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A three-year UBI study done in Germany shows that a guaranteed monthly check increased labor market exploration and increased work satisfaction.
(www.pilotprojekt-grundeinkommen.de)
About this, I will try to find it. But there is quite some research that shows productivity in simple jobs (think assembly line, or fruit picking) goes up in relation to reward. But complex an skilled jobs actually give diminishing returns quite soon with a higher reward, because with complex jobs most of the satisfaction comes from doing the job itself and excelling not the reward.