). All independent variables are lagged by 1 year and country-clustered (robust) standard errors are reported.
3 Empirical results
Our empirical results are reported in Table 2. With respect to our main variable of interest, we find that during the Cold War era left-wing terrorism was statistically significantly more likely. In fact, calculating the incidence-rate-ratios (based on specification 2) suggests that left-wing terrorism was approximately 4.32 times more likely in the Cold War era compared to the post-1991 period. Interestingly, for both nationalist-separatist and the “pooled” (total) terrorism variable, there is no statistically significant effect of the Cold War era. This points to heterogeneity in the determinants of terrorism for this variable. In fact, this heterogeneity can also be seen when education expansion is considered. Consistent with Sanchez-Cuenca (2009), education expansion fuels left-wing terrorism. However, we find it to negatively predict nationalist-separatist terrorism (albeit this effect is only weakly statistically significant). More importantly, heterogeneity in the determinants of terrorism is also present when ethnic fractionalization is considered as a potential terrorism determinant. This variable only matters to nationalist-separatist terrorism, potentially due to its close connection with ethnic grievances.
|Growth of tertiary educationt−1||0.020||−0.056||−0.030|
|Lagged dependent variable||0.007||0.001||0.007|
|Type of terrorism||Left-wing||Left-wing||Separatist||Separatist||Total||Total|
|No. of observations||666||608||666||608||666||608|
Constant not reported. Robust standard errors clustered over countries in parentheses.
*p<0.10, **p<0.05, ***p<0.01.
Still, there are also commonalities in the determinants of terrorism across the different types of terrorism. First, unfavorable economic conditions (low per capita income, high unemployment rates) are associated with more terrorism, irrespective of which type of terrorism we consider. Second, larger populations mean more terrorist activity. Third, facing a right-leaning government is conducive to both left-wing and nationalist-separatist terrorism. In the former two cases, the importance of these determinants is also not masked when using a “pooled” terrorism indicator as the dependent variable.
Our empirical analysis of left-wing and ethnic terrorism for 18 Western European countries for the 1970–2007 period suggests that the Cold War particularly influenced left-wing terrorism. This finding is intuitive, given the political proximity between left-wing terrorism and the Soviet bloc. More generally, our finding speaks to the idea of heterogeneity in the causes of terrorism (Kis-Katos, Liebert, and Schulze 2014), suggesting that ideology matters and may influence which systemic variables (e.g. the Cold War) and domestic conditions (e.g. ethnic fractionalization) incentivize terrorism. However, our findings also suggest that for a number of variables (e.g. per capita income) an approach that does not differentiate between different ideological backgrounds (i.e. “pooling”) may still have some merits. This is especially important for the validity of large-N analyses when there is no data available that allows to readily consider terrorist ideology.
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