In order to win elections political parties sometimes adapt their policy platforms to a changing society. But according to some scholars left-wing parties are in this regard more reluctant than right-wing parties. The former would show less programmatic flexibility than the latter. Other authors nuance this difference and state that leftist parties are ideologically more volatile at one moment and rightist parties at another time. In this article we empirically test whether rightist parties show more programmatic flexibility than leftist parties. We make use of an in depth quantitative analysis of the socio-economic policy proposals of the Belgian liberal and social-democratic parties between 1961 and 2010. We find that the right-wing liberal party indeed makes larger programmatic changes. The intensity of the ties with social groups such as trade unions is probably the most important variable to explain this difference.