  Incentive mechanisms for crowdsourcing have been extensively studied under
the framework of all-pay auctions. Along a distinct line, this paper proposes
to use Tullock contests as an alternative tool to design incentive mechanisms
for crowdsourcing. We are inspired by the conduciveness of Tullock contests to
attracting user entry (yet not necessarily a higher revenue) in other domains.
In this paper, we explore a new dimension in optimal Tullock contest design, by
superseding the contest prize---which is fixed in conventional Tullock
contests---with a prize function that is dependent on the (unknown) winner's
contribution, in order to maximize the crowdsourcer's utility. We show that
this approach leads to attractive practical advantages: (a) it is well-suited
for rapid prototyping in fully distributed web agents and smartphone apps; (b)
it overcomes the disincentive to participate caused by players' antagonism to
an increasing number of rivals. Furthermore, we optimize conventional,
fixed-prize Tullock contests to construct the most superior benchmark to
compare against our mechanism. Through extensive evaluations, we show that our
mechanism significantly outperforms the optimal benchmark, by over three folds
on the crowdsourcer's utility cum profit and up to nine folds on the players'
social welfare.
