In psychology, aversives are unpleasant stimuli that induce changes in behavior through punishment; by applying an aversive immediately following a behavior, the likelihood of the behavior occurring in the future is reduced. Aversives can vary from being slightly unpleasant or irritating (such as a disliked color) to physically damaging (such as an electric shock). It is not the level of unpleasantness, but rather the effectiveness the unpleasant event has on changing behavior that defines the aversive.
Types of aversive stimuli
There are two types of aversive stimuli:
Unconditioned aversive stimuli
Unconditioned aversive stimuli naturally result in pain or discomfort and are often associated with biologically harmful or damaging substances or events. Examples include extreme heat or cold, bitter flavors, electric shocks, loud noises and pain. Aversives can be applied naturally (such as touching a hot stove) or in a contrived manner (such as during torture or behavior modification).
Conditioned aversive stimuli
A conditioned aversive stimulus is an initially neutral stimulus that becomes aversive after repeated pairing with an unconditioned aversive stimulus. This type of stimulus would include consequences such as verbal warnings, gestures or even the sight of an individual who is disliked.
Use in Applied Behavior Analysis (ABA)
Aversives can be used as punishment during applied behavior analysis with autistic children to reduce unwanted behavior, such as self-injury, that poses a risk of harm greater than that posed by application of the aversive. Aversive stimuli may also be used as negative reinforcement to increase the rate or probability of a behavior by its removal. The use of aversives was developed as a less restrictive alternative to practices prevelant in mental institutions at the time such as shock treatment, hydrotherapy, straightjacketing and frontal lobotomies. Early iterations of the Lovaas technique incorporated aversives during therapy, though the use of aversives in ABA was not without controversy. Over time the use of aversives has become less and less necessary as less and less restrictive alternative treatments have been developed. Lovaas has since stated his disdain for the use of aversives.  Applied Behavior Analysis permits the use of aversives in limited cases, such as when a behavior is dangerous, especially when the reinforcing contingencies that maintain a behavior are unknown.
The Behavior Analyst Certification Board (BACB) Guidelines for Responsible Conduct state that voluntariness by the client or their surrogate is a necessary component of any behavior plan, and clients or families have the right to terminate a particular intervention if they see fit, including aversive treatments. The Behavior Analyst Certification Board issues credentials for behavior analysts nationwide, although state regulations vary as to whether or not a person can represent themselves as a behavior analyst. The use of aversive treatment is something that practitioners of applied behavior analysis take very seriously (see Professional practice of behavior analysis) due to the risks involved and the contrevesy surrounding their use (even among behavior analysts). Several National and International Disability Rights Groups have spoken against the use of Aversive Therapies, including TASH and AUTCOM.
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