Erikson (1902-1994) developed the Theory of Psychosocial Development, which describes eight stages in the life of a person (five until 18 and 3 during adult life). At each stage, the individual experiences an identity crisis and there are two possible outcomes at each crisis, one of them is positive and the other contributes negatively to personality formation.
The person sucessfully completes each stage if he/she is able to acquire the positive outcome and to incorporate it to his/her personality. On the other hand, when the individuals fails to complete the stage correctly it might lead to difficulties in further stages and a more unhealthy development of the personality.
In the photo attached, the different stages and the two opposing forces operating on each are described.
Stage number 6 is related to loneliness
The identity crisis experienced at this stage is Intimacy vs. Isolation, and it takes place during an adult life stage, comprised approximately between 18 and 40 years old. The sucessful resolution of the conflict would take place if the person is able to establish intimate and loving relationships with people which are not family members, and it will bring happy relationships plus a sense of safety, commitment and care.
On the other hand, if the stage is not sucessfully completed and the person cannot develop intimacy, and cannot build relationships, this might lead to him/her suffering isolation, loneliness and even depression.
