Safari911 is on the right line... it really depends on the severity of the impairment. I know that physically handicapped people do - i remember a case of a young lady affected by Thalidomide, born without limbs, successfully gave brith to and raised a son! On the other hand (no punn intended) Prof. Stephen Hawkins is so physically impaired that he is not able to raise children.

So it is the same with mental defects - it fully depends on the severity of the impairment and so generalisations can not and should not be made.