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Drug abuse is usually viewed as a behavior that occurs because of what the drugs do in the body. Most people assume that drug abuse is mainly driven by the immediate, attractive effects and sensations produced by the drugstheir physiological and psychological effects. But there is another, less direct way drugs can become of overwhelming importance to a person and dominate life. Excessive drug use can develop as a side effect of other strongly desired behavior that, for various reasons, cannot be engaged in or completed.

For example, for any one of a number of reasons a person may not be able to do something they are motivated to do: They may not be permitted to do it, the goals of the behavior may not be available to them, or the person may not have the skills or knowledge required to perform the behavior successfully. When blocked in any of these ways from performing a desired behavior, a person may turn aside to some sort of easy, satisfying alternativean adjunctive behavior. The following is a description of the first kind of adjunctive behavior that was produced in an experimental laboratory. It does not involve drug taking, but it has many of the exaggerated and compulsive characteristics usually thought to be a result of drug use.

Normal, adult laboratory rats were first reduced in body weight; then it was arranged for them to receive much of their daily food ration in individual chambers. The food was given as 45 milligram pellets, and each pellet was available only about once per minute over a 3-hour period each day. Although water was always freely available back in each animal's home cage, this schedule of intermittent food availability, in which the eating of small food portions was spread out over a 3-hour period, produced a strange result. When an animal received a food pellet, it quickly ate it and then took a large drink of water. Since an animal received a total of about 180 pellets during each daily, 3-hour session, and drinking occurred after almost every pellet, by the end of the 3 hours an animal had drunk an amount of water equal to about one-half its body weight. That is an excessive amount, and the overdrinking occurred day after day during each daily session. It seemed compulsive, for the animals did not lose interest in drinking water heavily after a few days. The exaggerated drinking that occurred under this feeding condition did not disappear as long as the intermittent schedule of feeding remained in force.

It was easy to prove that the excessive drinking was not a physiological effect of reducing the daily intake of food: If, instead of being given pellets spread across the session, rats were given the same 180 pellets all at once as a single ration, then they drank about 10 milliliters of water over the next 3 hours, rather than almost 100 milliliters. So, the rats did not really need the excessive amount of water they drink under the schedule of intermittent pellet delivery. Something about doling out bits of food to them over time drove the exaggerated drinking behavior. They got what they needed of something of crucial value (in this case, food), but the individual portions were rather small, and there was a time delay between these portions.

The details of how the size of the portions, the amount of time between them, as well as the state of food deprivation, affects the degree of excessive drinking have been worked out in many experiments. The details of these relations will not be considered here, but a few additional facts indicate the widespread nature of this excessive-behavior phenomenon. As described, this overdrinking of plain water is referred to as schedule-induced polydipsia. The term polydipsia means the food schedule can induce excessive drinking of many sorts of fluids. But in general, behaviors that become exaggerated under these sorts of conditions are known as adjunctive behaviors.

The phenomenon of adjunctive behavior is not limited to the rat species and it can be induced in ways other than food-schedule intermittency. And adjunctive behaviors other than drinking can occur. Adjunctive behavior can be induced in many animal species, for example, mice, monkeys, pigeons and chimpanzees, as well as in humans. The behavior can be induced by a generator schedule that is not based on doling out food; other kinds of incentives also are effective in inducing excessive behavior. For example, humans reinforced intermittently with money, by the opportunity to gamble, or even by maze solving, showed adjunctive increases in fluid intake, general activity, eating or smoking. In animal experiments, excessive aggression, overactivity, and eating are a few of the behaviors that occurred adjunctively owing to the intermittent availability of some important commodity or activity.

The adjunctive behavior of greatest interest with respect to the problem of drug abuse is, of course, the excessive seeking and taking of drugs. A few examples will be given. But first, as indicated above, it is important to understand that adjunctive drug taking is only one kind of excessive behavior that can be driven by a generator schedule. Thus, drug abuse is just one sort of adjunctive behavior; it is not a special problem driven exclusively by the pharmacological actions of drugs of abuse.

Once research had established the conditions inducing adjunctive drinking, it was of interest to determine if fluids other than water would be drunk to excess, especially drug solutions. Briefly, if the drug concentration of a drinking solution is not too high (a high concentration is usually too bitter), a much greater unforced (voluntary) oral drug intake can be induced by a generator schedule than would occur otherwise. Several classes of drugs have been investigated, and excessive intakes of alcohol, opioids (e.g., morphine), sedatives (e.g., barbiturates), anxiolytics (e.g., benzodiazepines), stimulants (e.g., cocaine), and other agents (e.g., nicotine) have been sustained for many months under these inducing conditions. For some of these agents, physical dependence results from the excessive amounts ingested by animals. However, if the generator schedule is discontinued, drug intakes decrease immediately to much lower levels. Thus, without the generator schedule, the high drug intakes do not continue. Similar generator schedules (intermittent food delivery) also can induce excessive intravenous drug self-administration.

