Psychologists have long sought to define and measure intelligence. In everyday terms, intelligence is the mental ability to learn from experience, solve problems, and adapt to new situations, although its precise nature remains debated. Charles Spearman proposed that a general factor, called g, underlies all intellectual abilities, alongside specific factors, called s, that account for performance on particular tasks. Howard Gardner challenged this unitary view with his theory of multiple intelligences, which proposes eight distinct types: linguistic, logical-mathematical, spatial, musical, bodily-kinesthetic, interpersonal, intrapersonal, and naturalistic. Robert Sternberg offered a third perspective, the triarchic theory, which distinguishes analytical intelligence (academic problem-solving), creative intelligence (generating novel ideas), and practical intelligence (adapting to everyday situations).
Development unfolds in qualitatively different stages, and Jean Piaget proposed four stages of cognitive development that children move through in a fixed order. The sensorimotor stage, from birth to about two years, is marked by the gradual development of object permanence, the understanding that objects continue to exist even when they cannot be seen. The preoperational stage, from about two to seven years, is characterized by egocentrism, the inability to see things from another person's perspective, demonstrated by Piaget's three-mountain task, and by a typical failure on conservation tasks, in which children do not yet understand that quantity remains the same despite changes in shape or appearance. The concrete operational stage, from about seven to eleven, brings mastery of such logical operations tied to concrete objects, and the formal operational stage, beginning around age twelve, enables abstract and hypothetical reasoning. Erik Erikson offered a complementary account in his eight stages of psychosocial development, each centered on a conflict that must be resolved, ranging from trust versus mistrust in infancy to integrity versus despair in old age. Adolescence brings the central conflict of identity versus role confusion, in which young people work to develop a coherent sense of self. Attachment theory, developed by John Bowlby and expanded by Mary Ainsworth, holds that early bonds with caregivers shape emotional development; Ainsworth identified secure, avoidant, and anxious-ambivalent attachment styles arising from these early experiences. Lawrence Kohlberg, meanwhile, described three levels of moral reasoning: preconventional reasoning based on self-interest, conventional reasoning based on social rules and conformity, and postconventional reasoning based on universal ethical principles.
Personality, the relatively stable pattern of thoughts, feelings, and behaviors that distinguishes one person from another, has been explained from several perspectives. The trait approach has produced the influential Big Five model, which identifies five broad dimensions often remembered with the acronym OCEAN: openness, conscientiousness, extraversion, agreeableness, and neuroticism. Freud, by contrast, proposed that personality is shaped by unconscious drives and develops through five psychosexual stages, oral, anal, phallic, latency, and genital, with unresolved conflicts at any stage producing a fixation. Freud also introduced defense mechanisms, unconscious strategies used by the ego to manage anxiety arising from conflicts between the id and superego. Repression, the pushing of threatening thoughts or memories into the unconscious, is the most fundamental, though projection, rationalization, and displacement also serve to protect the individual from distressing internal conflicts. Finally, the humanistic perspective, led by Carl Rogers and Abraham Maslow, focuses on conscious experience, free will, and the drive toward self-actualization, the realization of one's full potential, characterized by creativity, morality, and acceptance. Rather than seeing people as driven by hidden conflicts, humanism views them as inherently growth-oriented and capable of becoming what they are meant to be.