Environmental scientists from multiple disciplines have overwhelmingly acknowledged human-driven climate change as fact. Similarly indisputable is the fact that the effects of rising temperatures will be global in scope and resoundingly negative: droughts, coastal city flooding, decreased food production, and extreme weather, to name just a few. What you may not have considered, however, are some of the subtler psychological and social consequences of rapid climate change — including aggression and violent conflict. A growing body of evidence shows that rapid global warming can (and is) increasing violent behavior in three different ways.
Immediate Effect of Heat Stress on Aggression and Violence
When people get uncomfortably hot, their tempers, irritability, and likelihood of physical aggression and violence increase. This is perhaps best demonstrated in a series of laboratory studies conducted by APS Fellow Craig A. Anderson and his colleagues (Anderson, 1989, 2001; Anderson & Anderson, 1984, 1996, 1998; Anderson, Anderson, Dorr, DeNeve, & Flanagan, 2000; Anderson, Bushman, & Groom, 1997; Anderson & DeLisi, 2011). Across several studies, undergraduate participants completed measures of perceived hostility, anger, or behavioral aggression, but were randomly assigned to do so in one of several temperature-controlled rooms. For some, the room they sat in was a comfortable temperature (e.g., 75o F). For others, the room was particularly cold (e.g., 57o F) or hot (e.g., 97o F), and participants themselves indicated that these rooms were quite uncomfortable. In one study, participants in the cold and hot rooms perceived a series of filmed interactions as being more hostile and aggressive than participants in the comfortable room. In another, participants in the cold and hot rooms scored higher than those in the comfortable room on a state hostility scale. In a final study, participants in warmer and cooler rooms, relative to those in comfortable rooms, responded to an opponent’s ambiguous provocation during a competitive reaction-time task with outbursts of intense blasts of noise. The researchers conclude, based on the collection of studies, that uncomfortably hot temperatures increase aggression.
Although laboratory forms of aggression may seem trivial, other studies illustrate the deadly implications of these findings. Researchers Aldert Vrij, Jaap van der Steen, and Leendert Koppelaar (1994) randomly assigned 38 Dutch police officers to complete a firearms training simulator in either a comfortable-temperature room (70o F) or a warmer-temperature room (81o F). In the simulation, officers responded to a scenario being displayed life-size on a projector screen in front of them. The scenario involved approaching a shed in response to a burglary alarm and being confronted with a suspect brandishing a crowbar. Officers’ responses were recorded and coded by the researchers, along with the officers’ post-scenario impressions. The officers completing the simulation in the warmer room were more likely than those in the cooler room to perceive the suspect as being aggressive, were more likely to consider them to be a threat, and were more likely to draw their firearm from its holster (85% vs. 59%). Findings such as these illustrate the contributing role that temperature plays in escalating minor disputes into full-blown assault or homicides.
Numerous cross-sectional and time-series studies using real-world heat and violence data provide converging evidence. Cities and regions with higher temperatures tend to experience more violent crime than cooler regions, even after controlling for a dozen sociocultural factors such as age, race, poverty, and culture of honor. Further ruling out alternative explanations, some studies have assessed temperature and violence within the same geographic region over time. Across hours, days, months, and even years, similar trends emerge: When it is hotter, violence increases. From Chicago to Brisbane to Vancouver to Dallas, whether looking at domestic violence or physical assault, the same relationship emerges. In one of the most thorough and illustrative studies, Anderson and DeLisi (2011) compared data from the 1950–2008 FBI Uniform Crime Reports for violent crime (rates of homicide and assault per 100,000 people) and nonviolent crime (rates of burglary and motor vehicle theft per 100,000 people) with average annual temperature data from the National Oceanic and Atmospheric Administration for the same years. Average annual temperatures were significantly positively correlated with violent crime rates but not with nonviolent crime rates. More importantly, this relationship persisted even after controlling for numerous alternative explanations (e.g., incarceration rates). The researchers estimated, based on these findings, that a 1oC increase in average temperature — a fairly conservative estimate of climate change in the following decades — will likely yield a 6% increase in violent crime rates, as many as 25,000 more serious and deadly assaults per year in the United States alone.
