Scholars have long noted that couples are more likely to vote and that partners’ turnout behavior is strongly correlated, but so far, little causal evidence has explained why. Utilizing a large Danish administrative data set with detailed information about the timing of individuals moving in together, we tease out whether these correlations are causal effects of interpersonal influence or stem from other factors. To identify the causal effect, we argue that couples moving in together right before an election are comparable to households moving together right after the election. We find strong evidence for a substantial causal effect of interpersonal influence: Concordance increases by twelve to seventeen percentage points while turnout increases by four to nine percentage points as an effect of cohabitation. The results highlight the importance of social norms and how the household as a proximate social network plays an essential role in explaining couples’ turnout behavior.