Attached is an article arguing that the US primary system produces politically polarized candidates in general elections. While not a tight argument (i.e., why now?), increasing polarization seems to be an observable attribute of recent US politics. Also, (although measurement is tricky) this seems less the case in Canada and the UK, even though parliamentary candidates are chosen in local riding association meetings, presumably subject to the same forces as US primaries. Conventional wisdom among political scientists and economists both is that the parliamentary system (as opposed to presidential) influences the nature of the candidates one sees in elections.
