The world-systems theory stresses that world-systems (and not nation states) should be the basic unit of social analysis. World-system refers to the international division of labor, which divides the world into core countries, semi-periphery countries and the periphery countries. Core countries focus on higher skill, capital-intensive production, and the rest of the world focuses on low-skill, labor-intensive production and extraction of raw materials. This constantly reinforces the dominance of the core countries. Nonetheless, the system is dynamic, and individual states can gain or lose the core (semi-periphery, periphery) status over time.