Both biology and psychology have been discovering for some time now that the frontier between normal and abnormal is not as clear as we would like to think. The difference between perfectionism and obsessiveness, between nervousness and anxiety disorder, or between social awkwardness and autism, seems to be only a matter of degree. Something that popular wisdom had sensed long ago.
On the one hand, there are indications that the same genes that are at the base of certain disorders are also "responsible" for the normal personality traits that resemble them. Genomic analyses have discovered that the genes "suspected" of causing some disorders are normally distributed within the general population. And, oh, surprise! The greater the concentration of those genes, the greater the presence of the disorder.
Traits have also been found to "run" in families: relatives of patients with diagnosed disorders such as autism or schizophrenia often have similar personality traits, but within normal ranges .
This would imply the existence of "degrees" in the trait, which could be caused by a greater or lesser quantitative accumulation of the "responsible" genes. It would also imply that mental disorders are quantitative extremes of normal personality variations. In other words, there would not be "disorders", but rather "quantities" of a given trait.
Even psychiatry, used to well-defined diagnostic "categories," has begun to align itself with this view. The National Institute of Mental Health is changing its diagnostic strategy to a model of "dimensions" rather than the "categories" used in the past.
Danger alert! This continuity between "normal" and "abnormal" refers to the causes of a disorder, not to its consequences. Those who are afflicted with mental illness can experience it with great pain, and those around them as a great misfortune.
And of course, the degree to which a trait will be considered "pathological" will also depend, to a large extent, on the environment. A low level of "agreeableness" can be adaptive when the social environment is harsh. Entering a butcher shop and roughly beating freshly slaughtered animals hanging from a bloody hook is probably highly inappropriate, except if your name is Rocky Balboa, in your spare time you collect debts for a Philadelphia loan shark, and while doing so you listen to Bill Conti's music.