Overview In the next three chapters, we explore the causes of mental retardation. The disorder may arise from biological factors, either operating fairly independently as in Down syndrome, a chromosomal dis order, or interacting with the psychological environment as in what earlier was called "cultural-familial" mental retardation, now dubbed "psychosocial disadvantage." Biological determinants are presented in Chapters 3 and 4, and psychological ones in Chapter 5.