The law assumes that all people 18 and older, including people with disabilities, have the capacity to make decisions for themselves. This assumption can be overcome, however, when circumstances suggest otherwise.

One way to do so is after a hearing in court, where a judge decides, based on expert testimony, that the ability of a particular individual to make decisions is so diminished that he or she is unable to properly manage personal finances and/or properly protect personal health and safety. Such a decision can also be made by a government agency or by a treating physician. When such a determination has been made, we say that the person is incapacitated or incompetent. In these situations, another individual (the substitute decision maker or guardian) is usually given the power to make decisions about property management and/or personal health and safety on behalf of the incapacitated person. This substitute decision maker can be chosen either by the incapacitated individual, by the court, or by a government agency.