Developmental psychology covers cognitive development, moral development, personality and social development, and early development. Cognitive development theories include Piaget's stages of development and Vygotsky's zone of proximal development. Piaget's stages are the sensorimotor stage, preoperational stage, concrete operational stage, and formal operational stage. Kohlberg identified levels of moral development as pre-conventional, conventional, and post-conventional stages. Theories of development aim to understand how thinking, behavior, and understanding of society change throughout the lifespan.