In a band of 20 people where everyone is closley related to everyone else, people related simultaneously to both quarreling parties step in to mediate quarrels. In a tribe of 100 to 200 people, many people are still close relatives and everyone at least knows everybody else by name, mutual relatives and mutual friends mediate the quarrel. But once the threshold of "several hundred," below which everyone can know everyone else, has been crossed, increasing numbers of dyads become pairs of unrelated strangers. when strangers fight, few people present will be friends or relatives of both combatants, with self interest in stopping the fight. Instead many onlookers will be friends or relatives of only one combatant and will side with that person, escalating the two person fight into a general brawl. Hence a large society that continues to leave conflict resolution to all of its members is guaranteed to blow up. That factor alone would explain why societies of thousands can exist only if they develop centralized authority to monopolize force and resolve conflicts.
Guns, Germs, and Steel
By Jared Diamond