 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.