Large-scale societies rent when conflicts arise between tribes or other groups culturally bound by affinity. As narrow self-interest readily overrides comity, social conflict is as much a norm of human sociality as the tribal and economic bonding that creates large societies.
That fission in human tribes has often been stronger than fusion is helped by human adaptivity in being able to live in a broad range of biomes. This owed partly to cultural advance (knowledge of food sources, tools, clothing, et cetera).
The other aspect engendering fission over fusion is that human mental evolution led to imperative values based upon abstractions rather than practicalities. As survival became less problematic, mental constructs grew in importance. Belief systems became paramount, most notably religion. These were reinforced by rituals which fortified cultural identity.
If a subgroup did not like the conditions imposed by the tribe, they might leave. Human ability to migrate long distances facilitated diaspora and cultural diversity. This terminal fission distinguishes humans from the sociality of baboons, most of whom tolerate strict dominance hierarchies under which humans would chafe.
Here we encounter an ever-active facet in the gyre of human culture: a sense of individuality – conceived conceptually and expressed culturally – rubs against social restrictions imposed by established customs.
A subgroup forms adhering to a new culture. If existing tribal mores are considered repressive in opposition, the subgroup emigrates, forming its own tribe: whence cultural fission. Further, if limited resources become coveted and contested, the seeds the war are planted.
Beyond materialism, intertribal conflict is often driven by social grievance. A perceived insult breeds the desire for revenge. These dynamics remain in the cultures of modern societies, sowing the seamy underbelly of politics.
Fission-fusion as a social dynamic depends upon 2 determinants: shared goals and group cohesiveness. The depth of both depend upon the level of altruism in tribal members.
Evolutionary biologists make much of kin relations as mitigating the damage of conflicts. Pervasive domestic violence and intrafamily feuds throughout history puts paid too much of that notion.
At the societal level, civil wars demonstrate the bitterness of the adage that familiarity breeds contempt. Religious, political, and materialist ideologies tied to cultural identity are a potent concoction that can counter any inclination to civility.
The diaspora of humanity via male-provoked fission produced the profusion of human cultural diversity. Many cultures developed in ignorance of others, or, alternately, as derivatives of other cultures. People take what is useful to them.
The same dynamics drive cultural practices today as they have for hundreds of thousands of years. Technology has advanced, but not the motivations and modes that kindle culture.