A theory comprising limited numbers of variables, each of limited scope. Middle-range theories may be descriptive, explanatory (specifying relationships between two or more concepts), or predictive (envisioning relationships between concepts or effects of certain concepts on others). Examples include the Health Belief Model, the Theory of Maternal Attachment, Erikson’s Theory of Psychosocial Development, Watson’s Theory of Human Caring, and Maslow’s Hierarchy of Needs Theory.