From a white paper by the Afghanistan Research Reachback Center:
[T]he desire for “tribal engagement” in Afghanistan, executed along the lines of the recent “Surge” strategy in Iraq, is based on an erroneous understanding of the human terrain. In fact, the way people in rural Afghanistan organize themselves is so different from rural Iraqi culture that calling them both “tribes” is deceptive. “Tribes” in Afghanistan do not act as unified groups, as they have recently in Iraq. For the most part they are not hierarchical, meaning there is no “chief” with whom to negotiate (and from whom to expect results). They are notorious for changing the form of their social organization when they are pressured by internal dissension or external forces. Whereas in some other countries tribes are structured like trees, “tribes” in Afghanistan are like jellyfish.
(Hat tip: Matt Steinglass)