Final answer:
Alderfer's ERG theory differs from Maslow's hierarchy of needs in the number of levels and the possibility of regressing to lower levels of needs.
Step-by-step explanation:
Alderfer's ERG theory differs from Maslow's hierarchy of needs in two ways. First, the ERG theory only has three levels of needs, in contrast to Maslow's five levels of needs. Second, Alderfer suggests that when people are frustrated in achieving a certain level of need, they may regress to a lower level of needs. Maslow does not suggest that people move up and down through his hierarchy of needs this way.