Final answer:
Erik Erikson adds the stage of 'intimacy versus isolation' to describe human development during early adulthood.
Step-by-step explanation:
Erik Erikson's theory of psychosocial development includes eight stages of development, starting from birth and ending with death. The stage added by Erikson to describe human development during early adulthood is intimacy versus isolation. This stage occurs during the 20s through early 40s, and it is a time when individuals are concerned with forming intimate relationships and developing a strong sense of self. It is important for individuals to have a positive self-concept before pursuing intimate relationships with others.