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How and why do the policy positions of parties change? We have good theoretical reasons to expect that political parties adopt policy positions in response to the preferences of voters, since voters are likely to prefer parties that have ideological positions that most resemble their own. At the aggregate level, this leads to the prediction that parties shift their positions towards the median voter's bliss point. Extensive research has documented that parties adjust their ideological position in response to public opinion shifts. Yet, such empirical research has tended to lack sufficiently long time-series of party positions to systematically examine the dynamics of party positions over time. This leaves a number of questions still to be further explored, such as: when do parties shift their ideological positions? Are parties more responsive to the median voter or to party supporters? Does the responsiveness of parties to voters depend on the electoral cycle? This paper examines these questions using time-series data on party leader positions from 1973-2008 in Denmark and Britain. We find that party leaders adjust their positions in the direction of the median voter, and that the extent of this adjustment is greater in election years. ..PAT.-Unpublished Manuscript [ABSTRACT FROM AUTHOR]

An important property of any party system is the range of issues it presents to the electorate. Party systems are often characterized in terms of the level of ideological polarization, but rarely in terms of the relative amount of issue choice afforded to voters; what we refer to as issue diversity. In this paper, we present an innovate approach to measuring issue diversity and we examine what explains variation in issue diversity over time. Following the issue competition literature, we would expect that parties emphasize different issues (issue divergence is the norm). This theoretical approach does not, however, explain why issue diversity changes over time. We present an alternative explanation that draws upon the dynamic representation model. We argue that issue diversity is primarily driven by public opinion, and that parties in government as well as opposition parties are driven by the same incentives to diversify (or narrow) their issue appeal. These propositions are tested by analyzing party leader speeches in two parliamentary systems, Britain and Denmark, in the period 1953-2007, using time series analysis. ..PAT.-Unpublished Manuscript [ABSTRACT FROM AUTHOR]