Some civilizations and countries are very clannish, others are a bit clannish, and a few, such as the West, are not clannish at all.
Why such a divergence?
Some will answer: culture. In some places, the idea of clannism prevails, elsewhere it does not.
Others will answer: institutions. In some places, the rules of clannism took hold, and like the rules of the QWERTY keyboard, they persist.
Still others will answer: interests. In some places, the structures of property, power, or land gave people an interest in being clannish, elsewhere not.
But part of the answer is: genes. Clannism prevails where lineages are closely related owing to consanguineous marriage.
the “closer” the mating patterns in a society, the more “clannish” it tends to be — the more distant the mating patterns, the less “clannish.”
There is a spectrum of clannishness from Low:
very individualistic western societies characterized by nuclear families and, crucially, very little inbreeding (cousin marriage, for instance)
Through Medium, such as:
chinese whose society is built mostly around the extended familiy but in some regions of china also clans … characterized by mid-levels of inbreeding (typically avoiding the very close cousin marriage form of the arabs).
To High, notably
arab or bedouin societies characterized by nested networks of extended families and clans and large tribal organizations and having very high levels of inbreeding (specifically a form of very close cousin marriage which increases the degree of inbreeding).
If clannism is likely innate, does that imply it cannot be altered, cannot be reformed? No, or not entirely. Clannism can change but only slowly over several generations, and only if mating patterns change. Mating patterns probably will have to change if the demographic transition occurs in clannish lands and there are not enough cousins around to marry. A good reason to promote it.