• Many psychologists have proposed stage theories of development, which argue that people pass through stages in specific orders, with challenges related to age and different capacities emerging in each stage.
• Sigmund Freud first described personality development in terms of stages and believed personality developed by age five.
• Erik Erikson proposed a theory of psychosocial development that occurs in eight stages over a person’s lifetime. He proposed that people face new challenges at each stage: trust vs. mistrust, autonomy vs. shame and doubt, initiative vs. guilt, industry vs. inferiority, identity vs. role confusion, intimacy vs. isolation, generativity vs. self-absorption, and integrity vs. despair.
• Jean Piaget’s theory of cognitive development states that children develop schema or mental models to represent the world. He proposed four stages of cognitive development: the sensorimotor period, the preoperational period, the concrete operational period, and the formal operational period.
• Lawrence Kohlberg proposed a theory of moral development that includes three levels or stages: the preconventional level, the conventional level, and the postconventional level.

• Prenatal development occurs between conception and birth.
• Prenatal development is divided into three stages: the germinal stage, the embryonic stage, and the fetal stage.

• Motor development or increasing coordination of muscles improves rapidly in infancy and childhood.
• Maturation is genetically programmed growth and development. Maturation and experience influence motor development.
• Temperament refers to the personality features a person is born with. Alexander Thomas and Stella Chess proposed three basic types of temperament: easy, slow to warm up, and difficult.
• Attachment is the close bond between babies and their caregivers. Margaret and Harry Harlow concluded that attachment requires contact comfort, which is the comfort deriving from physical closeness.
• After conducting an experiment called the Strange Situation, Mary Ainsworth proposed three types of attachment styles: secure attachment, anxious-ambivalent attachment, and avoidant attachment.
• Separation anxiety is the emotional distress infants show when separated from people to whom they are attached.
• Gender is the learned distinction between masculinity and femininity. Gender stereotypes are societal beliefs about the characteristics of males and females.
• Depending on their perspective, researchers ascribe different causes for gender differences.

• Pubescence refers to the two years before puberty and entails growth spurts and the development of secondary sex characteristics. Secondary sex characteristics are sex-specific physical traits that are not essential to reproduction, such as breasts, widened hips, facial hair, and deepened voices.
• Puberty, the point at which sexual organs mature, occurs at the beginning of adolescence. Menarche refers to the first menstrual period.
• On average, puberty occurs between ages ten and fifteen for girls and eleven and sixteen for boys. Maturing before or after these ages can have adverse consequences.
• The search for identity is an important step in adolescence. James Marcia described four identity stages: identity foreclosure, identity moratorium, identity diffusion, and identity achievement.

• Adulthood usually includes experiences such as marriage, parenthood, the empty nest, the midlife crisis, menopause, and aging.
• Social clocks indicate the typical life events, behaviors, and concerns for a particular age.
• As people age, they tend to experience loss of neurons in the brain, a decline in vision and hearing, and decreased memory. People may also experience increased crystallized intelligence, which is intelligence based on accumulated knowledge and skills. Physical exercise and mental stimulation can create new neural brain connections, and older adults generally have a better sense of well-being.