Today, it may be difficult to find anybody browsing the Web who has never seen a CAPTCHA (completely automated public Turing test to tell computers and humans apart), a simple mechanism that many Web sites use to ensure that the visitors to the site are indeed human. In a typical CAPTCHA, the user is asked to identify and type in a word or some other sequence of letters and/or numbers based on a distorted image presented to the user. Today, users are more likely to encounter a reCAPTCHA, a modification of the same technique, in which two words are presented, one of which is generated from a scanned text that an optical character recognition system failed to recognize. By presenting the same unrecognized word to a large number of humans and asking them to type in the word that they see, a consensus emerges, thus solving a recognition problem that a computer system could not solve.

Law and von Ahn introduce the field of human computation, which leverages the collective force of human minds to solve simple tasks in order to solve computationally complex problems. reCAPTCHA is just one example of a human computation system that solves the problem of identity verification. Amazon’s Mechanical Turk is an example of a crowdsourcing marketplace, a human computation system where workers perform small tasks in exchange for monetary rewards. Other types of human computation systems include games that help tag audiovisual information, and distributed data collection for scientific purposes.

This book has several objectives. It surveys the nascent field of research and development in the area of human computation; offers a comprehensive review of many existing projects in this area; shows that a number of connections exist between the field of human computation and a broad variety of other disciplines, including machine learning, human-computer interaction, and psychology; and identifies emerging directions for further research in the field.

This book concludes with a comprehensive bibliography, to which the authors refer throughout the text. Given that human computation is a very young and rapidly developing research area, many sources in this bibliography may quickly become outdated. The authors should be commended for achieving two seemingly mutually exclusive tasks: presenting the material in a way that is accessible to a well-rounded generalist while not sacrificing the depth of coverage.