Rapid Climate Change and the Creation of Violence-Prone Individuals
In addition to the direct heat effect, there are least two indirect ways that rapid climate change (whether rapid heating or rapid cooling) increases the risk of violence. One involves known developmental pathways that lead infants, children, and adolescents to become violence-prone adults.
A major outcome of rapid climate change is food insecurity. Increased drought, extreme weather, and wildfires are all on the rise, and all of them represent threats to vital crop production and large-scale food shortages. And although the starvation associated with food scarcity is a problem in and of itself, it has the added detriment of contributing to individual-level aggression. Studies have shown that malnourishment — both prenatal and in early children — is a precursor to adulthood antisocial behavior, aggression, and violence. Illustrating this, in a longitudinal study of Mauritanian children conducted by Jianghong Liu and her colleagues (2004), 3-year-olds who were malnourished were found, more than a decade later, to be more aggressive and antisocial and more likely to show signs of conduct disorder than were sufficiently fed children. Given that hundreds of millions of people are estimated to be affected by climate-driven food insecurity, the magnitude of malnutrition effects on aggressive behavior should not be underestimated.
Increasingly frequent and extreme weather destroys homes and jobs and requires considerable emergency and recovery spending. The economic impact is disproportionately felt by disadvantaged and vulnerable populations, resulting in increased rates of poverty and income disparity. As with malnourishment, this is a problem by itself, but it also can lead to increased aggression: Income disparity can lead to life dissatisfaction, resentment, dissent, a desire for retribution, and even violence. In one example, political scientists Christopher K. Butler and Scott Gates (2012) studied the impact of adverse weather on East African cattle herders. They developed a model, grounded in game theory, of conflict in the region that takes into account available resources, their distribution, property rights, and the role of the state. The authors conclude that droughts and climate-driven resource shortages lead to income disparity increased among the herders, which, in turn, foments resentment and conflict that manifests itself as the banditry and retaliatory aggression often seen in the region.
In fact, many of these same climate-change-driven factors aid in terrorism recruitment: Uncertainty and frustration about one’s livelihood, seeing others who seem unfairly unaffected, and the belief that there are no other viable options to sustain oneself may all contribute to terrorism. These factors are thought to have played a role in regional conflicts such as those in Sierra Leone, Palestine, and Managua. And, given that droughts and other climate-driven natural disasters already are increasing in intensity and frequency (as was predicted years ago by climate-change models), it seems likely that conflict and violence will continue to worsen as resources become scarcer and wealth disparity increases.
Rapid Climate Change and Intergroup Conflict: War and Civil War
That rapid climate change will (and already is) negatively affecting the livelihoods and aggressive tendencies of individuals is obvious, but it is informative to consider how entire populations respond to these effects. Among the most prominent group-level effects anticipated is ecomigration, where entire groups migrate in response to the physical, economic, or political instability brought about by an ecological disaster. Although ecomigration is not, in and of itself, a sign of aggression, it can lead to hostility and conflict through a sudden increase in competition for an area’s resources, bringing together people with opposing or incompatible worldviews, concerns about the intentions of both the migrant group and the local population, and a host of socioeconomic issues. Indeed, there are numerous historical examples of climate disasters leading to ecomigration, war, and even dynastic collapse.
As a recent example, consider the possible role of droughts on ecomigration and conflict in the Syrian civil war. After a very unusual years-long drought (now seen by many as climate-change induced) destroyed much of the country’s arable land and cattle, rural farmers and herders migrated en masse to cities. Unrest regarding the government’s perceived role in the disaster and failure to do more to help grew, creating conditions that were fertile for conflict and terrorism. A comparable drought in Uganda similarly led to an escalation in food prices, violent internal strife, and mass migration of more than a million people, who clashed with armed cattle herders from Sudan who were fleeing from the same drought. Kenya, Sudan, and Ethiopia also have seen similar climate-driven conflicts, leading researchers to predict, based on models that include decades of data and dozens of countries, that civil wars, protests, coups, rebellions, riots, and large-scale conflicts are all likely to rise as temperatures increase and as changes in rainfall become increasingly extreme.
