We provide causal evidence on how people's beliefs about the labor market effects of immigration affect their attitudes toward immigration. In the study, we plan to elicit people's beliefs about how the Mariel boatlift, which caused a large influx of low-skilled Cuban immigrants to Miami, Florida, affected local labor market outcomes. Then, drawing on the results from Card (1990), half of the participants will receive information about the actual labor market consequences of the Mariel boatlift. Subsequently, we will measure the participants' support for immigration with self-reported and behavioral measures. In the attached pre-analysis plan, we outline our plan for analysis of the data, including the main specifications of interest, the dimensions of heterogeneity, and corrections for multiple hypothesis testing.