The prefix de- means not and opposite of. The prefix de- also means

Merriam-Webster's Eleventh Collegiate Dictionary (2003) lists several words in which the prefix un- negates a word that de- has previously negated:

Such instances are quite rare, however, in comparison to the number of words that use re- to override a de- negation. Thus, for example, if a doctor decontaminates a wound with antiseptic, but bacteria later reappear, we say that the new germs "recontaminate" the wound, not that they "undecontaminate" it.

disorders of the brain include both behavior disorders and other psychiatric illnesses. the primary difference between a behavior disorder and another type of psychiatric disorder is the presence of choice. psychiatric conditions are considered to be involuntary while in behavior disorders, choices are essential.