Ecomigration-driven conflict should not be seen solely as an African or Middle Eastern issue, nor is it one exclusively limited to droughts. A confluence of socioeconomic factors and environmental disasters in the past 6 decades has led to the cumulative migration of more than 10 million Bangladeshis into India. This influx of migrants was a source of continuous tension in the region, as many Indians believed the migrants were stealing farmland. Ultimately, the tension led to a rampage in 1983 that left 1,700 Bengali migrants dead. And more recently, Hurricane Katrina displaced hundreds of thousands of Americans, many of whom fled to neighboring states seeking refuge. Homicide rates in cities where refugees were taken in rose in the following months, and polls suggested that tensions were mounting between refugees and residents. Federal aid and other moderating factors prevented these tensions from escalating into armed conflicts, but the incident stands as an example of the role climate change plays in violent behavior. Indeed, there is a growing research literature examining the relationship between weather-related (and therefore climate-change related) disasters and outbreaks of violence. A recent study in Science reported on the huge increase in “water conflict events” that have occurred in the last decade or so.
It is easy to overlook the relevance of psychological science to climate change. Therefore, it is unsurprising that many people also overlook the important role that psychological scientists can play in reducing climate change and its effects. One obvious way is to apply what we know about attitude change, decision-making, and behavior change to help educate the general population (e.g., public service announcements, teaching modules), public policymakers, and politicians. For example, psychological studies show that fostering a long-term perspective in people makes them more likely to consider their legacy and engage in more proenvironmental behavior. Other psychologists have found that when you frame climate change in global terms, rather than in terms of specific, localized disasters, people become more peaceful and reconciliatory — something that could prove very useful as a means of counteracting the effects of climate change on aggression. Clearly, there is a need, and indeed there are many ways, for psychologists to weigh in on the issue of climate change and its relation to violent behavior.
In the future, psychological scientists also may find themselves conducting more interdisciplinary research — working hand-in-hand with climatologists, political scientists, and economists. Some of the best psychological studies on the relationship between temperature and aggression have proven just how fruitful it can be to integrate climatological data into analyses of behavioral data. Incorporating techniques and data from other fields may help to build more accurate models of climate-change effects that include subtler, less-frequently considered outcome variables. An interdisciplinary approach also may prove vital in bridging the gap between what scientists know, what the general public believes, and what government policies exist.
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A few risk factors for youth violence occur before birth. Others come into play as the child develops in response to his or her family and surroundings. Thus, most of the risk factors that exert an effect before puberty are found in the individual and family domains rather than in the larger world, a situation that changes dramatically in adolescence. Childhood risk factors are listed by domain in Box 4-1; effect sizes are listed in Table 4-1.
The most powerful early risk factors for violence at age 15 to 18 are involvement in general offenses and substance use before age 12. General offenses include serious, but not necessarily violent acts, such as burglary, grand theft, extortion, and conviction for a felony. Children engaging in such crimes often come to the attention of the police and juvenile justice system. Numerous studies have documented the overlap between serious nonviolent and violent offenses in adolescence, so early involvement in serious offenses carries a substantial risk for violence later.
Experimentation with drugs, alcohol, tobacco, or some combination of these substances is not particularly unusual by age 18, but use of these substances by children under the age of 12 is. Not only are these substances harmful to health, they are illegal. Thus, use of these substances signals antisocial attitudes and early involvement in a delinquent lifestyle that often comes to include violent behavior in adolescence (Fagan, 1993).
Two moderate risk factors emerge in childhood, being male and aggression. Boys (and young men) are far more likely than girls to be violent (see Chapter 2), yet some researchers have suggested that sex is a risk marker rather than a risk factor (Earls, 1994; Hawkins et al., 1998a; Kraemer et al., 1997). A risk marker is a characteristic or condition that is associated with known risk factors but exerts no causal influence of its own (Earls, 1994; Patterson & Yoerger, 1997). 5 For example, many more boys than girls are hyperactive, a risk factor with a small effect size, so some of the predictive power of being male may actually be the influence of hyperactivity. Moreover, boys have traditionally been exposed to more violence than girls, and socially approved male role models are more aggressive, suggesting that social learning plays a role in this risk factor. However, research indicates that being male confers risk even after accounting for other known risk factors. This suggests that being male is a risk factor rather than a risk marker, perhaps with some biological or biological-environmental interaction as the causal mechanism.
