Income inequality can cause all kinds of problems across the economic spectrum—but perhaps the most frightening is homicide. Inequality—the gap between a society's richest and poorest—predicts murder rates better than any other variable, according to Martin Daly, a professor emeritus of psychology at McMaster University in Ontario, who has studied this connection for decades. It is more tightly tied to murder than straightforward poverty, for example, or drug abuse. And research conducted for the World Bank finds that both between and within countries, about half the variance in murder rates can be accounted for by looking at the most common measure of inequality, which is known as the Gini coefficient.

The murders most associated with inequality, it seems, are driven by a perceived lack of respect. Like most killings, these are mostly perpetrated by males—and in societies with low inequality, there tend to be very few murders. To an outsider, these deaths, which make up more than a third of the homicides with known motives reported to the FBI, seem senseless: a guy looks at someone else the wrong way, makes a disrespectful remark, or is believed to have winked at another man's wife or girlfriend. These incidents seem too trivial to be matters of life and death. “A prosperous guy like me, if someone [insults me] in a bar, I can roll my eyes and leave,” Daly says. “But if it's your local bar, you are unemployed or underemployed, and your only source of status and self-respect is your standing in the neighborhood, turning the other cheek looks weak, and everyone soon knows you are an easy mark.”

Some argue that in these cases, the real issue is poverty, not inequality. For example, William Pridemore, dean of criminology at the University at Albany, S.U.N.Y., says that the inequity correlation is a methodological artifact. He gives a theoretical example of a country in which everyone is meaningfully employed, can afford vacations and other small luxuries, and lives in a safe neighborhood with free health care—but some of them are billionaires. He asks whether this sort of place would have the same level of violence as places where those at the bottom are in abject poverty instead. Whereas the size of gaps between rungs on the financial ladder may be identical in both cases, the level of relative deprivation experienced by people at the bottom rung may not be—inequality is not just about having less when others have more; it is about how low status is perceived.

That, Daly argues, is what can make status differences deadly. The living standards of poor people in developed countries today would be beyond the dreams of kings in the past because of technology—but we do not rate our social status by comparing ourselves with medieval lords; we do so by looking at those around us.

That specific and mostly local level of comparison may, in turn, explain one of the biggest mysteries in homicide and inequality research. Why, as inequality has skyrocketed in the U.S. in recent years, has the murder rate continued to fall? One explanation is a time lag: it takes a while for people to recognize their loss of status as the middle class erodes and they either plummet downward or join the tiny minority at the top. Indeed, murder rates have lately stopped falling and may even be ticking upward. And we have seen rises in what have been labeled “deaths of despair,” such as suicide and opioid overdoses, which research also links with inequality.

Another possible explanation is that as richer people retreat into ever more exclusive communities, their virtual disappearance masks rises in local inequality that are felt by former neighbors. A society in which millions struggle to pay their student loans and make a decent living while watching U.S. secretary of education Betsy DeVos—a woman of enormous wealth—cut the education budget and protect predatory for-profit schools is unlikely to be a safe and stable one. When men have little hope of a better future for either themselves or their kids, fights over what little status they have left take on outsize power. To break the cycle, everyone must recognize that it is in no one's interest to escalate such pain.

For more on “The Science of Inequality,” see our special report.