We are Americans, so maybe we'll vote for a little cash!
2014 Census: ~ 320 million people with 77% over 18
$50 tax credit, 100% voting, ~ $12.5 billion dollars
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Simple idea, probably really hard to make happen.
(1) Somehow know if people vote in Federal Elections (president, senator, rep). This step may be contentious.
(2) Receive tax credit when filing taxes. Either auto-magically or from what you filed.