His tortured and sadistic grin beamed like a full moon on that dark night. "Madness, as you know, is like gravity," he cackled. "All it takes is a little push." But once the house lights rose, the terror was lifted for most of us. Few imagined that the fictive evil on screen back in 2008 would later inspire a depraved act of mass murder by a young man sitting with us in the audience, a student of neuroscience whose mind was teetering on the edge. What was it that pushed him over?

In the wake of the tragedy that struck Aurora, Colorado last Friday there remain more questions than answers. Just like last time--in January, 2011 when Congresswoman Gabrielle Giffords and 18 others were shot in Tucson, Arizona or before that in April, 2007 when a deranged gunman attacked students and staff at Virginia Tech--this senseless mass shooting has given rise to a national conversation as we struggle to find meaning in the madness.

While everyone agrees the blame should ultimately be placed on the perpetrator of this violence, the fact remains that the United States has one of the highest murder rates in the industrialized world. Of the 34 countries in the Organisation for Economic Co-operation and Development (OECD), the U.S. ranks fifth in homicides just behind Brazil (highest), Mexico, Russia, and Estonia. Our nation also holds the dubious honor of being responsible for half of the worst mass shootings in the last 30 years. How can we explain why the United States has nearly three times more murders per capita than neighboring Canada and ten times more than Japan? What makes the land of the free such a dangerous place to live?

Diagnosing Murder

There have been hundreds of thoughtful explorations of this problem in the last week, though three in particular have encapsulated the major issues. Could it be, as science writer David Dobbs argues at Wired, that "an American culture that fetishizes violence," such as the Batman franchise itself, has contributed to our fall? "Culture shapes the expression of mental dysfunction," Dobbs writes, "just as it does other traits."

Perhaps the push arrived with the collision of other factors, as veteran journalist Bill Moyers maintains, when the dark side of human nature encountered political allies who nurture our destructive impulses? "Violence is our alter ego, wired into our Stone Age brains," he says. "The NRA is the best friend a killer's instinct ever had."

But then again maybe there is an economic explanation, as my Scientific American colleague John Horgan believes, citing a hypothesis by McMaster University evolutionary psychologists Martin Daly and his late wife Margo Wilson. "Daly and Wilson found a strong correlation between high Gini scores [a measure of inequality] and high homicide rates in Canadian provinces and U.S. counties," Horgan writes, "blaming homicides not on poverty per se but on the collision of poverty and affluence, the ancient tug-of-war between haves and have-nots."

In all three cases, as it was with other culprits such as the lack of religion in public schools or the popularity of violent video games (both of which are found in other wealthy countries and can be dismissed), commentators are looking at our society as a whole rather than specific details of the murderer's background. The hope is that, if we can isolate the factor which pushes some people to murder their fellow citizens, perhaps we can alter our social environment and reduce the likelihood that these terrible acts will be repeated in the future. The only problem is, which one could it be?

To Err is Non-Human

Just as it is with so many other issues in our species--infanticide, sexual coercion, or collective violence--I believe we can most successfully pinpoint the broad patterns in our behavior by thinking like a primate. In most social primates (including humans) males frequently engage in aggressive competition over status with other males in their group, maiming and sometimes even killing in the process. Naturally, there is a good reason for this: sex.

In chimpanzees, for example, Cristina Gomes and Christophe Boesch have documented that the two most common reasons females will choose to have sex with a male is if that male had shared meat with them in the past or if they were high-ranking. In some species, such as hamadryas baboons, the male obsession with status has taken an extreme form. Males of this species are nearly twice the size of females because, over evolutionary time, those males that were slightly larger than others had a competitive advantage and passed on more copies of their genes as a result. Of course, all of this male-male aggression comes at a price.

"The social life of a male baboon can be pretty stressful," writes Stanford University neuroscientist Robert Sapolsky in his book Why Zebras Don't Get Ulcers, "you get beaten up as a victim of displaced aggression; you carefully search for some tuber to eat and clean it off, only to have it stolen by someone of higher rank; and so on." Over time the build up of stress hormones, known as glucocorticoids, can cause serious physiological damage and the development of stress-related illnesses. But the most common result whenever a male loses a fight, or is harassed by a higher-ranking male, is to displace that aggression elsewhere (typically on someone smaller). "Stress-induced displacement of aggression," Sapolsky writes, "works wonders at minimizing the stressfulness of a stressor. It's a real primate specialty as well."

