Intellectual disability can be caused by any condition that impairs development of the brain before birth, during birth or in the childhood years. Several hundred causes have been discovered, but in about one-third of the people affected, the cause remains unknown. read more
I'm not a medical professional, but as I understand it, we as a species don't understand the brain/nervous system very well yet. That's why intellectual and neural disabilities so often have unknown causes. read more
Fragile X syndrome is a single gene disorder located on the X chromosome and is the leading inherited cause of intellectual disability. Problems during pregnancy - Use of alcohol or drugs by the pregnant mother can cause intellectual disability. In fact, alcohol is known to be the leading preventable cause of intellectual disability. read more