Normal is relatively new.

According to Lennard J. Davis, in Enforcing Normalcy: Disability, Deafness and the Body:

A common assumption would be that some concept of the norm must have always existed. After all, people seem to have an inherent desire to compare themselves to others. But the idea of a norm is less a condition of human nature than it is a feature of a certain kind of society.

…this concept ‘normal,’ ‘average,’ ‘abnormal’ – all entered the European languages rather late in human history. The word ‘normal’ as ‘constituting, conforming go, not deviating from, the common type or standard, regular, usual’ only enters the English language around 1840. (Previously, the word had meant ‘perpendicular’…)

We are taught that normal is good, disability is not normal and, so, disability is bad.

However, normal functioning is simply a line drawn arbitrarily. Normal itself is frequently held out in authority when it is an abstract concept that fails to provide an adequate definition of the human existence. Are people who are born with one arm or who have low IQ test scores abnormal? It depends on the set of criteria that one uses. By some standards one could say that these groups of people are rare in the general population. However, by others, if you look at how many people have one arm or low IQ scores, you will find that there are a great many of them, which may make them normal. Indeed, one could say that they are probably not typical or average. However, speaking of someone’s probable lack of typicalness in respect to one aspect of the person’s mind, body or identity does not ring with the same authority as saying that someone is not normal.

Even if one could establish what normal is or isn’t in relation to disability/impairment, there are other problems with normal. For instance, autistic people are commonly described as having “peaks of ability.” There are some things that many autistic people are very good at and other things that they cannot do. Michelle Dawson, an autistic woman takes issue with this view, saying in a Quirks and Quarks interview:

it isn’t that we have these sort of peaks and valleys it is that we are fundamentally different. The processes that we apply to these various tasks are different, our brains will do things differently…if you have normal as a baseline it will produce this result of peaks and valleys that isn’t a collections of excesses and deficits attached to a normal person it is just that it’s the product of a fundamental difference in how our brains process information

As Dr. Laurent Mottron, a psychiatrist who works with Dawson, put it in the same interview, “you do not describe a dog as a negative of a cat and we still do neuroscience research of autism by describing autism as defects of typical mankind” . In other words, you cannot take what is normal behavior for a cat (or an autistic person) and say it is abnormal because it is not what dogs (or non-autistic people) generally do. The same can be said of many groups, while some groups of peoples’ minds and bodies act differently than some other groups of people that does make them inferior, simply different. You cannot use the same scale to measure people in these groups as you do to measure people who are not members of those groups.