Psychologists have identified three main types of motives: biological motives related to basic survival needs like hunger and thirst, social motives related to needs for achievement and affiliation, and personal motives related to habits, goals, and interests. Several theories have attempted to explain human motivation, including drive-reduction theory which proposes that physiological needs create arousal states motivating behavior, arousal theory which suggests people seek an optimal level of stimulation, and incentive theory stating behavior is pulled by desires for rewards. Maslow's hierarchy of needs proposes basic needs must be met before higher-level needs, arranging needs into physiological, safety, love, esteem, and self-actualization.