Pagel makes an evolutionary case for the multiplicity of languages; language serves as an identifier of group membership. This is culture's darker role: defining group boundaries. Pagel sees language and other cultural institutions functioning to set limits for altruism. Humans are social -- but only to a degree. We are a species that engages in magnificent cooperation -- yet are capable of inflicting harm on a scale not found in any other species.
Human social nature is reflected in our capacity for self-sacrifice. A 'hero' gives her life and with it the further prospect for direct descendants in order to favor the propagation of the wider gene pool found within the community. Cultural forces (built on a fundamentally biologic foundation) support these extreme examples of sociability by fomenting solidarity.
Yet in-group identification may transcend the mere sharing of genes -- and at this point culture takes on an evolutionary life of its own. Culture, like genes themselves, is replicated and transmitted; cultures compete with other cultures. Cultures enable larger scale groupings -- creating new prospects for social cooperation. Nationalism -- in its many positive and sinister senses -- represents a recent and grand manifestation of the working of our cultural instincts to promote affiliation. National cultures stamp identities that some (many?) are willing to die for. Culture tells us who we are -- and who are enemies are (in international football matches, the Scot will cheer for 'anyone but England'). It is remarkable how 'natural' these identities feel.
Pagel borrows the 'green beard' thought experiment from Dawkins, but substitutes cultural markers for the expression of certain genes which trigger group identification responses (positive and negative). Family may be biologic; tribe is cultural -- but the effect is the same. Race (as a set of easily observable physical characteristics) may have served as crude 'green beards' in certain inter-group conflicts -- but cultural markers are much more discerning. Think of the genocides in post-breakup Yugoslavia where long repressed identities (Serbian, Bosnian, etc.) surged forth with deadly consequences.
Law can amplify these processes. Consider the uniform requirement from the Law of War. A combatant receives immunity from criminal prosecution for the mayhem he might wreak so long as he wears a uniform -- but a non-uniformed combatant (or worse, a combatant falsely wearing the uniform of the opposing belligerent) is a war criminal and may be (by tradition) shot on sight. These 'green beard' norms break down, of course, in civil wars, insurgencies and the like. A particular terror for the soldier is the inability to identify the enemy.
War may be an unavoidable consequence of group competition for finite resources. But Pagel makes the case that war is enabled by culture in profound ways. Much of our identities are demonstrably socially constructed. A baby adopted into any culture will acquire that culture's language, religious and other social beliefs, and identity. But the resulting adult will likely be unaware that these differentiated points of agreement are any less fixed than is the color of his eyes. We are constituted by the stubborn certainties of the rightness of the ways of the groups we belong to.
Signaling group allegiance has evolutionary value as well -- it is an appeal for the trust of others. But it has its toxic forms. Pagel explores the evolutionarily dubious practice of 'honor killings' where bloodthirsty fathers kill disgraced daughters -- and thus close off the prospect of passing genes to a future generation. Yet the cultural imperatives that drive such mad behavior can be given an evolutionary account: the heritability of 'reputation' may be more valuable than the heritability of genes.