Many studies have found aggression -- characterized as aggressive and disruptive behavior, verbal aggression, and aggression toward objects -- to be a moderate risk factor among boys, although there is some evidence that physical aggressiveness is actually responsible for most of the observed effect (Nagin & Tremblay, 1999). Additional research is needed to sort out the unique influence of each of these types of aggression.
The remaining individual risk factors have relatively small effect sizes. Various psychological conditions, such as hyperactivity, impulsiveness, daring, and short attention span, pose a small risk for violence. A consistent individual predictor is hyperactivity/low attention, the central components of attention-deficit/hyperactivity disorder (ADHD), a cognitive disorder that may be genetically influenced in some way (Hawkins et al., 1998a). ADHD is characterized by restlessness, excessive activity, and difficulty paying attention, traits that may also contribute to low academic performance, a risk factor in school. Hyperactivity is often found in combination with physical aggression, another risk factor. Some researchers question the independent effect of hyperactivity on later violence, suggesting that the effect is actually physical aggression (and perhaps low academic performance) that was not controlled for in earlier studies of hyperactivity (Nagin & Tremblay, 1999). There is little agreement about the mechanism linking hyperactivity to violence.
The effects of children's exposure to television and film violence have been studied extensively in regard to aggression, but there is relatively little research regarding the effects on more serious forms of violent behavior (for an extended discussion, see Appendix 4-B). Experimental studies have found that exposure to media violence has a small average effect size (.13) on serious forms of violence (Paik & Comstock, 1994); the average effect size in cross-sectional survey studies was very small (.06). Two frequently cited longitudinal studies have examined the effects that exposure to television violence in childhood produces on violent behavior during adolescence or early adulthood. One, in which participants reported having punched, beaten, or choked someone as young adults, found a significant predictive effect for women (.22) but no significant effect for men (Huesmann et al., (submitted)). The other study, in which teenage males reported being involved in a knife fight, car theft, mugging, gang fight, or similar delinquent behavior, found a statistically significant predictive effect in only one of nine tests (Milavsky et al., 1982). Exposure to violence appears to have a weak predictive effect on relatively immediate violence in experimental studies, but there is little consistent evidence to date for a long-term predictive effect.
Little research has been done on violence in other media -- video games, music videos, and the Internet. A recent meta-analysis by Anderson and Bushman (in press) reports that video game violence has a small average effect size (.19) on physical aggression in experimental and cross-sectional studies. Theoretically, the influence of these interactive media might well be greater than that of television and films, which present a passive form of exposure, but there are no studies to date of the effects of exposure to these types of media violence and violent behavior.
Problem behavior, another risk factor with a small effect size, refers to relatively minor problem behaviors such as stealing, truancy, disobedience, and temper tantrums. While not serious in themselves, antisocial behaviors may set the stage for more serious nonviolent or violent behavior later.
The medical or physical risk factor includes a number of conditions that as a group are somewhat predictive of violence. Prenatal and early postnatal complications, a more specific set of medical conditions, have been found to have inconsistent effects across a number of studies (Hawkins et al., 1998c). These complications encompass a broad group of genetic conditions or physical injuries to the brain and nervous system that interfere with normal development, including low birth weight, oxygen deprivation, and exposure to toxins such as lead, alcohol, or drugs (Hawkins et al., 1998b). Low resting heart rate, a condition that has been studied primarily in boys, is associated with fearlessness or stimulation seeking, both characteristics that may predispose them to aggression and violence (Raine et al., 1997; Hawkins et al., 1998c), but there is not enough evidence to establish this condition as a risk factor for violence. Some studies have even questioned its effects on aggression (Van Hulle et al., 2000; Wadsworth, 1976; Kindlon et al., 1995). There is also no evidence that internalizing disorders -- nervousness and withdrawal, anxiety, and worrying -- are related to later violence (Hawkins et al., 1998c).
Low IQ, or low intelligence, includes learning problems and poor language ability. This risk factor has a small effect size and is often accompanied by other risk factors with small effect sizes, such as hyperactivity/low attention and poor performance in school.
