erated in previous rounds of the game itself. It is important for the game’s designer to randomize which taboo outputs are presented in order to account for potential output-priming effects (in which the particular taboo outputs shown to the players influence the guesses they enter) and ensure wide coverage of all potential outputs for a given input.

other Design Guidelines

The general schemes we’ve presented here for designing GWAPs rely on the participation of two players per game session. Now we show that the games can be modified to accommodate single or more than two players.

Prerecorded games. Paired game play makes GWAPs social, meaning that players are able to validate each other’s computation. However, two-player games present logistical challenges. For instance, there may be times when an odd number of people want to play a particular game, meaning at least one of them cannot play. In addition, when a game is just beginning to gain popularity, it is difficult for game adminis-

trators to guarantee that many people will be able to play at the same time. We thus recommend that game developers apply a technique—prerecorded game play—introduced by the ESP Game. ²² A dyadic game, normally played by multiples of two players, can be transformed into a single-player game by pairing a single player with a prerecorded set of actions.

In the case of an input-agreement game or output-agreement game (such as the ESP Game), implementing automated players is relatively easy. When two people are playing, the game should simply record every action they make, along with the relative timing of each action. Then, when a single player wishes to play, the system can pair that single player with a prerecorded set of moves.

In inversion-problem games, implementing prerecorded game play is more complex because one of the players (the guesser) must dynamically respond to the other (human) player’s actions. Peekaboom, Phetch, and Verbosity have each implemented a single-player version using techniques cus-

tomized to each game. ^23–25

More than two players. The three GWAP templates can be extended to include more than two players; for example, output-agreement games can be extended to incorporate more players by modifying the winning condition such that the first two players who agree on the output are the winners of the round (and granted a higher number of points than the nonwinners). Similarly, the template for inversion-problem games can be extended to incorporate multiple players by substituting an individual guesser with an arbitrary number of players in the role of guesser, all racing to be first to correctly guess the input (winning condition).

These extensions change the nature of the games considerably. Whereas the two-player versions of each template are cooperative in nature ( players work together to obtain points), the multiplayer versions are competitive. Cooperative, as well as competitive, games involve advantages and disadvantages. For certain players, competitive games may be more enjoyable than

A Sampling of GWAPs
The ESP Game and Verbosity

Two of the most popular GWAPS—ESP and Verbosity— can be played online at www.gwap.com.

The ESP Game has generated millions of labels for random images located throughout the Web. In it, two players are randomly paired for two-and-a-half minutes as they are shown a series of images to label. The game does not directly ask them to label the images. Rather, both players must try to enter the same word as their partner for each image on the screen; neither player can see the partner’s words. When both players agree on a word, each is given a new image. The goal is to agree with the partner on words for as many images as possible. The words the players agree on for each image are extremely accurate labels that can be used to improve image search throughout the Web. To increase the quality of these labels, as well as to motivate player engagement, the game

forbids the use of “taboo words” from being entered. In the screenshot (see Figure a), players cannot use the words “dog” or “pillow” when trying to agree on a word with their partner.

Verbosity is a word-guessing game in which two players alternate roles. The describer is given a secret word the guesser

must figure out as quickly as possible. The describer helps the guesser by providing clues about the secret word using sentence templates that must be completed without using the secret word itself. In the example here (see Figure b), the secret word is “sock,” and the sentence template “It is a kind of _____”

has been instantiated to the clue “It is a kind of clothing.” The describer sees all of the guesser’s inputs and indicates which ones are “hot” and which are “cold.” The computational purpose of the game is to collect a database of commonsense facts about the secret words (such as “Sock is a kind of clothing”).

figure a: Players of the esP Game try to guess what their partner is typing on each image.

figure b: Players of Verbosity enter commonsense facts to help their partner guess a secret word.