Looking at the OECD data, one thing that is striking is that Belgium "rejoins the pack" if you look at the tax burden for a two-parent single-earner family.
Looking at the list (OECD Taxing Wages), I'm going to hazard a guess that it's a cultural/religious thing - it looks like the drop in the average tax rate as a proportion of household income for a single-earner vs. the same income for a single person is far greater in countries with a significant proportion of Catholics.
Looking at the list (OECD Taxing Wages), I'm going to hazard a guess that it's a cultural/religious thing - it looks like the drop in the average tax rate as a proportion of household income for a single-earner vs. the same income for a single person is far greater in countries with a significant proportion of Catholics.