The reviewed literature suggests that terrorist attacks are a risk factor for the development of MDD, mainly in the first months after its occurrence, and in certain at-risk populations. The risk of MDD ranges between 20 and 30% in direct victims of terrorist attacks and between 4 and 10% in the general population in the first few months after terrorist attacks. These prevalence rates are 2-3 times higher than might be expected according to general population surveys [14
]. These results are consistent across studies that have used separate methodologies and assessment instruments after different terrorist attacks occurring in various cities [as in the case of the studies 35 and 37, or the study 30]. This suggests that the consequences of terrorist attacks may be universal and, in some respects at least, independent of context.
It is not easy to perform longitudinal investigations after the occurrence of terrorist attacks and the scarcity of studies in this area limits clear inference. Thus, whereas in some studies the prevalence of MDD in victims has decreased over time [34
], in other studies, it has remained relatively stable [33
There are several risk factors that have consistently been shown to be associated with the risk of suffering from MDD after a terrorist attack. These include having undergone stressful situations before or after the attack, having suffered a panic attack during the attack, being female, and having borne a greater loss of psychosocial resources. Although high perceived social support has been shown to be a protector factor for the onset of other psychological problems in several studies [1
], this result is inconsistent in relation to MDD. This inconsistency in the published studies may suggest the existence of moderating and/or mediating variables in the relation between social support and MDD after terrorist attacks.
Previous literature has noted that severe levels of impairment are most likely to occur in people exposed to terrorism than to any other types of disaster, such as natural disasters [1
]. Consistent with these observations, the prevalence of MDD reported in our review appears to be higher than that reported after natural disasters [38
]. Terrorism has been distinguished from natural disasters by its capacity to produce greater sense of fear, loss of confidence in institutions, unpredictability and pervasive experience of loss of safety [4
]. These characteristics may be associated with the increased risk of psychiatric morbidity after terrorism. However, the different rates of MDD after natural disasters and terrorist attacks may be due to differences in the samples assessed among studies. After natural disasters, it is difficult to classify persons as either direct or indirect victims [3
] and, consequently, the study sample may include persons who were more or less directly exposed to the disaster [6
Together with MDD, some of the reviewed studies assessed the prevalence of PTSD and some examined the comorbidity between MDD and PTSD. Given the evidence indicating the high rates of comorbidity between MDD and PTSD following trauma [52
], it is likely that MDD seldom happens in isolation after terrorist attacks. In this respect, one study examined in this review [37
] reported high rates of comorbidity in general population, with around 50% of individuals with MDD having comorbid PTSD one month after S-11, and around 30% with MDD having comorbid PTSD one month after M-11. Similar results were reported in direct victims exposed to the Oklahoma City Bombing (55% of subjects with PTSD were also diagnosed as having MDD) [29
]. The mechanisms linking PTSD and MDD remain unclear, with alternative explanations including PTSD and MDD as a single general traumatic stress construct [54
], comorbid MDD developing as a secondary reaction [55
] or MDD and PTSD as relatively independent posttraumatic disorders [56
]. Reviewed studies show that MDD is not always concurrent with PTSD and suggest that, consistent with previous studies carried out after other traumatic events [52
], both disorders can be considered related but different posttraumatic reactions. In this regard, Rubacka et al. [57
], examining the specific association of PTSD cluster symptoms (re-experiencing, avoidance, and hyperarousal) and MDD in a sample of mothers directly exposed to the WTC attacks, showed that only higher arousal symptom scores were significantly correlated with persistent MDD. Furthermore, if we compare the rates of MDD and PTSD found in some reviewed reports, we can reach some interesting conclusions. Whereas in direct victims the probability of developing PTSD after terrorism is higher than that of MDD (with percentages of PTSD usually over 30%), this tendency is reversed in the general population. For example, the prevalence of PTSD and MDD in the general population was 7.5% and 9%, respectively, after S-11 [35
], and 2.3% and 8%, respectively, after M-11 [37
]. These results support those found in previous research [53
], suggesting that the pathways to MDD and PTSD may by somewhat distinct; whereas the intensity of the attack and the degree of exposure may be more closely involved in the development of PTSD, bereavement and psychosocial loss may underlie MDD after a terrorist attack [10
The aim of the current work was to review the evidence regarding MDD following terrorism. There are some limitations to the literature in the field and to our review that need to be taken into account when interpreting the results herewith presented.
