A Concept of Developmental (Mental) Disabilities
Developmental (Mental) Disabilities, commonly abbreviated to DD and formerly known as Mental Retardation, is more a socially defined concept than a medical diagnosis. A medical diagnosis suggests that there is some biological agent that precipitates the illness, that an etiology can be defined, and that some treatment can be suggested from this etiology or cause. DD is a social construct in that it is defined by government regulation. A person who is said to have DD becomes eligible for certain social programs. The person or child with DD could have DD from many different sources that could fall into three different categories.
Before discussing the categories, consider a common definition of DD: a person is at least two standard deviations below the mean of intellectual functioning as measured by a standard intelligence test and the person is at least two standard deviations below the mean in social and adaptive functioning as measured by a standard assessment of social and adaptive functioning and the person is below the age of 18.
First, statistically at least 2.5% of any population will be at least two standard deviations below the mean, or average, on both intellectual assessments and social/adaptive assessments. Items on these assessments are selected based on the idea that they will distinguish as a group between people who are average, people who are below average, and people who are above average, and average is defined as a certain percentage of people able to pass the items. So, theoretically only 2.5% of the population will be two standard deviations below that mean. In reality, about 8% of the population is at least two standard deviations below the mean, and the reason for that is given by the next two categories of people with DD.
Second, some people with DD have DD for, let's call it, "biological reasons". People who have DD of this sort have genetic disorders (Trisomy, three chromosomes instead of the usual two; a missing chromosome) and metabolic disorders (such as mucopolysaccharydosis and lipofuscinosis); a biological illness that results in DD.
Another cause of DD is "acquired", such as traumatic brain injury as a result of mechanical accidents (i.e., automobile accident) or other accidents (i.e., oxygen deprivation).
Finally, children may have DD as a consequence of psychiatric illnesses. DD as a sequelae of psychiatric problems may or may not be related to biological illnesses. Whether or not it is dependent on biological causes appears to be dependent upon knowledge of the source of the psychiatric problem.
The determination of DD is not always clear. Certain other problems can lead one to assume a person has DD, but they do not, such as drug-induced dementia, cerebral palsy, and epilepsy. DD is often based on a person's ability to respond appropriately to assessment items and the response is interpreted as an ability to comprehend. People with cerebral palsy or epilepsy, however, may have a temporary or permanent inability to respond as expected on a given assessment even though they comprehend, in much the same way as a person who cannot see is at a distinct disadvantage when given a Picture Vocabulary Test.