The hypothesis to explain the often puzzling variability of political language in Parliament is that phenotypical elements of political speech such as emotionality, hostility, and oratorical style can be explained by structuralelements, that is by various aspects of the issues under debate. The data led us to criticize the more common cultural hypothesis, which would explain the variations of political language by linking it to the political culture of the members of parliament. As a result the evolutionof political language would be a global one, appearing in all policy domains. This was by no means the case. The evolution of political language between 1950 and 1970 is on the contrary a strongly differentiated one. The structural hypothesis performs much better both for the short- and for the medium-term variations. The expression of hostility during parliamentary debates can be explained satisfactorily by the stakes of the game. The expression of emotion seems to stem both from these stakes and from the level of aggregation of the issue. Finally, various aspects of oratorical style, such as the invocation of values, the principled motivation of proposals and the deductive logic used to legitimize the position taken, all seem to be derived from the level of aggregation and, most of the time, from the ideological contents of the issue as well. As a result we get a profile of political language which is rather tightly controlled by the opportunities and the necessities of the political agenda and much less by all kinds of extraneous factors.