Isnt it kind of scary that one man could wreak this kind of hell? Timothy J McVeigh said of the 1995 Oklahoma City bombing, which killed 168 people. Indeed, it is. Automatic weapons and potent bombs allow the deranged and begrudged to slaughter scores of innocents in mere seconds. Witness the shootings at the University of Texas in 1966 (14 killed), Columbine High School in 1999 (13 killed), Virginia Tech in 2007 (32 killed), or, for those with a sense of history, see Bath School disaster of 1927 in which an angry school board member blew up 38 children and 6 adults in Michigan. The recent massacre in Norway fits this nefarious pattern, although in number of victims it eclipses all of its antecedents except Oklahoma City. It is not only the firepower available to the unhinged that is scary. Our inability to confront a simple but unacknowledged truth is equally troubling. Most threats and violence tend to emerge from within a society, not from outside it. John F Kennedy, Anwar el-Sadat and Yitzhak Rabin were all assassinated by their fellow countrymen. Cautious citizens may push for better street lighting, but they have more to fear from a spouse, ex-spouse, friend or co-worker than from a stranger on the street. We prefer, however, to imagine threats as emanating from aliens and foreigners. Talk of a clash of civilisations is, while inaccurate, oddly reassuring because it suggests that the enemies are outsiders who can be easily identified. In Oklahoma City as in Oslo, authorities initially assumed the perpetrators were extremists or jihadists. This reflex continues to bedevil both popular and academic thought. Those who hate President Obama cannot accept that he was born and raised an American; in their eyes, he has to be a foreigner. Edward W Saids 1978 book Orientalism engendered a small industry of scholars documenting how we perceive the outsider or other. But in Oklahoma City as in Oslo the actors are natives and citizens. Increasingly, civil wars and strife have become more common and more lethal than wars between states, which have grown ever less frequent. The Syrian army shoots Syrians and like any good Orientalists, the authorities attribute the protests roiling the region to outsiders. Syria is a target of a big plot from outside, its president, Bashar al-Assad, has charged. Brutal civil wars are hardly unique to Africa or the Middle East. More American lives were lost in the Civil War when the population of the United States was a tenth of what it is today than in any other conflict. In the 20th century, civil wars in Russia, China and Spain collectively led to millions of deaths. Nor does the situation change if one adds genocide to the equation. With some exceptions genocide involves closely related groups, not outsiders. See Rwanda or Cambodia or even Europe, where the term genocide originated. The Jews of Germany were extraordinarily assimilated, as they were generally throughout Europe. Far from standing out as oddballs, they excelled in conventional German pursuits and professions. Not for nothing did the Princeton historian Jan T Gross title his book about the killing of Jews in a Polish town Neighbors. Despite the commandment to love thy neighbor, why do we frequently hate him (or her)? Proximity is certainly one reason. A neighbors barking dog or loud music elicits more anger than the imagined threat of an unseen stranger. The evolutionists would probably highlight competition over limited resources. Foreigners might endanger our livelihood abstractly, but countrymen do so directly. In both mythology and reality brothers compete over the patrimony, be it a cow or a castle. Things easily turn violent. Yet perhaps something more important is at work, what the maligned master, Freud, dubbed the narcissism of minor differences. Small variations frequently elicit more rage than large ones because they imperil identity. They challenge our sense and love of self. We like to believe that big conflicts require big causes, but we ignore that in fact it is small differences that most regularly spark hatred. The first act of violence in the Judeo-Christian narrative is a fratricide. Cain seemed upset that his offerings were less appreciated than those of Abel, but Cains anger even puzzled God. Fratricidal violence opens the book of history and reappears constantly. Similarities among people give rise to antagonism, not harmony. This might appear to take us far from the killings in Norway. And yet, to the degree that Anders Behring Breivik is guilty and acted alone, he illuminates this uncomfortable truth that the rancor originates very often among kith and kin, not among strangers and targets fellow citizens. A Norwegian citizen with Norwegian parents slaughtered some 76 of his countrymen. Hes one of us, a Norwegian scholar said of Mr Breivik. That makes the tragedy both scary and familiar. Russell Jacoby, a historian at the University of California, Los Angeles, is the author of Bloodlust: On the Roots of Violence From Cain and Abel to the Present. International Herald Tribune