Interventions to counter misinformation are often less effective for polarizing content. We found that crowdsourcing accuracy judgements by adding a "Misleading count" (next to the Like count) reduced sharing inaccurate information about partisan issues by 25% royalsocietypublishing.org/d… The Misleading count was also more effective when it reflected in-group norms (from fellow Democrats/Republicans) compared with the norms of general users. The normative intervention was roughly 5X as effective as another popular misinformation intervention (i.e. the accuracy nudge reduced sharing misinformation by 5%). And extreme partisanship did not undermine the effectiveness of the intervention. Our results suggest that identity-based interventions based on the science of social norms can be more effective than identity-neutral alternatives to counter partisan misinformation in politically polarized contexts (e.g. the US). Led by @ClaraPretus @alijaveed_ @DianaRHughes1 @KobiHackenburg @manos_tsakiris @OVilarroya