Abraham Maslow posited that there are five stages of need: physiological, safety, social, self-esteem, and self-actualization. He stated that each need had to be met before people could progress to the level. Later, he added a sixth need, self-transcendance, which refers to the need to give to others.
Clayton Alderfer, a psychologist, amended Maslow's hierarchy of needs to produce ERG Theory, which refers to existence, relatedness, and growth. In Alderfer's hierarchy, there are only three levels instead of five. He posited that people can be trying to meet needs on more than one level at one time and that they can move from one level back and forth to another more fluidly than in Maslow's hierarchy of needs. If one's needs are not met at one level, they can regress to a lower level to have their needs met. This is the frustration-regression principle and can be applied to organizational analysis to think about why employees whose needs for growth are not met may decide to spend more time socializing to have their needs for relatedness met. In addition, Alderfer stated that people's needs can change at different times of their lives, so that they are not static.