Discrimination is appropriate and necessary in the interests of many people with a mental handicap. For example, my 45-year-old son has the mental age of a three-year-old: he cannot speak, he has some neurological impairment, his behaviour can be very erratic and challenging. To satisfy his needs requires discrimination in a manner that is not appropriate for his normal siblings. There are many thousands like him.
There is a world of difference between handicapped people and the larger number whose disability is relatively slight. The World Health Organisation classification of a number of learning disabilities specifically excludes conditions which are attributable to mental retardation. I am not surprised that 50 per cent of the public use the term "mentally handicapped". I find that groups of parents and relatives usually use the term just because it is more precise and not euphemistic. Changing names does not change reality.