Estimates of the labour supply effects of recent UK reforms in the area of direct taxes and benefits show that policy can have significant influence on the level of employment. We confirm this in a simulation of an in-work support system introduced into the German tax and benefit system. Our simulation results suggest that introducing in-work tax credits in Germany would increase the employment of single individuals by over 105,000 but would result in a reduction of labour supply among individuals living in couples by about 70,000, among both women and men. The result found for men is especially important as it is markedly different from all results for the UK, where the net response among men has always been found to be positive. Our estimation results call for a high degree of caution as far as "importing" UK-style tax credits to Germany is concerned. In-work support based on family income would reinforce the existing work disincentives for secondary earners, reducing the employment levels of both men and women living in couples.