There are a few different types of mental retardation syndromes that can affect people, of which Down syndrome and autism are some of the most commonly known. Mycrocephaly and cretinism are some forms of mental retardation that are caused by underdevelopment of the brain. Finally, dyslexia is a type of mental disability that is not often associated with retardation because it has so little effect on intelligence.

Down syndrome is one of the first things that come to mind when one thinks of types of mental retardation syndromes. This disorder is genetic and occurs when the individual has more than 46 chromosomes, which causes developmental problems in the brain. Down syndrome is easily identified by abnormal facial features such as a flat nose, thick eyelids, and large ears, among others.

Autism is among the most enigmatic of the different types of mental retardation syndromes. Autism seems to affect people in very different ways. Research has brought some understanding of this disorder, but there is still much to learn about the brain function of people with autism. It is understood that people with autism think differently than those with normal brains, which diminishes the social interactive abilities, but also sometimes increases abilities in areas like math. Verbal skills are typically underdeveloped, and eye contact is rare in people with this disorder.

One of the mental retardation syndromes that is caused by impaired brain development is called mycrocephaly. The name literally means small headedness, because the brain, and consequently the head, is underdeveloped during pregnancy. The appearance of each baby with mecrocephaly will vary quite a bit, but affected individuals tend to have a head about 17 inches (43.18 cm), compared to a normal head size of about 22 inches (55.88 cm), and they often have a cone-shaped skull as well. These children have very limited mental capacity and tend to develop very little language skills.

Cretinism is a type of mental retardation syndrome that is caused by a thyroid deficiency. This disorder is sometimes caused during fetal development, when the thyroid is underdeveloped, but can also be caused in adulthood from injury to the thyroid gland. Some diseases, such as measles, diphtheria, and whooping cough, can cause this disorder when bleeding enters the thyroid. Early treatment has shown some success in aiding mental development, but if no treatment is received within the first year of life, the effects are often permanent.

One of the most common of the mental retardation syndromes is dyslexia. Fortunately, it is also a very mild form of mental disability but is quite difficult to identify because it involves the interpretive processes in the brain, which can't be seen by others. This disorder primarily affects reading, writing, and arithmetic skills. A common sign of dyslexia involves the person consistently mixing up the order of letters and numbers.