Family-controlled firms (FCFs)' prevalence, strategies, and performance differ across countries. We explain these differences through the lens of informal institutions, suggesting that different countries have different levels of appreciation for family business. To capture this effect, we introduce the construct of family business legitimacy (FBL) and an associated index (FBLI). We empirically measure FBLI scores for 83 countries spanning both developed and emerging economies. By combining meta-analytic and archival data, we show that FCFs prevail, follow unique strategies, and outperform non-FCFs in countries with high FBLI scores. As a new contingency variable, FBL advances the literature on the informal institutional embeddedness of organizations and family business.