This paper sheds light on the links between media and political polarization by looking at the introduction ofbroadcast TV in the US. We provide causal evidence that broadcast TV decreased the ideological extremism ofUS representatives. We then show that exposure to radio wasassociated with decreased polarization. We interpretthis result by using a simple framework that identifies two channels linking media environment to politicians’incentives to polarize. First, the ideology effect: changes in the media environment may affect the distribution ofcitizens’ ideological views, with politicians moving theirpositions accordingly. Second, the motivation effect: themedia may affect citizens’ political motivation, changing the ideological composition of the electorate and therebyimpacting elite polarization while mass polarization is unchanged. The evidence on polarization and turnout isconsistent with a prevalence of the ideology effect in the case of TV, as both of them decreased. Increased turnoutassociated with radio exposure is in turn consistent with a role for the motivation effect.

• The role of partisan media Boston Globe, Oct 06, 2013