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A negative control group is chosen so that the proposed causal mechanism should not operate in it. Reproducing the target association in that group is evidence for residual bias or an alternative mechanism.

Cultural abstainers form a negative control group: ending rationing should not materially change their sugar intake. A diabetes discontinuity at the cutoff among consumers but not abstainers supports the proposed sugar pathway, whereas a similar discontinuity in both groups points to cohort confounding. The interaction between cutoff exposure and consumer status, or a difference-in-differences contrast, formalizes this comparison, provided abstention itself is not differentially selected across cohorts.