Interpersonal influence shapes political behavior and attitudes. So far, however, little efforts have been devoted to testing this mechanism comparatively in Europe. This paper aims at explaining differences in influence patterns in three European countries (Italy, Germany, and the UK). Based on works in demography, we argue that in Mediterranean cultures (characterized by high degrees of familism), people are more prone to be affected by attitudes and behaviors of their relatives with respect to other strong ties (e.g., spouses), while in continental, northern Europe, and the UK, this effect is less important. We test this argument using longitudinal data. Consistent with expectations, results show that the influence of relatives in familistic contexts is stronger than in non-familistic ones. At the same time, the spouse effect (namely, the effect of an intimate, non-relative discussant) is particularly strong in non-familistic contexts (and vice versa). In sum, we demonstrate that public opinion studies can be strengthened by theories developed in other social sciences, such as demography.