My theory is that because of the big influx of immigration in the 1900-1920 period the size of the Voting Age Population increased faster than number of actual registered voters.
From 1920 on, the VAP doubled, since it was no longer gender constrained, but the turnout did not double since women were less likely to get out and vote at the time.
It is not a particularly good article and I can't vouch for its accuracy but I do think it is interesting: http://en.wikipedia.org/wiki/Voter_turnout_in_the_United_States_presidential_elections
I have to say it is full of interesting information. Notice how turnout figures in the nineteenth century often seem to correspond with turning point elections and it is interesting to consider that 1876 still records as the highest turnout in American history. I wonder to what effect various disenfranchisements had on those figures; Blacks it seems not a lot, Women (which is o/c easier to record) a lot more at least initially.
But what really caught my eye was this stretch from about 1896 to 1924:
1896 79.3%
1900 73.2%
1904 65.2%
1908 65.4%
1912 58.8%
1916 61.6%
1920 49.2%
1924 48.9%
In the Nineteenth Century turnouts into the 70s were the norm, the lowest turnout of any election between 1840 and 1900 was 71.3% in 1872 - a non-contest where one of the two major parties effectively refused to run a candidate. 1896's was relatively high for the era but that was a decisive and highly polarized election and then you see this rather steep decline. What is notable about is doesn't seem to correlate very well with 'major' elections or ones where there was a high degree of polarization and a wide range of choices (ie. Third party candidates). 1912's is well down on 1908's despite being a more of a contest in every single possible way. I suspect the incredibly low numbers for the 1920s can be explained by a combination of the implosion of the Democratic 'Ethnic' vote due to Woodrow Wilson's foreign policy, the effects of female suffrage (passed in time for 1920, many women may have been initially reluctant to vote? I'm speculating here) and the recognition that those elections were going to be massive Republican landslides. But earlier? I can't explain that plunge. I'm not an expert at all in this period of American history, so any ideas?
