Research on the 'contagion effect' suggests that the dynamics of diffusion and competition influence a political party's propensity to adopt candidate gender quotas. Led by the work of Matland and Studlar (1996), 'contagion theory' suggests that the adoption of gender quotas by small (generally leftist) parties on the political periphery will incentivize other rival parties to follow suit, especially in countries with PR electoral systems. Yet while the concept of 'contagion theory' is widely used in the women and politics field, subsequent research has tended to apply it descriptively and uncritically, even when the original conditions of the theory are not met. Much of this work also continues to focus on single case studies, limiting the broader applicability of their findings (see for example Kolinsky 1991; Baldez 2004; Meier 2004; Davidson-Schmich 2010). This paper seeks to fill this gap, bringing together new and comparative empirical research to reevaluate the central propositions of contagion theory. Building on recent work in the field, it moves beyond the theory's traditional focus on inter-party competition to also assess the impact of intra-party dynamics on party change (cf. Kittilson 2006; Cowell-Meyers 2011; Kenny and Mackay 2012). Drawing on a cross-sectional sample of West European countries, we attempt to identify the conditions and characteristics that facilitate or inhibit processes of quota contagion. Our findings challenge several of the key tenets of contagion theory as it is currently conceptualized – including the relative importance of small catalyst parties, party competitiveness, and electoral system variables – and point to the need to consider how both external and internal factors impact on party decisions to adopt and effectively implement gender quotas.