Theory is the bridge between research questions and study design. Theoretical perspectives establish a philosophy that guides research perspective, thinking about initial areas of research and research questions, and applying approaches specific to research with older populations. Gerontological theories are divided into several groups: early and/or general theories; biomedical and health-based theories; biopsychosocial and biological theories; psychological, developmental, and cognitive theories on aging; social gerontology and life-course perspective; critical gerontology; ecological theories; social construction and phenomenological theories; and identity-based theories on aging. Early and/or General theories include: disengagement theory, continuity theory, activity theory, age and modernization theory, and gerotranscendence theory. Biological theories seek to explain how we age and debate the difference between "normal" and non-normative aging. Social-gerontology theories seek to add the role of larger society to the experience of aging and older people. Both social construction and phenomenological theories share a common approach in understanding how an older person would attach meaning to his or her social world.