Boundary between Normal & Abnormal
One of the tenets of developmental psychology is that a knowledge of normal development informs psychopathology partly because the boundaries between normal and abnormal are sometimes vague, diffuse, or continuous. Many disorders (eg, conduct disorder, dysthymic disorder) are defined on the basis of cutoffs in dimensional criteria rather than on qualitative distinctions that are more easily recognizable. Criteria such as “low energy” and “low self-esteem” (for dysthymic disorder) and “marked or persistent fear” (for social phobia) are matters of degree. One of the central questions is where to locate the boundary between normal and abnormal when the criteria of psychopathology are dimensional.
In some cases the boundary is arbitrary. In other cases the “true” boundary might be identified on the basis of three considerations: (1) a noncontinuous pattern of the distribution of scores, (2) a qualitatively distinct change in functioning that accompanies a quantitative difference in a score, or (3) unique etiology at the extreme of a distribution.
The first consideration is whether the population of scores is distributed normally with a single mode or bimodally with an unusually large number of cases at one extreme. A large number of cases at one extreme would suggest that a second causal agent is operating, beyond whatever agent caused the normal distribution. A second causal agent might suggest a deviant (ie, psychopathologic) process. Consider the relation between the intelligence quotient (IQ) score (a continuous measure) and mental retardation. The distribution of IQ scores in the U.S. population is not normal. Far more cases of IQs below 70 occur than would be expected by a normal distribution. Thus the distinction between normal and abnormal IQ scores is not merely one of degree.
The second consideration is whether qualitative differences in functioning occur with quantitative shifts in a criterion. For example, if a decrement of 10 IQ points from 75 to 65 makes it significantly more difficult for a child to function in a classroom than a decrement from 100 to 90, then a case can be made for locating the cutoff point near an IQ of 70.
The third consideration is the possible distinct etiology of scores at an extreme end of the distribution. A single set of causes will ordinarily lead to a normal distribution of scores. A disproportionate number of scores at an extreme often suggests a separate etiology for those scores. In the case of IQ scores, one set of forces (eg, genes, socialization) leads to a normal distribution, whereas a second set of forces (eg, Down syndrome, anoxia, lead toxicity) leads to a large number of cases at the low extreme.