Antisocial beliefs and attitudes, including dishonesty, rule-breaking, hostility to police, and a generally favorable attitude toward violence, usually constitute a risk factor in adolescence, not childhood (Hawkins et al., 1998c). Only dishonesty in childhood is predictive of later violence or delinquency, and its effect is small.
There are no known strong risk factors for youth violence in the family domain, but low socioeconomic status/poverty and having antisocial parents are moderate factors. Socioeconomic status generally refers to parents' education and occupation as well as their income. Poorly educated parents may be unable to help their children with schoolwork, for example, and children living in poor neighborhoods generally have less access to recreational and cultural opportunities. In addition, many poor families live in violent neighborhoods, and exposure to violence can adversely affect both parents and children, as described above. Limited social and economic resources contribute to parental stress, child abuse and neglect, damaged parent-child relations, and family breakup -- all risk factors with small effects in childhood.
Studies suggest that antisocial parents -- that is, violent, criminal parents -- represent an environmental rather than a genetic risk factor (Moffitt, 1987). In other words, children learn violent behavior by observing their parents rather than by inheriting a propensity for violence. In fact, attachment to parents, a possible protective factor, can have the opposite effect if the parents are violent (Hawkins et al., 1998c).
Among the early risk factors with small effect sizes on youth violence is poor parent-child relations. One specific risk factor in this class -- harsh, lax, or inconsistent discipline -- is also somewhat predictive of later violence (Hawkins et al., 1998c). Children need reasonable, consistent discipline to establish the boundaries of acceptable and unacceptable behavior. Children who are treated harshly may view rough treatment as acceptable, those who are given no guidance may engage in whatever behavior gets them what they want, and children who receive mixed signals are completely at sea regarding appropriate behavior. Other family conditions, such as high stress, large size, and marital discord, also exert a small effect on later violence.
Another childhood predictor with a small effect size is broken homes, a category that includes divorced, separated, or never-married parents and a child's separation from parents before age 16. Separation from parents also operates as a distinct risk factor, again with a small effect size.
Abusive parenting in general and neglect in particular are predictors of later violence, but they have very small effect sizes. Neglect operates as a distinct risk factor, possibly because neglected children are less likely to be supervised or taught appropriate behavior. This is not to imply that child abuse and neglect do not cause serious problems in adolescence: Indeed, they have large effects on mental health problems, substance abuse, and poor school performance (Belsky & Vondra, 1987; Cicchetti & Toth, 1995; Dembo et al., 1992; Esbensen & Huizinga, 1991; Silverman, et al., 1996; Smith & Thornberry, 1995). This finding is discussed in more detail below, in the section on unexpected findings and effects.
The only early risk factor in the school domain is poor attitude toward and performance in school, and its effects are small. Numerous individual and family factors may contribute to poor performance, making it a fairly broad measure. For example, a child who is physically aggressive and is rejected by peers or who has difficulty concentrating or sitting still in class may understandably have difficulty performing academic tasks. Children who have been exposed to violence, as noted earlier, may also have trouble concentrating in school.
Young children do not socialize extensively with other children and are not strongly influenced by peers. Peers become more important as children progress through elementary school, although school-age children still look primarily to parents for cues on how to behave. Nonetheless, weak social ties to conventional peers and associating with antisocial peers both exert small effects in childhood.
Children with weak social ties are those who attend few social activities and have low popularity with conventional peers. School-age children often reject physically aggressive children because of their inappropriate behavior (Hann & Borek, in press; Reiss & Roth, 1993). The combination of rejection and aggressiveness exacerbates behavior problems, making it more difficult for aggressive children to form positive relationships with other children. Indeed, recent research indicates that children who are both aggressive and rejected show poorer adjustment in elementary school than children who are aggressive, rejected, or neither (Hann & Borek, in press).
Being drawn to antisocial peers may introduce or reinforce antisocial attitudes and behavior in children. Indeed, aggressive children tend to seek each other out (Hann & Borek, in press).
Community risk factors, such as living in socially disorganized neighborhoods or neighborhoods with high rates of crime, violence, and drugs, are not powerful individual-level predictors in childhood because these external influences have less direct impact on children than on adolescents. They may well exert indirect influences through poor parenting practices, lack of family resources, and parent criminality or antisocial behavior.