  Crowdsourcing offers unprecedented potential for solving tasks efficiently by
tapping into the skills of large groups of people. A salient feature of
crowdsourcing---its openness of entry---makes it vulnerable to malicious
behavior. Such behavior took place in a number of recent popular crowdsourcing
competitions. We provide game-theoretic analysis of a fundamental tradeoff
between the potential for increased productivity and the possibility of being
set back by malicious behavior. Our results show that in crowdsourcing
competitions malicious behavior is the norm, not the anomaly---a result
contrary to the conventional wisdom in the area. Counterintuitively, making the
attacks more costly does not deter them but leads to a less desirable outcome.
These findings have cautionary implications for the design of crowdsourcing
competitions.

Social and Information Networks;Computer Science and Game Theory;Physics and Society