Advances in the field of genetic research have helped us gain an understanding of how conditions that cause mental retardation, such as Down and Williams syndromes, occur. During the 1930s, genetic researchers Waardenburg and Bleyer were the first to postulate that the cause of Down syndrome was genetic in nature; however, conclusive research to establish the genetic cause of Down was not available until 1959. At that time, two researchers, Jerome Lejeune and Patricia Jacobs, each independently demonstrated the cause of Down syndrome to be an abnormality on chromosome 21, specifically Trisomy 21.

Williams syndrome was first recognized in 1961, but the genetic cause of the disorder was not discovered until the 1990s. Dr. Colleen Morris and her associates from the University of Utah discovered that Williams syndrome was caused by a genetic deletion on chromosome 7.

Other mental retardation syndromes now understood to be caused by genetic problems have similar associated stories. Working at the University of Florida, Dr. Charles Williams' research into Angelman syndrome was able, in 1987, to link the cause of that disorder to missing portions of chromosome 15. Williams' discovery was further refined in 1997 by Drs. Joseph Wagstaff and Arthur Beaudet, who pinpointed the cause of Angelman syndrome as a mutation (or more typically, an absence) of a specific gene known as UBE3A. Similarly, researchers Martin and Bell were able to link X chromosome problems to Fragile X syndrome forms of mental retardation in 1943, and research Herbert Lub was able to provide confirmatory data for this finding in 1969.