Children are so tremendously variable that it is sometimes hard to believe that they are all members of the same human family. You can crush some children with nothing more than a stern look; others seem to require strong and even painful disciplinary measures to make a vivid impression. This difference usually results from the degree to which a child needs adult approval and acceptance. As I said earlier, the primary parental task is to get behind the eyes of the child, thereby, tailoring the discipline to his unique perception.
In a direct answer to the question, it is not this individual variation that causes spanking to be ineffectual in most cases. When disciplinary measures fail, it is usually because of fundamental errors in their application. It is possible for twice the amount of punishment to yield half the results. I have made a study of situations where the parent has told me that the child ignores the spankings he receives, going back to violate the same rule. There are four basic reasons for the lack of success:
The most recurring problem results from infrequent, whimsical punishment. Half the time the child is not punished for a particular act of defiance; the other half of the time he is cuffed about for it. Children need to know the certainty of justice.
The child may be more strong-willed than the parent, and they both know it. If he can outlast a temporary onslaught, he has won a major battle, eliminating punishment as a tool in the parent's repertoire. Even though Mom spanks him, he wins the battle by defying her again. The solution to this situation is obvious: outlast him; win, even if it takes a repeated measure. The experience will be painful for both participants, but the benefits will come tomorrow and tomorrow and tomorrow.
The parent suddenly decides to employ this form of punishment after doing nothing for a year or two prior to that time. It takes a child a while to respond to a new procedure in this manner, and the parent might get discouraged during the adjustment period.
The spanking may be too gentle. If it doesn't hurt it isn't worth avoiding next time. A slap with the hand on the bottom of a multi-diapered thirty-month-old is not a deterrent to anything. It isn't necessary to beat a child, but he should be able to feel the message.