In 1969, researchers C.R. Schuster and T. Thompson initiated a study to explore drug-seeking behaviors in monkeys. The experiment aimed to determine if primates would voluntarily self-administer psychoactive substances, providing insight into the nature of addiction. This investigation became known not just for its scientific conclusions but also for the significant ethical questions it raised within the scientific community.
The Experimental Design
The subjects were primates, including rhesus monkeys and baboons, chosen for their physiological and behavioral similarities to humans. To facilitate the delivery of drugs, the animals underwent surgery to implant intravenous catheters. These catheters were connected to a system that allowed for the direct administration of substances into their bloodstream within an operant conditioning chamber where each animal was housed.
The monkeys learned that pressing a lever would result in an injection of a drug. This method of self-administration was designed to gauge the reinforcing properties of each substance. After this initial conditioning phase, the experimental parameters shifted significantly.
The animals were then given continuous, unrestricted access to the drug-dispensing lever. This meant they could self-administer as much of the drug as they desired, 24 hours a day. Researchers recorded their behavior on video, observing how patterns of drug use and general conduct changed over time. Each monkey was exposed to a different substance, including cocaine, morphine, and amphetamines, to compare their effects.
Observed Behaviors and Results
The results of the unlimited access phase were dramatic. Monkeys with access to stimulants like cocaine and amphetamines quickly escalated their intake. They would press the lever repeatedly, leading to severe behavioral changes. Some animals experienced convulsions and exhibited hallucinatory behaviors, such as picking at their own fur and skin. In several cases, this self-administration led to death within weeks.
The group of monkeys given access to opioids like morphine also demonstrated a clear progression into dependency. Their behavior became overwhelmingly focused on pressing the lever to maintain their drug supply. This drive often superseded other natural behaviors and instincts. The study documented the powerful reinforcing nature of these drugs.
Some of the most unsettling observations came from the extreme physical and psychological distress. There were reports of monkeys suffering from drug-induced psychosis, leading to self-mutilation. In one instance, a monkey broke its arm while trying to escape its enclosure during a state of agitation.
The data collected demonstrated the addictive potential of these substances. One monkey self-administering cocaine was recorded to have given itself over 40 injections per day until it died from an overdose. Another monkey, provided with codeine, experienced convulsions and died after reaching a high daily dosage.
Scientific and Ethical Implications
The study furnished compelling evidence that addiction is a powerful behavioral driver. By showing that monkeys would self-administer drugs to the point of severe harm and death, the experiment helped to shift the perception of addiction from a moral failing to a biological condition. It demonstrated that the compulsion to use certain substances could override fundamental survival instincts. The use of intravenous self-administration in this study also helped establish it as a standard model for assessing the addictive potential of drugs.
The extreme suffering of the animals became a focal point for the growing animal rights movement, with the monkeys’ distress serving as a powerful example of cruelty in scientific research. The ethical controversy contributed to significant changes in the regulation of animal research. It was instrumental in prompting stricter governmental controls, including the strengthening of the Animal Welfare Act, and its legacy includes the now-mandatory establishment of Institutional Animal Care and Use Committees (IACUCs) to prevent unnecessary suffering.