Can electoral reform lead to a reduction in the number of terrorist incidents? Economists have shown that the introduction of constitutional institutions such as courts in the early eighteenth century had a direct effect on investment. Could there be a similar link between the introduction of proportional representation (PR) electoral systems and a reduction in the number of terrorist attacks? Previous studies using cross-sectional data have found a negative correlation between the presence of PR-electoral systems and the number of terrorist incidents. However, earlier studies were based on aggregate figures, not on time-series data. They did not provide a direction that could be used to measure the possible effect of the introduction of PR. This research note addresses this problem. Using a paired samples t-test it is possible to show that the introduction of proportional representation in Northern Ireland and Algeria led to a marked reduction in the number of terrorist attacks. The note thus adds strength to earlier studies.