This paper examines how panelists serving on interdisciplinary funding panels produce evaluations they perceive as fair, drawing on 81 interviews with panelists serving on multidisciplinary fellowship competitions. We identify how peer reviewers define "good" interdisciplinary proposals and the rules they follow: respect for disciplinary sovereignty, deference to expertise and methodological pluralism. These rules ensure the preponderance of the voices of experts over non-experts. Panelists also adopt strategies to make other reviewers who lack expertise trust that their judgments are disinterested and unbiased, while reviewers who lack expertise are not afraid to make decisions based on idiosyncratic tastes rather than substantive quality.