Mental Retardation is mostly a legal term. It means that a person scores below a particular level on a particular test agreed upon by a specific state’s legislature, or the federal government’s subsidiaries which provide funding, such as Medicare, SSI, and other such bodies.
What causes the developmental delay, and behavioral deficits that are grouped together as Mental Retardation by the lay population is much more complex than you might think.
There are over a thousand named syndromes which are often associated with mental retardation, the most common of which is Down’s Syndrome. In addition, whether or not it is correct, many people with other syndromes are grouped with mental retardation, because they are unable for physical reasons to perform the mandatory behaviors of normal daily living.
Just being unable to hear makes it harder to learn to speak. It doesn’t mean you have an impaired intellect, but it does mean that your stimulatory environment will be less diverse unless deliberate actions by other make up for the deficit of deafness. If that doesn’t happen, thousands of other variables will tend to make it harder for a person to learn. Cerebral Palsy is another example. The delay in learning might not be caused by CP, but rather by the barrier that CP causes in the process of learning. Blindness is another. All of these things make normal development more difficult. Without well managed intervention, they can result in a cumulative delay in learning, and physical growth.
Anoxia associated with birth, neural damage during, or shortly after gestation, fever during illness that interferes with neural development, heart malformations during development that impair circulation, or oxygenation of the blood, drugs, many specific diseases, and a fairly large number of micronutritional deficiencies are all possible causes. Severe epilepsy during childhood can prevent, or drastically slow down normal cognitive development. Neglect or abuse can do it as well. Hundreds of identified genetic factors, and many unidentified ones as well cause impairments that can result in mental retardation. Macronutritional deficits, (big word for chronic hunger) will almost always limit neural development in children.
What can be done? Huge steps could be taken. Micronutrition alone could entirely eliminate five or six percent of mental retardation, world wide, at a cost of less than a billion dollars for three or four years, and less than a million a year thereafter. Raise the average IQ of the entire human race by five points, mostly at the bottom ten percent. The only things preventing that are ignorance, and politics. Well baby care and prenatal nutrition could double or triple that effect, although that would cost much more.
The cold facts of life in the industrialized world are counterintuitive. We don’t let as many folks just die in childhood as in the third world. We save babies at any cost. And there are strong political forces, which impel, or even compel us to continue to do that, even when individual choices are not enough. That means we will have progressively more and more mentally retarded people. Yet the same forces which encourage that growth do not combine to provide social infrastructure to meet the increasing demand for services.
It is a problem that is much more complex than your question anticipates.
Tris