There are distinct personality styles in baboons that influence how they will react to this form of social stress. Some males are what are called "high-reactors" and see potential threats everywhere whereas others, even if they lose a struggle over status, are able to shake it off and contentedly groom another member in their troop. High-reactors can further be divided into those who externalize this stress by attacking at every opportunity and those who internalize, nervously withdrawing from others or even displaying behaviors that, if they were human, would be indications of neuropathology.

Similar results have been found in rhesus macaques by Stephen Suomi at the National Institutes of Health in Bethesda, Maryland, who determined that approximately 20 percent of these monkeys were high-reactors. What's more, he found that infant monkeys were likely to share this trait with their fathers even when the father wasn't around to influence their behavior, suggesting a genetic component to highly reactive personalities. But there was an equally strong case to be made for environmental factors. When the sons of highly reactive males were placed with unusually nurturant mothers this personality trait was completely prevented. This means that borderline personalities in primates are formed by a combination of nature as well as nurture.

This influence that the social environment can have on primate behavior was dramatically demonstrated by Sapolsky in his 2004 paper co-authored with Lisa Share in the journal PLoS Biology. In a unique natural experiment a group of baboons known as Forest Troop began feeding at the contaminated dump site of a Western safari lodge. As had occurred elsewhere, the largest and most aggressive males dominated the food source. But this time their despotic behavior resulted in untimely death after they all contracted tuberculosis. In the intervening years Forest Troop developed a culture in which cooperation was rewarded more than aggression and adolescent males who migrated into the troop adopted this culture themselves. Remarkably, the level of stress and stress-related behaviors in low-ranking males were dramatically reduced after the outbreak (and remained significantly lower than the nearby Talek Troop that retained its most aggressive males).

"Males had high rates of affiliative behaviors, and low-ranking males were subject to low rates of aggressive attack and subordination by high-ranking males," wrote the authors. "Precedent for this unexpected implication comes from the social epidemiology literature concerning 'social capital,' in which health and life expectancy increase in a community as a function of communitywide attributes that transcend the level of the individual or individual social networks."

In other words, a culture emphasizing less aggression and with closer bonds between individuals throughout the community formed the basis for a more egalitarian society.

The Exceptionalism of American Violence

As it turns out, the "social capital" Sapolsky found that made the Forest Troop baboons so peaceful is an important missing factor that can explain our high homicide rate in the United States. In 1999 Ichiro Kawachi at the Harvard School of Public Health led a study investigating the factors in American homicide for the journal Social Science and Medicine (pdf here). His diagnosis was dire.

"If the level of crime is an indicator of the health of society," Kawachi wrote, "then the US provides an illustrative case study as one of the most unhealthy of modern industrialized nations." The paper outlined what the most significant causal factors were for this exaggerated level of violence by developing what was called "an ecological theory of crime." Whereas many other analyses of homicide take a criminal justice approach to the problem--such as the number of cops on the beat, harshness of prison sentences, or adoption of the death penalty--Kawachi used a public health perspective that emphasized social relations.

In all 50 states and the District of Columbia data were collected using the General Social Survey that measured social capital (defined as interpersonal trust that promotes cooperation between citizens for mutual benefit), along with measures of poverty and relative income inequality, homicide rates, incidence of other crimes--rape, robbery, aggravated assault, burglary, larceny, and motor vehicle theft--unemployment, percentage of high school graduates, and average alcohol consumption. By using a statistical method known as principal component analysis Kawachi was then able to identify which ecologic variables were most associated with particular types of crime.

The results were unambiguous: when income inequality was higher, so was the rate of homicide. Income inequality alone explained 74% of the variance in murder rates and half of the aggravated assaults. However, social capital had an even stronger association and, by itself, accounted for 82% of homicides and 61% of assaults. Other factors such as unemployment, poverty, or number of high school graduates were only weakly associated and alcohol consumption had no connection to violent crime at all. A World Bank sponsored study subsequently confirmed these results on income inequality concluding that, worldwide, homicide and the unequal distribution of resources are inextricably tied. (see Figure 2). However, the World Bank study didn't measure social capital. According to Kawachi it is this factor that should be considered primary; when the ties that bind a community together are severed inequality is allowed to run free, and with deadly consequences.