Limitations of the literature in the field
First, although we only included studies that assessed MDD based on diagnostic criteria, most of them used instruments that did not include an assessment of either manic or psychotic symptoms, therefore we could not classify the disorder beyond probable MDD [8
]. Although the prevalence of bipolar affective disorder is not much higher than 1% [14
] it could be inflating the MDD percentages. Future investigations should take this into account in order to help improve our understanding of the psychopathological processes involved. Second, as we mentioned in the selection criteria section, the reviewed studies were not designed to ensure that persons were free from psychopathology before the occurrence of the terrorist attacks, which means that prevalence, instead of incidence, was assessed. Moreover, none of them included a control group that would enable the comparison between exposed and non-exposed populations. We overcame this challenge by comparing the prevalence of MDD following terrorism with the prevalence reported in other general population surveys. However, it is important to be cautious when interpreting this comparison because the majority of these epidemiologic studies are referred to a whole nation's population (e.g. Spain or United States) [15
] and not to the city where terrorist attacks occurred (e.g. Madrid or New York), hence there could be differences between the two. Future studies should attempt to analyze the incidence of MDD by establishing baseline psychopathological assessments that may be used as population cohorts to document MDD incidence in the event of terrorism exposure. Third, there are several challenges facing longitudinal studies that aim to document the course of MDD. Several studies suffer from attrition, that is, the reduction in the number of people who participated in the follow-up studies. This may have biased the prevalence estimates of MDD, especially in the case of small samples [33
]. Fourth, we have to be careful when extracting conclusions with respect to some correlates of MDD, mainly the pre-traumatic factors. In the reviewed studies, the assessments documented always took place after the terrorist attack in question. It is possible that pre-event reports are biased in the sense that depressed persons may selectively recall stressful situations that occurred before the attack to a greater extent than non-depressed persons.
Limitations of the review
In relation to the characteristics of our review, we only considered studies that had assessed samples of direct or indirect adult victims. Even though we did not include studies that assessed children or adolescents, further work with this age group is clearly warranted. We also limited our search to studies that analyzed the consequences of terrorist attacks and not other kinds of disasters; knowing and comparing the prevalence and course of MDD after natural, technological, or other disasters linked to interpersonal violence (such chronic exposure to trauma) could help us understand the onset of mental disorders after mass traumatic events. Finally, we have focused our review on the examination of MDD using diagnostic criteria. This enables us to compare prevalence rates of MDD with previous epidemiological surveys and between studies carried out after different terrorist attacks. However, MDD is not the sole disorder within the unipolar spectrum and extant research after terrorism has also highlighted the high prevalence and impairment associated with other forms of depression, such as mild or minor depression. Including these other forms of depression, together with the risk factors associated with it, could be of research and public health interest.
Implications for future research in this field
Our review highlights some key areas that are important for future research and may serve to guide intervention. First, the course of MDD after terrorist attacks remains unclear. That is why greater efforts are needed to elucidate the course of MDD after terrorist attacks. Second, there is very limited literature about psychological constructs that may be associated with MDD after terrorist attacks [3
]. It would be interesting, in this context, to analyze the role played by other variables that have been shown to be related to MDD, such as attributional style [59
], self-esteem [60
] or response styles to depression [61
], and to examine the way in which certain psychological variables interact with other sociodemographic variables to predict the onset of MDD. For example, it is possible to analyze which psychological factors mediate the relationship between MDD and gender. This line of research will be useful in helping to identify the persons with higher probability of developing MDD following a terrorist attack and to improve the efficacy of the interventions from which they will benefit. Third, more research is needed on the role of MDD in psychiatric comorbidity after terrorist attacks. Although some reviewed studies have reported high rates of comorbidity between MDD and PTSD, more works are needed to have a better understanding of this relationship. For example, an interesting objective would be to examine the form in which both pathologies vary over the time after terrorism. In this line, some authors have recently documented the important role that depressive symptoms plays in the development and persistence of stress post-traumatic symptoms after different traumatic events [62
]. Fourth, some of the studies in this revision included victims who had been bereaved [33
]. Although not reported in these papers, differences in the prevalence of MDD may exist between victims who have been directly injured by a terrorist attack and those who have been bereaved. Moreover, bereaved people could develop other psychological problems, such as complicated grief syndrome. A number of studies support the differentiation between complicated grief and MDD [63
], and some authors have shown that it is a usual reaction in bereaved people after terrorism [65
]. A clear definition of victims in future works could provide us with a better understanding of the psychological consequences in people directly and strongly exposed to terrorism.