L. von Ahn, and L. Dabbish. CHI '04: Proceedings of the SIGCHI conference on Human factors in computing systems, page 319--326. New York, NY, USA, ACM Press, (2004)
L. Steels. Artificial Life V: Proceeding of the Fifth International Workshop on the Synthesis and Simulation of Living Systems, page 179--184. Cambridge, MA, The MIT Press, (1996)