German psychologist Erik Erikson,came up with his own eight-stage theory of development,that goes all the way from infancy to old age.He based these stages on the key challenge of each period of life.When you're a toddler,for example,your biggest challenge is getting what you want or as Erikson puts it,gaining autonomy,which helps you build skills and confidence in your abilities.But once you're a young adult,you've got plenty of autonomy.Now a bigger challenge is developing intimate relationships.Every life stage features different expectations that inform what we see a markers of social development.Moving out,getting married,having kids - they're all societal markers of social development as an adult.