Personality reflects individual differences that determine how people respond to their environment and can be used to segment consumers, though it also depends on social relationships and can change over time. Common personality theories include Freudian concepts of the id, ego, and superego, traits like innovativeness, and how self-image relates to brand preferences and choices that express the self. Marketers aim to understand personality to develop targeted products and appeals that relate to consumers' traits, needs, and self-perceptions.