How do attitudes and social affiliations co-evolve? A long stream of research has focused on the relationship between attitudes and social affiliations. However, in most of this research the causal relationship between views and affiliations is difficult to discern definitively: Do people influence each other’s views so that they converge over time or do they primarily affiliate (by choice or happenstance) with those of similar views? Here we use longitudinal attitudinal and whole network data collected at critical times (notably, at the inception of the system) to identify robustly the determinants of attitudes and affiliations. We find significant conformity tendencies: individuals shift their political views toward the political views of their associates. This conformity is driven by social ties rather than task ties. We also find that, while individuals tend to associate with similar others, political views are notably less a basis for associational choices than demographic and institutional factors.