Political parties are often regarded as unitary actors that pursue policy and office in parliament. As members of either the government or opposition, their role in proposing, supporting or opposing policy seems to be set. However, there are instances in which individual members work together on legislation across party lines. While this behavior might seem irrational from a party unity perspective, I propose three mechanisms that might explain it. First, as MPs see themselves as surrogate representatives of social groups, they might be motivated to work together with other members of those groups on legislation. Second, candidate-centred electoral systems might motivate MPs to look for collaboration along geographic or policy lines rather than within party. Third, situations that weaken the government parties' cohesiveness, such as minority governments, oversized majorities and ideologically broad coalitions might lead to MPs looking for policy outside their party. I test these mechanisms using a dataset of bill cosponsorships in 14 European countries between 1990 and 2020.