Significance Research on violence has mainly focused on its consequences on individuals’ health and behavior. This study establishes the effects of exposure to violence on individuals’ short-term memory and cognitive control. These are key factors affecting individual well-being and societal development. We sampled Colombian civilians who were exposed either to urban violence or to warfare. We found that higher exposure to violence significantly reduces short-term memory and cognitive control only in the group actively recalling emotional states linked with such experiences. This finding demonstrates and characterizes the long-lasting effects of violence. Existing studies have found effects of poverty on cognitive control similar to those that we found for violence. This set of findings supports the validity of the cognitive theory underpinning these studies.

Abstract Previous research has investigated the effects of violence and warfare on individuals' well-being, mental health, and individual prosociality and risk aversion. This study establishes the short- and long-term effects of exposure to violence on short-term memory and aspects of cognitive control. Short-term memory is the ability to store information. Cognitive control is the capacity to exert inhibition, working memory, and cognitive flexibility. Both have been shown to affect positively individual well-being and societal development. We sampled Colombian civilians who were exposed either to urban violence or to warfare more than a decade earlier. We assessed exposure to violence through either the urban district-level homicide rate or self-reported measures. Before undertaking cognitive tests, a randomly selected subset of our sample was asked to recall emotions of anxiety and fear connected to experiences of violence, whereas the rest recalled joyful or emotionally neutral experiences. We found that higher exposure to violence was associated with lower short-term memory abilities and lower cognitive control in the group recalling experiences of violence, whereas it had no effect in the other group. This finding demonstrates that exposure to violence, even if a decade earlier, can hamper cognitive functions, but only among individuals actively recalling emotional states linked with such experiences. A laboratory experiment conducted in Germany aimed to separate the effect of recalling violent events from the effect of emotions of fear and anxiety. Both factors had significant negative effects on cognitive functions and appeared to be independent from each other.

Violence and warfare are a persistent characteristic of most human societies, from the lowest levels of complexity (1). Approximately 650,000 people are estimated to have been victims of homicide around the world in 2010 (2). In addition to its direct economic costs (3), exposure to violence (ETV) has significant negative effects on both physical and mental health (4⇓–6). Approximately 5% of male and 10% of female US citizens suffer from posttraumatic stress disorder (PTSD) after ETV (4, 5). ETV has significant effects on a wide array of individual behavior and socioeconomic outcomes (7), ranging from changes in risk preferences (8, 9), to higher temporal discounting and impulsivity (9), higher prosociality (8, 10, 11), and increased political participation (12).

This study addresses the direct impact of ETV on short-term memory and cognitive control. Short-term memory is an individual’s capacity to store information. It is distinct from working memory, which requires both storing and a mental manipulation of information (13). Short-term memory is a valid predictor for job and training performance (14). Better forward digit span as a measure of short-term memory at preschool age predicts better math and reading abilities during the first years of primary school (15). Deficits in short-term memory, by contrast, are associated with several medical conditions, such as Alzheimer’s disease (16) and PTSD (4⇓–6, 17).

Cognitive control, also referred to as executive function, has been defined as a family of top-down mental processes needed when one has to concentrate and pay attention, when going on automatic or relying on instinct or intuition would be ill-advised, insufficient, or impossible (13). Extensive empirical research identifies three components of cognitive control that seem crucial in problem-solving. These components are inhibitory control (which can be broken down into self-control and selective attention), working memory, and cognitive flexibility. These three components of cognitive control, in turn, enable fluid intelligence (13). Cognitive control, in general or in some of its components, is positively associated with and temporally precedes a wide array of aspects of individual well-being and indicators of societal development. Lack of cognitive control is correlated with and precedes mental disorders (18), unhealthy behavior (19), and early mortality (20). Cognitive control predicts school readiness and positive educational attainment (21). Self-control in children has been shown to predict physical health, personal wealth, and criminal offenses 30 y later (18). Low cognitive control also predicts unemployment and affects wages (22). Specific neural systems are activated when individuals exert self-control (23), and these neural systems are linked to genetic components that have also been associated with violent behavior (24). Ascertaining the impact of ETV on cognitive control is therefore important to further understanding the social, economic, and health effects of violence.

The main site of our research was Colombia, a country affected by extensive warfare and violence for several decades. In the first experiment, participants were residents of urban areas where violence, street crime, and ordinary criminality are widespread. In the second experiment, participants were internally displaced civilians, living in rural areas that had been the theater of armed conflicts between irregular militia groups up to 10 y ago. Given the variety of these environments, we were able to compare short- and long-term effects of violence, as well as the impact of different types of violence—warfare in the rural sample and ordinary criminality in the urban one.

The key methodology of our research relied on the randomized and experimentally induced recall of experiences of violence in a portion of our sample, and of emotionally neutral or joyful experiences in the remaining portion, before the execution of cognitive tests (9, 25, 26). We analyzed the direct effects of violence recall (VR), ETV, and their combination on measures of short-term memory and cognitive control. This design controlled for confounding factors and allowed us to better investigate the issue of causality.

