A welfare state’s tax system does not solely redistribute from rich to poor (vertical) but also between family types (horizontal). Different types of families are treated differently due to gendered (de)familialization policies in the tax code, such as joint filing for spouses or single-parent relief. In this study I aim to examine the tax system’s modification of horizontal income inequality between the six most prevalent family types of non-retiree households. To answer my research aim I draw on harmonized data from 30 countries provided by the Luxembourg Income Study (LIS). I estimate pre- and post-fiscal income inequality measured as between-family-type Theil indices. Using multivariate linear regression, I examine the association of the percentage change in inequality and the prevalence of family type-related tax characteristics. The results show that welfare states with familialization tax policies reduce less horizontal income inequality compared to welfare states without familialization tax policies. As familialization tax policies provide additional benefits for breadwinners with dependents, they discourage labour market participation of secondary earners and might exacerbate gender inequalities.