An aversive stimulus is “something we might find unpleasant or painful. A behaviour followed by an aversive stimulus results in a decreased probability of the behaviour occurring in the future. This both defines an aversive stimulus and describes the form of conditioning known as punishment” ((Boerre, G. C.). In other words, to discourage the undesired response occurring again, it is followed by something the recipient thereof would consider as punishment. Several decades ago a school decided to have a class run a pilot program with respect to punishment for students mishaving during class. So, when Johnny threw a spit ball again, the teacher asked him to go and sit in the hallway for 15 minutes. After a few days, the teacher realized that instead of throwing fewer spitballs, Johnny was throwing more. The reason for this was that he had become somewhat of a hero in the hallway as his peers lauded him for his behaviour that resulted in his time out and, besides, he did not mind at all being out of class for awhile.
Skinner did not actually approve of aversive stimuli because he felt that it did not work. For example, if children know that punishment will occur if they pass notes during an exam, they are not very likely to carry out such behaviour. However, if the teacher leaves the room for a little while, they may revert to their former ways and pass some notes, simply because the teacher is not there to catch them; and, therefore, cannot punish them. This happens because whatever was causing the bad behaviour was only covered up but not removed, as it would be if extinction had occurred. If the desired behaviour were continually reinforced, perhaps beginning with the reinforcment of baby steps (shaping), the individual would form a bond with the desired behaviour. If the individual learns the desired behaviour, it will be reflected in his behaviour, which must be observable and measurable.
Boerre, G. C. Personality Theories. B. F. Skinner 1904 – 1990. Retrieved on February 13, 2012, from http://webspace.ship.edu/cgboer/skinner.html.