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Spontaneous remission is the natural tendency for many medical conditions and psychological problems to gradually improve over time without any formal treatment or intervention. This phenomenon acts as a significant confounding variable in studies that lack a control group; researchers evaluating a treatment using a simple pretest-posttest design may falsely attribute a patient's recovery to their specific intervention, when in reality, the condition would have naturally improved on its own.

Spontaneous Remission

In a 2001 meta-analysis of depression treatments, researchers Michael Posternak and Ivan Miller highlighted the powerful effect of spontaneous remission. They discovered that participants assigned to wait-list control conditions—who had not yet received any formal therapy—still showed an average improvement of $$10\%$$ to $$15\%$$ in their depression symptoms. This finding underscores the necessity of control groups to isolate true treatment effects from natural recovery.

Posternak and Miller's 2001 Spontaneous Remission Study

To effectively rule out threats to internal validity—such as history, maturation, testing, instrumentation, regression to the mean, and spontaneous remission—researchers can add a control group to a pretest-posttest design. A control group does not receive the experimental treatment but is subjected to the same external events, passage of time, and repeated measurements as the treatment group. By comparing the posttest changes between the two groups, researchers can isolate the actual effect of the treatment from these alternative explanations, although doing so means the study is no longer a one-group design.

Control Group in Pretest-Posttest Designs

A wait-list control condition is an experimental setup where participants are informed they will receive the intervention, but must wait until those in the primary treatment condition have already completed it. This approach enables researchers to compare outcomes between participants who have received the treatment and those who are currently untreated but maintain the expectation that they will eventually improve.

Wait-List Control Condition

In a classic 1952 article, researcher Hans Eysenck challenged the assumed effectiveness of psychotherapy by evaluating the results of existing pretest-posttest designs. Although the original clinical studies showed patient improvement after receiving therapy, Eysenck compared these results to archival data from state hospitals and insurance companies for untreated patients. He found that both the treated and untreated groups recovered at similar rates. This highlighted that the improvements observed in the treated group were likely due to spontaneous remission rather than the psychotherapy itself, underscoring the absolute necessity of including a proper control group to accurately measure treatment efficacy.

Eysenck's 1952 Psychotherapy Effectiveness Study

A clear example of spontaneous remission is the natural recovery from the common cold. If researchers measure the symptom severity of $$100$$ people with colds, feed them chicken soup daily, and then re-evaluate them a week later, the patients will likely feel much better. However, because colds naturally run their course and improve over time without any medical intervention, this recovery is an instance of spontaneous remission rather than proof that the chicken soup cured the illness.

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