These and related studies clarify the nature of what sustains excessive intake of drugs (drug abuse). It is important to note that, by this analysis, drug abuse arises from a background of excessive behavior that is induced by a generator schedule. It is produced by the limited and intermittent availability of crucially important environmental events. Such schedules are similar to conditions in natural and social environments that provide commodities we need, and activities we desire, but only in little bits at a time, and with delay intervals in between the bits. The adjunctive behavior generated may be largely noninjurious, like water drinking. Or the adjunctive behavior may be creative, like an intense hobby. But it also can result in aggression or drug taking, depending upon personal history, the skills one possesses, and currently available opportunities.

The pharmacological consequences of drug taking are only one factor sustaining drug abuse. The environmental generating conditions and the context of alternative opportunities are of crucial importance. Drug abuse occurs out of conditions already generating behavioral excesses. Although drug abuse often is described as if it is a direct consequence of exposure to a drug with abuse potential, the great majority of people experimenting with such drugs do not become abusersthey simply lose interest and turn to other pursuits. (This is not to approve of such hazardous experimentation.) Those self-administering such drugs for medical reasons (opioids are prescribed on a long-term basis for controlling chronic pain) almost never become drug dependent or motivated to abuse these agents.

Adjunctive behavior studies teach us that drug abuse stems more from environmental generating conditions, together with a lack of, or poor utilization of, other opportunities, than from any overwhelming intrinsic attractiveness of agents with abuse liability. A few drug-abuse situations can be described briefly to clarify how drugs come to be so attractive to some individuals, and what causes this behavior to persist in the face of the trouble it causes for them.

An example frequently given is that of a poorly educated youth, living in an urban ghetto, with minimal job prospects, who deals in drugs for economic reasons, and comes to use them owing to the sparse schedule of conventional opportunities and satisfactions available in that environment. Although that may be true, drug abuse is a problem that occurs not just for disadvantaged persons. The conditions of life for the affluent rich do not strike us as being extreme conditions that can lead to drug abuse. But consider the overprivileged young person, sent to a superior boarding school by parents with great expectations (i.e., demands), but with little time to provide their children with direct social reinforcement. They may be too engaged with their own lives, management responsibilities, and range of personal advantages, to afford their children much of their valuable time. The lean schedule of social reinforcement for the children, the often competitive nature of social interactions in private schools, the young person's migrant status in a new or isolating school, together with a high disposable income, can lead to engaging in drug abuse, particularly if this activity gains one local power and social status, while the threat of enduring legal consequences is negligible. And the person, although perhaps expected to take over the family business, together with its social obligations, is not yet empowered to do so, or to have their opinions taken seriously, so that their current social status is weak even if they appear privileged.

Consider a sales representative who travels for a corporation, making intermittent sales agreements (which may be subject to cancellation), with little effective influence on company policies or politics, with little to do in brief stop-overs in strange towns. Given the demand characteristics of the job, the uncertainty and intermittency of reinforcement, and the sparse opportunities for creative efforts when "on the road," a person so exposed may be vulnerable to drinking too much in the easy ambiance of hotel bars.

Studies on the schedule-induced production and chronic maintenance of excessive intake for a variety of drugs indicate that drug abuse is a problem that has its roots in the behavioral effects induced by environmental conditions, as well as in the pharmacological consequences of these elevated intakes. The exaggerated and problematic behavior designated as drug abuse is not behavior that is specific to, or the outcome of, a person's interaction with drugs. It is one possible adjunctive outcome of a set of conditions that comprise economically and/or socially restricted schedules of reinforcement that generate and sustain a host of possible exaggerated and persistent behaviors.


Falk, J. L. (1971) The nature and determinants of adjunctive behavior. Physiology and Behavior, 6, 577-588.

Falk, J. L. (1977) The origin and functions of adjunctive behavior. Animal Learning and Behavior, 5, 325-535.

Falk, J. L. (1981) The environmental generation of excessive behavior. In S. J. Mulé (Ed.), Behavior in Excess: An Examination of the Volitional Disorders (pp. 313-337). New York: Free Press.

Falk, J. L. (1993) Schedule-induced drug self-administration. In F. van Haaren (Ed.), Methods in Behavioral Pharmacology (pp. 301-328). Amsterdam: Elsevier.

Falk, J. L. (1996) Environmental factors in the instigation and maintenance of drug abuse. In W. K. Bickel & R. J. DeGrandpre (Eds.), Drug Policy and Human Nature: Psychological Perspectives on the Prevention, Management and Treatment of Illicit Drug Abuse (pp. 3-29). New York: Plenum.

Falk, J. L. (1994) Schedule-induced behavior occurs in humans: a reply to Overskeid. The Psychological Record, 44, 45-62.

Falk, J. L. (1998) Drug abuse as an adjunctive behavior. Drug and Alcohol Dependence, 52, 91-98.

Falk, J.L. and Tang, M. (1988) What schedule-induced polydipsia can tell us about alcoholism. Alcoholism: Clinical and Experimental Research, 12, 576-585.

John L. Falk

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Adjunctive Drug Taking

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