By ancestry, John McCain is a Scots-Irishman. That is to say, he comes from one of the oldest, most admirable and most worrying ethno-cultural traditions in the US. To a remarkable extent, that tradition is reflected in McCain's character traits: his obstinancy; his tendency towards unshakeable friendship and implacable hatred; his hair-trigger temper; his deep patriotism; his obsession with American honor; and his furious response to any criticism of the US. These are not just the products of his military upbringing and experiences as a prisoner in North Vietnam, but also the result of his being the proud descendant of Indian-fighters and Confederate soldiers.

Non-Americans are not used to thinking of white Americans in terms of old ethno-cultural traditions, except when it comes to imported immigrants such as Italian-Americans. Yet the Scots-Irish cultural traits live on everywhere, from evangelical religion to country music. They have been examined by several great American scholars, including David Hackett Fischer and Kevin Phillips, as well as more popular authors like Walter Russell Mead.

Both sides of McCain's family come from the old Confederate southwest: his father's side from Missouri, his mother's from Tennessee, Texas and Oklahoma. McCain's great-great-grandfather, William Alexander "Fighting Bill" McCain, was a Confederate soldier. His paternal family took the classic Scots-Irish route in the 18th century, from Scotland, down from Virginia through the Carolinas to the old frontier in the Appalachians and beyond. McCain's mother was born in Muskogee, Oklahoma, the setting for Merle Haggard's iconic anthem of patriotic, conservative small-town America, "Okie From Muskogee," where: "We don't smoke marijuana in Muskogee/We don't take our trips on LSD/We don't burn our draft cards down on Main Street/We like livin' right, and bein' free."

The Scots-Irish tradition has been praised, with reservations, by another Scots-Irishman, Democratic senator Jim Webb, in his book Born Fighting: How the Scots-Irish Shaped America. Until he ruled himself out in early July, Webb was considered a favourite for Obama's vice-presidential pick. This would have given the election a flavour of a Scots-Irish family feud-and you can't get more combative than that.

The American Scots-Irish are the descendants of the Scottish Protestants settled in 17th-century Ulster by the Stuart kings, a process that involved the ethnic cleansing of much of the native Irish Catholic population. In the 18th century, those of the Scots-Irish who moved on to the western frontier of Britain's colonies in North America took with them a prior experience of frontier fighting and a fundamentalist identification of their cause with God. The American frontier's lawlessness, high levels of violence among white males and ferocious conflicts with the Native Americans perpetuated this culture into modern US society. In the words of a biographer of McCain's personal hero, Scots-Irish president Andrew Jackson: "It appears to be more difficult for a North-of-Irelander... to allow an honest difference of opinion in an opponent, so that he is apt to regard the terms opponent and enemy as synonymous." Similar things have often been said about McCain.

The Scots-Irish tradition belongs above all to the "greater south," and is indeed at the core of most white southern traditions. The southern historian Grady McWhiney has gone so far as to attribute most of the cultural difference between the south and the rest of the US to the Scots-Irish heritage. The most enduring political reflection of this has been "Jacksonian nationalism," named after President Jackson, whose career was shaped by ruthless conflict with Native Americans and their British, French and Spanish backers. Jacksonian nationalism has been described by Walter Russell Mead as one of the four key historical strands of US foreign and security policy.

The first defining character of the frontier was of course conflict with Native Americans, in which both sides committed appalling atrocities. This has bred in sections of the American tradition a capacity for ruthlessness and a taste for unqualified victory. The second was constant expansionism, often pushed for by the white frontier populations against the wishes of Washington administrations.

The frontier also helped keep alive a cult of personal weaponry associated with a certain kind of egalitarianism and belief in every man's right to defend his honor-a classic theme of Hollywood westerns, but one with real roots in the southern and frontier traditions.

One of my favourite stories of upper-class southern violence in the 19th century comes from the family history of William Faulkner (an old Ulster Protestant name; Faulkner added the "u")-a history which renders the lurid subject matter of some of his novels more comprehensible. In 1848, Faulkner's great-grandfather William C Falkner stabbed and killed a friend of his, another Mississippi gentleman, in an election dispute-one of several killings in the course of his life. The unusual aspect of this otherwise commonplace occurrence was that the election in question was to the local chapter of the Sons of Temperance.

As someone remarked of Appalachian society in the 1890s, "It has been found impossible to convict men of murder... provided the jury is convinced that the assailant's honor was aggrieved and that he gave his adversary notice of his intention to assail him." John Shelton Reed has described this as a tradition of "lawful violence": a socially sanctioned response to certain actions that observes codes and limits. This is related to the idea of the community right to administer "justice" when the state is unwilling or unable to follow the popular will. Lynching is most associated with the terrorization of blacks in the south in the century after the civil war, but the practice on the frontier was much older, and was usually deployed against deviant whites (as well as Native Americans).

Of course, these attitudes have faded greatly in the south and west, but they still stand out in these regions compared to the rest of the US, let alone the rest of the developed world. According to one 2003 study-reportedly based in part on heroic researchers at US universities bumping into students, shouting "asshole" at them, and then comparing their reactions to their states of origin-"students from the southern part of the US reacted far more aggressively than those from the north... and in tests regularly suggested more belligerent solutions to problems. America, it seems, remains culturally divided along the Mason-Dixon line."

This has had obvious effects on US attitudes in the wake of 9/11, when the world has come more than ever to seem to many Americans like a lawless frontier populated by alien savages.

Of course, it would be wrong to see the Scots-Irish of today as forming some tight community with uniform cultural traits. As their culture has spread to influence much of white culture in the US heartland, so it has weakened. Though both McCain and Webb stress their faith, neither espouses the fundamentalist religion at the core of the Scots-Irish tradition.

Nonetheless, there is enough of the Scots-Irishman in McCain to make me nervous about how he would behave as president. This is less because of his policies, which differ less from Obama's than it may appear from the election campaign (with admittedly the immense exception of Iran). Rather, my nerves stem from McCain's possible reaction to unexpected events, shocks and provocations, of which there will be plenty in the years to come.

For example, McCain's frequently expressed loathing of Putin's Russia and his deep attachment to Georgia and its president Mikheil Saakashvili could have serious implications if Georgia and Russia go to war over Abkhazia and South Ossetia. McCain may not set out to attack Pakistan, but how would he respond in the face of an increase in attacks on US troops by Taliban forces based there? In a crisis, where would McCain's explosive temper take him?

McCain's temper is the stuff of legend in Washington. Republican senator Thad Cochran recalls a confrontation with a Sandinista rebel in which McCain "got mad at the guy and he just reached over there and snatched him."

To sum up both McCain and his ethnic tradition in an old nutshell, one might say that there is no one better to have on your side in a fight--and no one more likely to get you into one.