This chapter contains sections titled: Introduction Definition of Disability Causes of Intellectual Impairment Evolving Terminology for Intellectual Impairment The Bell Curve and IQ: Levels of Intellectual Disability Cognitive Developmental Thresholds Intellectual Disability and the Brain: Fluid and Crystallized Intelligence Neuroplasticity in Brain Development Developmental Perspective on Intellectual Disability: Cognitive Developmental Thresholds Moral Development Implications of Recognizing Personhood and Autonomy Participation in Research Summary References.