We study how peers affect in-group bias. Exploiting several umpiring reforms in international cricket matches - where two umpires make independent decisions in each other’s presence - we show that home-team umpires are less biased when working with a neutral colleague, i.e. one who is neither a national of the home nor the foreign team. This temporary debiasing is driven by the social pressure umpires feel to be impartial in the presence of neutral peers. Performance evaluation by visually non-salient monitors does not reduce bias, suggesting that physical presence is an important component of debiasing and peer influence.