Note that I'm not asking what distinguishes disability from non-disability, or normalcy from lack of normalcy. These are entirely different questions.
Many readers probably assume I am asking a scientific question. But I do not believe it is. I think it's a question of convention, a philosophical question, and ultimately a matter that belongs in the realm of Ethics. Let me elaborate.
There are conditions that no one would dispute are pathologies, e.g. various forms of cancer. When cancer is detected in someone, a considerably lowered life expectancy is practically assured. Additionally, treatment of the condition can be demonstrated to improve the survival rate. Continued medical research will no doubt improve the survival rate even further.
Then there are conditions such as ADHD that many people intuitively find dubious. When about 10% of one end of a normally distributed behavioral spectrum are labeled with a brain pathology, and most of those labeled appear unremarkable to the untrained eye, it's not surprising that people are not going to take the condition seriously. In fact, ADHD is often made fun of. Another such example is Oppositional Defiant Disorder (ODD), a psychiatric condition that is diagnosed when a child strongly disagrees with his or her parents much of the time.
It seems that the pathology status of conditions defined by conventional medicine is rarely controversial (although some examples exist, such as hypoglycemia). On the other hand, the pathology status of conditions defined by Psychiatry is not only often controversial, but it tends to overlap with politics. Some of this could be due to the scientific methodology used in each case, but I think it mostly has to do with the way individuals experience psychiatric conditions and the consequences of their pathology status.
Admittedly, I do not understand what distinguishes pathology from non-pathology all that well, but I do not believe the "experts" do either.
When expert psychiatrists try to apologetically argue that a condition such as ADHD is a pathology, they will invoke a number of arguments that to me appear rather weak under scrutiny. I will go over some of them.
ADHD increases the risk of injury.
Research shows this to be the case and it's not surprising that an over-active child would have a higher chance of getting injured. It's not clear how this demonstrates pathology, though. I could make up a syndrome on the spot with this characteristic.
Extreme Sports Disorder: This is a neurological disorder that causes people to pursue extreme sports. No cause or cure have been identified.
If you find this counter-argument far fetched, consider increased risk of trauma and other medical problems in left-handedness (Canacki et al., 2003; Bryden et al., 2005). Is this enough to classify left-handedness as a pathology like it once was believed to be?
Making up new syndromes is not just a theoretical exercise. For example, the APA has been considering the inclusion of video game addiction in the DSM, a manual that grows considerably with every version that comes out.
There are differences between the brains of people with the "disorder" and those without.
So? Do you seriously expect there to be no differences between the brains of two groups of people who behave differently? The brains of men are different to the brains of women. I would be surprised if the brains of those with Extreme Sports Disorder are the same as those of the general population.
Another example (that Kassiane brought to my attention) is that alien abductees have abnormal EEGs in the temporal lobe. Alien abduction is not in the DSM-IV, AFAIK. While I don't doubt psychiatrists would be in theory willing to make up a syndrome related to alien abductions, it would be interesting to see how they handle the conspiracist circus that would result.
Treatment with neuroleptics controls the symptoms of the "disorder" and has effects on outcome.
There are drugs that can affect the way people behave and feel; no one disputes that. This does not demonstrate pathology either. How difficult would it be to invent a drug that causes people with Extreme Sports Disorder to be afraid of danger, for example? I'm sure it would save lives.
The spoof website on the "anti-effeminate" Hetracil also illustrates this point.
The nature of the risk factors demonstrates pathology.
I do not buy that it's about causation. Not many deaf people would consider deafness a pathology, even though there are no doubt a huge variety of causes of deafness, including injury, viruses and so on. It could be, though, that people in the general population tend to see deafness as a pathology. In that case, consider the relatively low heritability of another condition, left-handedness (Orlebeke et al., 1996).
Medical correlates demonstrate pathology.
Again, it's easy to demonstrate that many ways of being not considered pathologies have medical correlates.
Often times the "medical correlates" of a made-up disease are other made-up diseases. For example, the "scientific rationale" for depathologizing homosexuality was that it didn't correlate with psychiatric disorders. (Incidentally, I think the gay community is asleep at the wheel if they buy this as the proper rationale).
Calling something a pathology shouldn't be a big deal. Does it matter to Halle Berry, for example, whether her diabetes is called a disease or not?
The short answer is that it is problematic when the so-called disease refers to your way of being.
If it were no big deal, then shouldn't the gay community be OK with homosexuality being called a pathology?
The reasons why it is a big deal should be self-evident: (1) It redirects efforts and resources into finding an often mythical "cure", which many of the individuals with the condition do not want; (2) It discourages accomodation; (3) It provides a justification for discrimination; and (4) If the condition is an important part of someone's way of being, calling the condition a disease is effectively the same as calling the individual a disease.
This is the case whether we're talking about a disability or a difference.