This paper presents a model of ingroup favouritism in collective decisions. The focus is on the role of the institutional set-up for whether individuals' discriminatory preferences are mitigated or exacerbated. When are collective decisions less biased than individual decisions? Do diverse committees discriminate less than homogeneous ones? I consider the interaction between committee composition (homogeneous or diverse) and decision rule (unanimity or majority). The analysis suggests that homogeneous committees can be expected to discriminate more than individual decision makers both under unanimity rule and under majority rule, but the reasons are different under the two rules. Diversity in committees may help mitigate or avoid own group favouritism and can be expected to lead to less discrimination than decisions by homogeneous committees or by an individual decision maker.