Interest groups are collections of people with essentially the same self-interest, about which they feel so strongly that they collectively form an organization to promote and defend it through the political process. The development of the political philosophy of the social contract underlying civilization prioritizes individual self-interest and protects the pursuit of that self-interest through politics. The political and economic philosophy that assigned such a fundamental role to individual self-interest emerged after Europe had achieved enough political stability that factions grounded in economic differences could emerge. American society is extremely diverse, and so the number of different interests that are felt intensely enough to motivate people to form an interest group is probably unknowable. Growing public concern about the physical and social health of America during the Industrial Revolution also spurred the formation of cause-oriented interest groups. Real interest groups represent some portion of the public, not just their own leaders and CEOs.