There are many causes of mental retardation, including genetic and chromosomal disorders, infections during pregnancy, drug and alcohol use during pregnancy, problems with the birth or delivery, prematurity and low family IQ in general.
While for every newborn baby there is at a small risk of MR, babies born into a family which already has a child with MR are much more likely also to develop MR. This suggests a genetic component that is not, at present, fully understood. I'd like to review the current knowledge about risk factors for and causes of MR in order to help people with concerns about the risks make better informed decisions.
MR Basics
The American Psychiatric Association defines MR as the combination of (1) tested IQ at or under 70, (2) problems with learning and social adaptation and (3) symptoms that begin before 18 years of age. MR is further subdivided into two categories: mild (IQ 50-70) and moderate-severe (IQ under 50). While there are great differences between these subcategories, genetics plays at least some role in both. MR is a common problem, affecting as much as 2% of the world's population.
For many individuals and families, the cause of mental retardation is not yet known. One reason for the uncertainty is that mental retardation covers a number of different problems with different causes.
Looking for a Cause
Many studies have attempted to find out how and why MR occurs. Unfortunately, most were done in the 1980s or earlier, before the recent advances in our ability to identify and test for certain genetic and chromosomal disorders. Chromosomes are the materials within human cells that contain our genes. Damaged or abnormal genes, and extra or missing chromosomes, are known to be a prime cause of MR.
