Abraham Maslow's hierarchy of needs theory states that people are motivated to fulfill five basic needs: physiological needs, safety needs, social needs, esteem needs, and self-actualization needs. Clayton Alderfer built on this by proposing the ERG theory, which argues that there are three core existential, relatedness, and growth needs that motivate people, and that frustration in meeting higher-level needs can cause people to focus on lower-level needs.