Psychologists propose a “dual-process” framework in which an “automatic system” and a “reflective system” interact to produce decisions (27, 28). The former is uncontrolled by our conscious thinking, is effortless, follows associations rather than deductions, and is constantly active. The latter requires conscious cognitive control, demands deliberate effort by the individual, and can use deductive logic or rules. Our main hypothesis was that ETV triggers immediate psychological reactions whereby the automatic system becomes predominant over the reflective system. We conjectured that the effects may be long-lasting, especially when individuals are faced with traumatic, life-threatening ETV. Furthermore, we posited that recalling episodes of violence, even many years after their occurrence, may lead to the same effect, especially when the experience that is being reenacted is particularly traumatic. Support for these hypotheses comes from extensive neurobiological and clinical research documenting the effects of traumatizing stress (29, 30) and PTSD (4⇓–6) on the neuroendocrine network underlying cognitive control. In particular, the release of catecholamine induced by acute stressors impairs cognitive control of the prefrontal cortex (PFC) (31), while at the same time enhancing the emotional and habitual responses of the amygdala, which also regulates fear conditioning, and the tonic firing of the noradrenergic locus coeruleus (32, 33). The dopamine system is also involved, because acute stress triggers enhanced dopamine efflux in the medial PFC. Upon modulation by glucocorticoid receptors, this system decreases cognitive control (34, 35). Although most of the above studies involved animals, similar transmission mechanisms seem to operate in humans. In particular, exposure to natural situations of mild distress led to deficits in both attentional control and in dorsolateral PFC functional connectivity with other areas of the fronto-parietal attentional network in humans (36). On the basis of this evidence, we hypothesized that ETV may impair cognitive control through the induction of acute and chronic stress.

Discussion This study shows differential effects of continuing exposure to higher and lower levels of violence over years on short-term memory and cognitive control in both urban and rural samples from Colombia. We found that higher ETV significantly reduces both short-term memory and cognitive control, but only in the group actively recalling emotional states of fear and anxiety linked with experiences of violence. On the contrary, when such emotional states were not recalled, accuracy on memory and cognitive tasks was no different between groups highly and lowly exposed to violence. The patterns we found were similar in all of the measures of cognitive abilities that we used—although the results fell short of statistical significance on the Raven test. This set of findings is noteworthy because our samples varied greatly in terms of the type of ETV—street crime violence in the case of Bogotá residents and warfare in the case of the rural sample—as well as the time frame of ETV—short-term in the case of Bogotá and long-term in the case of the rural sample. Research conducted on animals and humans showed that the effects of mild stress is reversible after a few weeks (36). We found that ETV seemed to have cognitive effects even after more than a decade had passed from the main traumatic event that was recalled. The effects of ETV thus seemed to be persistent. Although other studies found significant behavioral differences between groups that have either been exposed to traumatic experiences of violence or that have not been affected by violence (11), in this study, we found effects of ETV within the former group. This finding suggests that not only the mere ETV, but also its intensity, has the capacity to severely affect human behavior. Our results have two additional implications. On the one hand, our results suggest that a traumatic experience that is unresolved, and that one keeps bringing up in memory, may significantly negatively affect a person’s ability to exercise short-term memory and cognitive control, thus affecting his or her ability to function well in life. In particular, when memories of past trauma are triggered, they might also impact the cognitive functioning of victims who would otherwise not suffer the effects of trauma. Recovering from past traumas, although possible, might be fragile and highly vulnerable. On the other hand, our results also suggest that, even if a person has experienced traumatic violent events, this person’s recovery may be helped by not dwelling on these memories, but rather by recalling happier memories. His or her ability to exercise the critical mental skills that we studied will not be affected, allowing him or her to potentially succeed in arenas of life in which these skills are important. More research is needed to better understand the psychological, but also the social, mechanisms behind our findings. We cannot rule out that the observed effects reflect the combined influence of ETV and other social factors such as poverty. Moreover, our results may be culture- or context-specific. Although the evidence coming from Germany seems to be consistent with what we found in Colombia, replication is needed under different circumstances, in other parts of the world, or for other types of violence to corroborate fully our findings. Recent research using similar methodologies to ours shows that poverty—understood as the psychological perception of a gap between one’s needs and one’s material resources—exerts a decay of cognitive functioning and alters preferences (26, 46). Our results demonstrate that ETV appears to have a similar effect to that exerted by poverty. These findings support the robustness of the theory of mind that lies behind these analyses.

Materials and Methods Sampling procedures, experimental protocol, instructions read to subjects, and ethnographic information are discussed in SI Appendix, section 1. All our econometric analyses of accuracy on cognitive tasks deployed Tobit models. This method takes into account the truncated nature of the accuracy variable. The econometric models controlled for demographic factors—namely, gender, age, education, and socioeconomic status. Our main variable of interest in experiments I and II was the measure of ETV, categorized in two groups of people more or less exposed to violence. Its effects on cognitive performance were analyzed both on the whole sample and separately for experimental treatment. Data from our experiments are shown in Datasets S1–S4. Variables are defined in Dataset S5. The commands to replicate our econometric analyses with the Stata package are reported in Dataset S6. The research committee of the Konrad Lorenz University granted ethical approval for experiments I and II. The German Psychological Foundation approved the research design of experiment III. Written informed consent was obtained from every subject.

Acknowledgments We thank Suelen Castiblanco, Laura Jiménez, and Daniel Reyes for providing research assistance and the community leaders in Montes de María (who prefer to remain anonymous) and the personnel in Cedecampo for providing logistical support. We also thank Barbara Bobba, Benedetto De Martino, and Navah Kadish for comments. This work was supported by Open Evidence Grant 008-Tierra-Colombia; Fundación Universitaria Konrad Lorenz Grant 7INV3131; Universitat Jaume I Grant P1.1B2015-48; Spanish Ministry of Economics and Competitivity Grant ECO 2015-68469-R; and Fondazione Franceschi.