Information effects scholarship argues that knowing more about relevant issues changes how people vote, although findings demonstrate that no particular ideological position consistently benefits from these shifts. Newer work extends this to consider how knowledge is relevant for other outcomes, including attitudes and policy preferences. Yet observational surveys often lack questions testing respondents’ factual knowledge about particular topics. Consequently, extant work relies on batteries of general political knowledge questions to distinguish more informed voters from less informed ones. This invokes a potentially strong assumption: possessing general knowledge should be diagnostic of holding issue-specific knowledge. We empirically test this assumption using 2018 survey data from seven European countries (Germany, Hungary, Poland, Romania, Spain, Sweden, UK) that, unusually, includes knowledge questions about general politics as well as EU immigration (N=11,429). After building knowledge scales using item response theory, we construct counterfactual models that simulate how becoming more informed about either general politics or EU migration changes respondents’ immigration attitudes and preferences (an ‘information effect’). Then, we compare the sizes and directions of these effects across both types of knowledge questions. Our results make two contributions: (1) novel cross-national evidence of how and to what extent EU immigration attitudes and preferences are sensitive to citizens’ levels of different kinds of knowledge, and (2) empirical evidence about the validity of using general political knowledge questions as a proxy for respondents’ specific knowledge about issues. This presents implications for information effects research that depends on these common survey question types.