Fundamental income tax reforms are usually justified by or opposed because of large employment implications. The employment gains and losses are supposed to originate from various behavioural and dynamic effects of tax reforms over the medium to long term. To test the limits of such arguments, we study hypothetical radical measures designed to have potentially large employment effects in the context of Slovakia. A close inspection of the different implications of such tax reforms for adjustment on the extensive margin of the labour market reveals that promises or worries of large employment effects have little empirical support. This is because labour supply responses to ‘making work pay’ are small, the requirement of revenue neutrality limits the extent to which (dis)incentivising work is feasible, and because income effects arising from positive assortative mating within families counteract total individual-level effects. Our framework suggests the focus of tax reformers should be on the variation in effective labour supply coming from intensive margin effects.