Clayton Alderfer further expanded Maslow's_hierarchy_of_needs by categorizing the hierarchy into his ERG theory (Existence, Relatedness and Growth). Alderfer categorized the lower order needs (Physiological and Safety) into the Existence category. He fit Maslow's interpersonal love and esteem needs into the relatedness category. The growth category contained the Self Actualization and self esteem needs.

Alderfer also proposed a regression theory to go along with the ERG theory. He said that when needs in a higher category are not met then individuals redouble the efforts invested in a lower category need. For example if self actualization or self esteem is not met then individuals will invest more effort in the relatedness category in the hopes of achieving the higher need.