Studies of individual differences embrace a range of perspectives. Research into individual differences looks at why people differ along certain dimensions such as intelligence, personality, and mental health. The psychodynamic approach focuses on early childhood experiences and unresolved conflicts. The psychodynamic model contrasts sharply with the biological model by suggesting that mental illness "arises out of unresolved unconscious conflicts in early childhood". Most psychologists adopt a multi-dimensional approach to abnormality. Psychoanalysis has tended to be used predominantly to treat people suffering from neuroses such as anxiety disorders or from depression. The treatments derive from psychoanalytical theory, which focuses on conflicts between the ego, id, and superego and the use of defence mechanisms such as repression and regression. The enables them to gain insight into what produced the emotional conflict or deficits in their psychological development.