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What Are the Causes of Mental Retardation?

Nicole Madison
Updated May 17, 2024
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There are many possible causes of mental retardation, but the most common are infections that develop in an unborn baby or soon after birth. In other cases, chromosome abnormalities are at fault. Inherited disorders, traumatic injuries, and nutritional deficiencies can sometimes lead to mental retardation as well. Basically, anything that occurs before a person is born, soon after birth, or during childhood that alters brain development can lead to mental retardation. It is important to keep in mind, however, that the causes of many cases of retardation are never discovered.

Infections that affect a baby while he is still in the uterus, soon after childbirth, or during childhood may lead to mental retardation if they adversely affect the development of the brain. Among the infections that can cause or contribute to the development of mental retardation are tuberculosis, a lung disease; encephalitis, which is inflammation of the brain; rubella, also called German measles; and toxoplasmosis, which is caused by parasites. Additional infection-related causes include listeriosis, which is caused by bacteria; meningitis, marked by inflammation of brain and spinal membranes; and cytomegalovirus, a type of herpes virus. Even human immunodeficiency virus (HIV), the virus that causes acquired immune deficiency syndrome (AIDS), has the potential to cause mental retardation.

Chromosome abnormalities are also among the causes of mental retardation. For example, abnormalities that are present before birth, such as in Down syndrome, may cause mental retardation. Inherited chromosome problems, such as in an abnormality called fragile X syndrome, can cause it as well. Additionally, a person could be born with mental retardation when a gene is located in the wrong place on a chromosome.

Some inherited disorders are among the possible causes of mental retardation as well. For example, a person may be mentally retarded as a result of Hunter syndrome, a disorder in which sugar molecules build up too much, or Hurler syndrome, a rare disease that involves the metabolism of sugar. Rett syndrome and Tay-Sachs disease, which both involve the nervous system, can also lead to mental retardation. Phenylketonuria (PKU), which is marked by the inability to break down a substance called phenylalanine, is also among the causes of mental retardation.

There are also nutritional causes of mental retardation. If a child is malnourished, for example, he may be more likely to suffer from mental retardation. Malnutrition can result from diets that don’t include a good balance of nutrients, problems with nutrient absorption, lack of food, and certain medical conditions that are left untreated.

Traumatic injuries sometimes cause mental retardation as well. An injury to the brain during birth can cause it, as can severe head injuries during childhood. Deprivation of oxygen to the brain may cause it as well. Additionally, bleeding in the brain that occurs before or after birth can lead to mental retardation.

WiseGeek is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Nicole Madison
By Nicole Madison , Writer
Nicole Madison's love for learning inspires her work as a WiseGeek writer, where she focuses on topics like homeschooling, parenting, health, science, and business. Her passion for knowledge is evident in the well-researched and informative articles she authors. As a mother of four, Nicole balances work with quality family time activities such as reading, camping, and beach trips.

Discussion Comments

By Animandel — On Jan 21, 2015

There is a lot of speculation about what causes autism in children. I know some people still believe vaccines are to blame in a lot of these cases. However, one of the possible reasons that we are seeing more cases of autism in children could be that more and more older people are becoming parents. For whatever reasons, many people are waiting to have children.

Many scientific studies show that one older parent increases the chances of a child having some symptoms of mental retardation. So a couple with two older partners and a couple with one older partner are pretty much in the same boat. Of course, there are other studies that would discount this, so I guess it depends on which study you choose to put your faith in.

By Drentel — On Jan 21, 2015

@Laotionne - I have a friend whose wife got pregnant when he was 55 years old. He had three kids by his first wife, and this was his second marriage. His second wife was in her early 30s, so they never thought their would be a greater risk beyond the normal risk that comes with having a baby.

As it turned out, the child was born mentally retarded. At some point they learned that a baby who is born with a father over 40 has a greater chance of having a mental disorder. Strangely enough, they were told that the age of the mother didn't really matter in this case.

By Laotionne — On Jan 20, 2015

I have heard that being older and having children can make the children more likely to have some type of mental retardation. I don't know why this would be so, but has anyone else heard this. And if so then what is the reason for this?

Nicole Madison

Nicole Madison


Nicole Madison's love for learning inspires her work as a WiseGeek writer, where she focuses on topics like...
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