But what about guns? Multiple studies have shown a direct correlation between the number of guns and the number of homicides. The United States is the most heavily armed country in the world with 90 guns for every 100 citizens. Doesn't this over-saturation of American firepower explain our exaggerated homicide rate? Maybe not. In a follow-up study in 2001 Kawachi looked specifically at firearm prevalence and social capital among U.S. states. The results showed that when social capital and community involvement declined, gun ownership increased (see Figure 3).

Kawachi points out that it is impossible to prove whether one factor caused the other, but the most reasonable interpretation is that people who don't trust their neighbors are more likely to think guns will provide security. In this way the number of guns and the number of homicides both stem from the same root, suggesting that guns don't cause murders anymore than cars cause fatal accidents. This was also the conclusion of a policy paper conducted by the London-based Centre for Economic Policy Research in 2005 that found no support for the argument that more guns cause more homicides. "The appearance of such an effect in past research," wrote the authors, "appears to be the product of methodological flaws." Unfortunately, gun control may not save us after all.

The same can also be said for violent movies as a cause of violence, according to Texas A&M University psychologist Christopher Ferguson in his 2009 book Violent Crime: Clinical and Social Implications. "Does violent media availability and exposure in a culture relate to levels of violence in that culture?" he asks. "If so, then removing violent media would appear to be an easy way to reduce societal violence. Disappointingly, the answer is clearly no."

University of California economists Gordon Dahl and Stefano DellaVigna go even further and conclude that violent movies actually decrease the amount of violent crime. Because those who are more violence prone are more likely to seek out violent entertainment as a substitute, "violent movies deter almost 1,000 assaults on an average weekend." Furthermore, since Hollywood films make up to three times more money internationally than they do at home, it's hard to understand how these movies would only influence violence in the United States. Anyway, despite the clear increase in violent entertainment during the last forty years, the level of violent crime in the United States has decreased (though it still remains high compared to other countries). It would appear that Batman is not to blame either.

Rebuilding Gotham

The clear implication is that social capital followed by income inequality are the primary factors that influence the rate of homicidal aggression. Does this mean that John Horgan's "modest proposal" of state-sponsored socialism is the answer? Is aggression caused by inequality, and can we reduce its prevalence by imposing a level playing field?

"My tendency would be to reverse the causality," said Frans de Waal, C. H. Candler Professor of Primate Behavior at Emory University in Atlanta, Georgia, via e-mail. "In order to have a highly skewed distribution of resources or reproductive privileges you will need a lot of aggression to maintain it. So, it's not the inequality that causes aggression, but the other way around." As primates we don't simply respond to our environment; we actively build it through our interactions with others and the shared culture we create in a process known as niche construction, to use the technical jargon. And, as any baboon can tell you, what we construct isn't always good for the least among us. Fortunately, as Forest Troop has demonstrated, there is no law of nature forcing things to stay that way.

The high level of inequality, both within the United States as well as between countries globally, was constructed through a process of social interactions. It can be deconstructed the same way. If the interpretation from social capital is correct, it suggests that building relationships through our schools, labor unions, farmers' markets, and gun ranges, at City Hall and the State House, or active participation in our churches, temples, and mosques, can ultimately make us all more secure. But at the same time it means collectively challenging the policies of those high-ranking members in our society whose obsession with status leaves the rest of us completely stressed out.

Remarkably, this kind of social activism is the single most important factor associated with reduced violence for any neighborhood in the world. According to University of Washington sociologists Blaine Robbins and David Pettinicchio, in the first global study to examine social capital and homicide, only social activism consistently predicts homicide at the national, neighborhood, and individual levels.

This is because politically oriented individuals are also more likely to serve the needs of their community and assist in collective endeavors aimed at reducing crime. All of which follows the classic Tocquevillian premise: a willingness to take part in political affairs generates a willingness to contribute to the common good, including the production and maintenance of a safe and secure society.

Are we up to the challenge? In case anyone doubts it just consider the selfless acts of heroism that were on display at around 12:20am Friday morning. With smoke filling the air and bullets flying, Jon Blunk, Matt McQuinn, and Alex Teves all sacrificed their lives to protect the people who were most important to them. Each reacted instantly by covering their loved ones with their own bodies and taking the bullets that were otherwise intended for the person beneath.

It is this capacity for altruism that distinguishes us from nearly all other primates. How many of these lone and deranged gunmen, quietly secluded from the world like brutalized baboons, could have been redirected along a different path if there had been a community that made them feel secure? Our species is uniquely qualified to engage in activities that promote the public good; all we need is a little push.