Compared with the rest of the world, the United States is a young country. Its people left many of their traditional social structures behind, crossed vast oceans, and started anew. So to understand the lives of the majority of people around the world, who live within institutions that have shaped human existence for centuries, Americans need to make a special effort to see things from a very different perspective.

All too often, the US carries out foreign policy with little comprehension of the societies it confronts. This can lead to unintended – often destructive – results.

One central element of the Iraqi social fabric that most Americans know little about is its astonishing rate of cousin marriage. Indeed, half of all marriages in Iraq are between first or second cousins. Among countries with recorded figures, only Pakistan and Nigeria rate as high. For an eye-opening perspective about rates of consanguinity (roughly equivalent to cousin marriage) around the world, click on the "Global Prevalence" map at www.consang.net.

But who cares who marries whom in a country we invade? Why talk to anthropologists who study that arcane subject? Only those who live in modern, individualistic societies could be so oblivious. Cousin marriage, especially the unique form practiced in the Middle East, creates clans of fierce internal cohesiveness and loyalty. So in addition to sectarian violence in Iraq, the US may also be facing a greater intensity of inter-clan violence than it saw in Vietnam or the ferocious Lebanese civil war.

The US can't deal with a problem it doesn't recognize, let alone understand.

Anthropologist Stanley Kurtz has described Middle East clans as "governments in miniature" that provide the services and social aid that Americans routinely receive from their national, state, and local governments. No one in a region without stable, fair government can survive outside a strong, unified, respected clan.

But still, what does this have to do with marrying cousins? Cousin marriage occurs because a woman who marries into another clan potentially threatens its unity. If a husband's bond to his wife trumped his solidarity with his brothers, the couple might take their property and leave the larger group, weakening the clan. This potential threat is avoided by cousin marriage: instead of marrying a woman from another lineage, a man marries the daughter of his father's brother – his cousin. In this scenario, his wife is not an alien, but a trusted member of his own kin group.

Wives are also bound tightly to their clan because their in-laws are not strangers but aunts and uncles who have a strong interest in supporting their marriages. (The risk that cousins' offspring will suffer genetic anomalies is somewhat mitigated by genetic benefits too complex to discuss here.)

Thus, to many Iraqis, nepotism in government and business isn't a bad thing – it's a moral imperative. The flip side of favoring relatives is that, as Steven Sailer observed in The American Conservative in 2003, it leaves fewer resources "with which to be fair toward non-kin. So nepotistic corruption is rampant in countries such as Iraq."

The corrupt dictatorships that rule much of the Muslim Middle East often function more like self-interested clans than as national governments. That, in turn, motivates people not to trust the state, but to instead remain loyal to the proven support of kin and tribe.

Clan loyalty and nepotism strengthened by centuries of cousin marriage were always bound to undermine President Bush's fantasy of creating a truly national democratic government in Iraq. Never again should the US blithely invade a country knowing so little about its societal fabric.

I have been struck since early on in the Iraq war by how little Americans know about the groups the US so vaguely labels "insurgents." US ignorance is now further camouflaged by the label "chaos." I wonder whether, if US citizens took the time to "know thy enemy," they would learn that there are many forms of logic in the layers of Iraq's so-called chaos. I wonder if the almost daily discovery of 40, 50, or even 60 Iraqi bodies, kidnapped and tortured before being murdered, are clans battling one another.

The debacle in Iraq reinforces the idea that to have a positive relationship with any foreign society, America needs to know how its various elements work and interrelate. It must fully understand the social glues that sustain human life within particular geographic, economic, and social constraints – especially the adhesives that seem strangest and least comprehensible to us.

• Anne Bobroff-Hajal is the author of "Working Women in Russia Under the Hunger Tsars: Political Activism and Daily Life."