Voter turnout in US presidential elections: does Carville's law explain the time series?

We estimate a time series model of voter turnout for 34 US presidential elections, 1880-2012. Employing a variety of econometric techniques, our major results are as follows. (1) A negative and significant structural shift in voter turnout occurs in 1972 and is too large to be explained by the lowering of the voting age. (2) The 1972 shift is the only statistically significant structural shift to occur since the first decade of the twentieth century. (3) Short-term macroeconomic conditions significantly impact turnout, with unemployment having a positive effect. (4) Turnout in recent presidential elections has not deviated significantly from the post-1972 norm. (5) Turnout is positively related to the expected closeness of the election outcome, but contrary to some theoretical predictions, closeness exhibits no trend over time.