Definitions of aging differ between biologists and behavioral scientists. The biologists regard aging as reflecting the sum of multiple and typical biological decrements occurring after sexual maturation; the behavioral scientists view it as reflecting regular and expected changes occurring in genetically representative organisms advancing through the life cycle under normal environmental conditions. It is difficult to define normal aging, since many changes observed in older adults and previously perceived as concomitants of normal aging are now recognized as effects of disease in later life. The behavioral science view allows for incremental as well as decremental changes with aging. Senescence is not always equated with aging; it is viewed as the increasing vulnerability or decreasing capacity of an organism to maintain homeostasis as it progresses through its life span. Gerontology refers to the study of aging. Geriatrics refers to the clinical science that is concerned with health and illness in the elderly.