Quantifying The Small Body Problem

When advocates urge people to eat less meat, do they accidentally steer consumers from cows and pigs toward chickens and fish? This is the “small body problem”: reducing red meat sounds like a win, but if it shifts demand onto smaller animals, the body count goes up. It takes roughly 200 chickens to replace the meat from a single cow. Health and environmental appeals — two of the movement’s most common pitches — both tend to single out red meat, making the concern more than hypothetical.

To find out whether the problem is real in practice, we teamed up with Faunalytics to analyse 38 articles containing 78 large-animal, 64 small-animal, 57 fish, and 36 bird intervention outcomes. We took a review-of-reviews approach, drawing data from studies housed in meta-analyses and systematic reviews published between 2020 and 2024.

No evidence of small-body substitution. Across the literature, reduction interventions did not increase consumption of small-bodied animals, fish, or birds on average. Advocates worried they are inadvertently fueling chicken consumption can exhale — at least a little.

But also no evidence of reduction. Interventions did not, on average, meaningfully decrease consumption of any animal category either. Positive and negative effects across studies appear to cancel out. This is less damning than it sounds: the variability between studies was enormous (τ values up to 2.9, where 0.8 is already considered “large”), which means some interventions worked, some backfired, and pooling them flattens real signal.

Choice architecture stood out. Among 20 interventions using choice architecture — plant-based defaults, menu redesigns, strategic product placement — small-bodied animal consumption dropped significantly (SMD = -2.63, p = .025). It was the single intervention feature that held up under scrutiny.

Everything else is too thin to judge. The planned meta-regression to identify which intervention features drive substitution had to be abandoned: the literature simply doesn’t contain enough comparable studies to support it. Many intervention types (health appeals, vegan asks, university-course formats) had fewer than 10 corresponding studies — below the threshold for reliable inference.

For advocates: Use choice architecture wherever you can. It appears to be the safest bet for moving consumption in the right direction without triggering substitution. Other intervention types aren’t ruled out — we just can’t yet tell you which ones to trust.

For researchers: Replicate. The field has breadth but not depth. We are comparing leaflets to lectures to eco-labels and calling them the same thing. Future studies should also measure specific animal products (not “meat” in aggregate) so that small-body substitution can actually be tracked.

Caveats worth naming: The review-of-reviews approach lags behind the most recent literature, screening was done by a single researcher, and included studies were all English-language.