Introduction

Since the 15th Century, the oxymoron serio ludere has suggested the idea of combining the concepts of games and serious subjects. Humanist literature would extensively use this concept to denounce all kinds of social problems. Then, until the development of computing, the armies of the world would exploit this concept through war games, which are ludic simulations for developing new tactics and training officers1. The modern concept of the serious game dates back to 2001-2002, with the video game AMERICA'S ARMY2, developed for the US military to simulate training exercises. But almost simultaneously, the notion of the serious game would be popularized by its application to the educational world. Today, the title serious game refers to such diversity in terms of support, concepts, intentions, approaches and target audiences that it is difficult, if not impossible, to confine this concept into an exhaustive definition. One of the least restrictive definitions refers to a computer application that combines serious intent, education-oriented, and that is informative, communicative, marketing, ideological, or for training, with recreational parameters issued from video games or computer simulations.

Karen Chabriac, in her review of all the attempts of definition of serious games3, concludes that the most synthetic is that of [MIC 06]: any type of game whose purpose is other than mere entertainment.

Due to the diversity of themes, objectives and approaches, there are numerous attempts regarding the classification of "serious games", depending on their purpose (advertising, recreational-educational, military, simulation, prevention, training, rehabilitation, etc.), or even depending on the serious function associated with the ludic basis (informative broadcast, educational, persuasive, military, etc.), provide training to improve the physical or cognitive potential of players, promote the exchange of data between players (or between the designer of the game and players), and/or the associated market sector (the type of public targeted). There is even an interactive site dedicated to the research of serious games based on several simultaneous criteria, divided into three categories: intention, market and public4.

The general idea is that there are only two broad categories of games: ludic games designed for entertainment and serious games in which the player learns. In fact, in parallel to this concept, a third category of games has been emerging for slightly more than a decade, the Games With A Purpose (GWAPs), in which it is the player who teaches something to the machine. The concept of GWAPs is based on the idea of harnessing human skills for purposes of research and/or data production, whether destined to support programs to progress in their understanding of the world or more simply to use home computers to increase the computing power at the service of a research project. In either case, the ludic component is essential to motivate the public. The applications are numerous and the sector is growing. The concern is to exploit the available brain time (available and not only willing, but enthusiast if possible) to perform tasks that machines are (still) unable to do. A non-negligible or even crucial aspect is that this type of games, directed toward the production of all kinds of data, makes it possible to, therefore, use the creativity, the imagination, the knowledge and the know-how of hundreds or even thousands of users at a lower cost. It should be noted that unlike the AMT system5 (resource collection tool that uses crowdsourcing and offers a derisory remuneration as well as conditions not complying with the French labor law) the principle of GWAP does not raise any ethical problem, as long as it remains free and does not offer prizes that look like disguised salaries [SAG 11].

Originally, at the formalization of the concept of GWAP, there were CAPTCHAs, which were invented by Luis Von Ahn [AHN 06a], an American academic: these are small tests based on the deciphering and the input of a sequence of characters, which are employed to differentiate a human from a computer on the Internet, and thus to prevent spam, phishing or any other malicious activity by automatic means. Luis Von Ahn realized that the 10 s spent by a human to decipher a CAPTCHA (therefore to do something which a computer does not know how to do) could be usefully employed. He then created RECAPTCHA: from now on, when a captcha is decrypted, not only does it identify the user as a human being, but it also helps to digitize books by deciphering sequences of characters that the optical character recognition (OCR) is unable to decipher. The principle of the GWAP was born, and will be illustrated by ESP GAME [AHN 04]: the father of CAPTCHAs invented a game that consists of presenting the same image to two players who will score points and progress as soon as they suggest the same keywords to define it. The interest is naturally to make searching for images by keywords in a search engine more powerful, accurate, fast and relevant.

The use of GWAPs, either to collect data, annotate images or documents, or to solicit the public for solving major scientific problems, is currently expanding and it concerns all areas. Nonetheless, it gives rise to the most high-profile experiences and results in the life sciences and medical fields. The concept of citizen science reflects not only a change in the way that scientific issues are perceived by the public, but also a willingness to take science out of laboratories and researchers out of their ivory towers. By making the challenges of research accessible and understandable by ordinary people, science is demystified and desecrated. For researchers, the ludo-collaborative approach is a powerful way to involve the public and to mobilize its support and its empathy, while soliciting and evaluating its non-specialist outside perspective. It should be noted that, according to Luis Von Ahn and during the years 2005-2010, almost 1010 (i.e. 10 billion) hours have been spent annually by individuals playing on the Internet (which globally corresponds to an average slightly higher than 1 h per person and per year). Why not try to deviate from it, at least a tiny portion, for useful games allowing the acquisition of resources?

GWAPs are thus numerous in multiple disciplines, and it is now obvious after a careful overall review that the effort of giving a really recreational dimension to a useful task is very uneven. As a result, games where the interest is all the more stimulated by a real challenge, by emulation among players, by a motivating classification system and, above all, by a real interest of the underlying task are found less often than games stimulated by the idea of helping science and/or of doing good work. Frequently, various parameters are borrowed from the universe of games (design, avatar, sounds, etc.) to give a ludic polishing to a monotonous and repetitive task. It is clear that in many cases game designers primarily rely on the players' civic zeal by means of the excitement caused by participative science and the extremely rewarding feeling to achieve something useful in a field which remains prestigious in the eyes of the general public.

With regard to GWAPs, and since they are all based on the principle of crowdsourcing, large discrepancies can be observed according to the nature and the magnitude of the task, the target audience, the field of research, the skills required on the part of the players, the ludic and/or the educational dimension, and the manner in which the data generated by the game are processed. The choice of criteria for establishing a classification is, therefore, difficult; however, the classification of [GOO 13], which is based on the nature, the extent and the complexity of the task, can be considered as interesting. In fact, these authors make the distinction between microtasks and macrotasks in systems using crowdsourcing for scientific purposes. Microtasks are tasks that can be solved within seconds by anyone who is able to read some simple instructions, and macrotasks, on the contrary, concern complex problems that are resistant to the qualified experts of the institutional research. The first category of tasks require a large number of people who will process a huge volume of data in a short period time, and whose contributions, (strongly) redundant between players, will be aggregated in order to provide data of a quality as good as experts' annotations. For the second category, resorting to crowdsourcing makes possible to detect the few talented people within a large population of potential candidates with very heterogeneous skills. Through the provided interactive environment, they will not only demonstrate but also develop the inventiveness, the curiosity and the creativity necessary to meet the challenge and allow real scientific progress.

It should be noted that among GWAPs, a huge majority concern microtasks.

It should also be noted that games with a large audience whose spectacular results are subject to a wide media coverage, and which are generally macrotasks (FOLDIT6, EYEWIRE7), all have a dedicated website while GWAPs, with a more modest ambition, instead related to microtasks, are often proposed through portals. The most famous and the oldest is ZOONIVERSE8.

At the origin of the ZOONIVERSE portal, there is GALAXYZOO9, an online astronomical project which somewhat symbolizes the beginnings of the type of science known as...