Clayton Paul Alderfer (b. 1940) is an American psychologist who further developed Maslow's Hierarchy of Needs into his own ERG Theory. ERG Theory posits that there are three groups of core needs: existence (E), relatedness (R), and growth (G)—hence the acronym "ERG." These groups align with the Maslow's levels of physiological needs, social needs, and self-actualization needs, respectively.
The "existence" needs describe our basic material requirements for living. These include what Maslow categorized as physiological needs (such as air, food, water, and shelter) and safety-related needs (such as health and secure employment and property).
The "relatedness" needs concern the maintaining of important interpersonal relationships. These needs are based in social interactions with others and align with Maslow's levels of love/belonging-related needs (such as friendship, family, and sexual intiamcy) and esteem-related needs (such as respect of and by others).
Finally, the "growth" needs describe our intrinsic desire for personal development. These needs align with Maslow's levels of esteem-related needs (such as self-esteem, confidence, and achievement) and self-actualization needs (such as morality, creativity, problem-solving, and acceptance of facts).
Alderfer proposed that if an individual's needs in a certain category are not met, then they will redouble their efforts toward fulfilling needs in a lower category. For example, if an individual's self-esteem is suffering, they will invest more effort in the relatedness category of needs.