An updated overview of the public facts of the Trump-Russia scandal, and five theories (from seven last year) mostly consistent with those facts
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The USA in particular has one of the lowest voter turnouts and the lowest registration levels of most developed countries.
http://www.pewresearch.org/fact-tank/2018/05/21/u-s-voter-turnout-trails-most-developed-countries/
In 2016 only 61% of eligible citizens voted and only 70% were registered.
https://www.census.gov/data/tables/time-series/demo/voting-and-registration/p20-580.html
And that was a good year.
https://en.wikipedia.org/wiki/Voter_turnout#Trends_of_decreasing_turnout_since_the_1980s