MacCurdy discusses the evolution and cohesion of social groups, the origin and nature of group ideals, the concept of ethos, and the analogy between ethos and personality. A group's system of rules, conventions, and beliefs comes to symbolize group membership and is unwittingly adopted by the component individuals. Plasticity in societies is retained because the group's control over members is indirect and incomplete, each member elaborating his personal version of the group's mandate. Group ideals are characteristically never precisely formulated, and operate, in part at least, at the unconscious level. Shifts of group ideals in modern society from the economic to the altruistic, and the quasi-mystical belief in the immortality of many groups, are considered. 'National ethos is tolerant of slow evolutionary change but resilient to revolution.'