Self-injurious behavior (SIB), displayed by individuals with autism and mental retardation, involves the occurrence of behavior that results in physical injury to one's own body. Common forms of SIB include, but are not limited to head-hitting, head-banging, and hand-biting. In the most severe cases, SIB can result in retinal detachment, blindness, broken bones, bleeding, or death. SIB is displayed by 10% to 15% of individuals with autism and mental retardation. These estimates are higher among individuals living in institutions and among those with greater cognitive impairments. SIB is also associated with certain genetic disorders, such as Lesch-Nyhan and Rett Syndromes.

Individuals may engage in SIB for a variety of reasons. In some cases, SIB may occur because it results in favorable outcomes, such as attention from caregivers or the termination of academic or instructional demands. SIB may also be biologically based. For example, some research has suggested that SIB may result in the release of chemicals in the brain that produce pleasurable effects. Although there is considerable evidence to support of all of these explanations, current thought indicates that SIB is a highly complex, heterogeneous phenomenon that is often attributable to a combination of factors.