An interesting paper at NIPS aims to combat the lower quality of Turkers with gamification. Instead of yes/no answers they offer a choice of: absolutely sure, yes, don't know, no, absolutely not. Monetary reward is doubled if they answer a question correctly with a high certainty, and it is zero'd when they are wrong.

http://arxiv.org/abs/1408.1387 "Double or Nothing: Multiplicative Incentive Mechanisms for Crowdsourcing"