Historically, only a small minority of murders of unrelated adults have had their origins in long-term, hostile relationships. People who killed non-relatives who lived with them—boarders, landlords, slaves, servants, masters, mistresses—had deep-seated, personal reasons for doing so. Killer and victim were bound together emotionally, interacted with each other almost daily, and could not easily sever ties. The vast majority of murders committed by women have always been personal.However, in the vast majority of homicides of non-relatives in both America and Western Europe there was no long-term, hostile relationship between murderer and victim. The violence wasn’t generated by the relationship between the victim and the perpetrator. The killers were already predisposed to violence. They were already prepared to view people as enemies or rivals or prey.The strength and prevalence of that predisposition determines whether men in a given society are homicidal or non-homicidal, whether they are emotionally prepared to be violent at the slightest provocation or whether they refrain from violence even if they are brutalized or humiliated.Where does the predisposition to violence come from? What causes men to be so alienated that they can kill passersby for money or sex? What causes men to view every encounter with another man as having the potential to be a life-and-death struggle for supremacy or self-preservation?The predisposition to violence is not rooted in objective social conditions. Men who are poor, oppressed, or unemployed can be disposed to violence in one historical situation and nonviolence in another. The same is true of men who are well-off. The predisposition to violence is rooted in feelings and beliefs.Four correlations emerge from an examination of homicide rates in parts of the United States and Western Europe throughout the past four centuries.One, a correlation with a belief that government is stable and that its legal and judicial institutions are unbiased and will redress wrongs and protect lives and property;Two, a correlation with a feeling of trust in government and the officials who run it, and a belief in their legitimacy;Three, a correlation with patriotism, empathy, and fellow feeling arising from racial, religious, or political solidarity;Four, a correlation with a belief that the social hierarchy is legitimate, that one’s position in society is or can be satisfactory and that one can command the respect of others without resorting to violence.The feelings and beliefs in these correlations are closely related—especially the first three; the absence of one usually means at least a partial absence of another. They also have synergistic relationships with the homicide rate. When the homicide rate rises, for instance, because of a loss of government legitimacy or a decline in fellow-feeling, the rise in homicide itself can undermine the belief that government can protect lives and that citizens care about each other and thereby bring about a further increase in homicide.An increase in homicide can also change the character of a society’s social hierarchy and make violence a means of winning respect. Homicide rates can then soar into hundreds per 100,000 adults per year. Alternatively, when nearly all citizens believe their government is stable and legitimate, when they feel a strong bond with their fellow citizens, and when they believe their society’s social hierarchy is just and violence is not necessary for respect, homicide rates can fall below 2 per 100,000 adults per year. In most societies, these beliefs and emotions have been neither entirely absent nor widely shared, which is why most historical homicide rates have fallen between the extremes.Encouragingly, these conclusions are supported by the findings of scholars in other fields—notably Tom Tyler in psychology, Gary LaFree in criminology, and the later Roger Gould in sociology—who have discovered independently that political legitimacy and political stability can deter homicide (and other crimes) among unrelated adults.Criminologist Gary LaFree, for example, confirms the fundamental importance of feelings and beliefs when he points out that of all the variables social scientists have collected data on in the past fifty years, homicide rates among unrelated adults in the United States have correlated perfectly with only two: the proportion of adults who say they trust their government to do the right thing and the proportion who believe most public officials are honest. When those proportions fell, as they did in the late 1960s and early 1970s, the homicide rate among unrelated homicide rate rose.As I say in the foreword to American Homicide, the theory we are developing is after all a “working hypothesis.” I plan to test it further, and I hope other researchers will put it at risk in against new evidence.