= Solution
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.
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