Under the social investment paradigm, formal childcare services are heralded as being the policy instrument par excellence to combat social exclusion. However, it was shown that a Matthew effect (ME) in its use is present in almost all European countries: disadvantaged children are less likely to use childcare than more advantaged children. In this contribution we aim to uncover the cause of the ME by distinguishing between supply-side and demand-side explanations. This refers to constraints in the availability or affordability of childcare and to dominant cultural norms on motherhood. In doing so, we take due account of the role of employment. The results show that the ME in formal childcare cannot be explained by class differences in employment. Moreover, the ME is related to the supply-side and much less to the demand-side. Structural constraints in childcare provision matter everywhere and tend to limit the uptake of childcare, especially for disadvantaged children. In contrast, cultural norms on motherhood are a less important predictor of the ME in childcare use. This means that more investment in the provision of childcare services is necessary in order to achieve its ambitious policy goals.