Mutualism explains why we form groups and coalitions (there is strength and safety in numbers), and hence why we value unity, solidarity, and loyalty. Social exchange explains why we trust others, reciprocate favours, feel guilt and gratitude, make amends, and forgive. And conflict resolution explains why we engage in costly displays of prowess such as bravery and generosity, why we defer to our superiors, why we divide disputed resources fairly, and why we recognise prior possession.