The characterizing goal of a serious game is closely linked to the application area. In educational games, the characterizing goals include learning or training effects. In games for health, the characterizing goals include changes in vital status or general changes in attitude and behavior (eg, nutrition or mobility behavior). It may also be beneficial to combine physical and cognitive training. For example, in the game Lü (Lü Interactive Playground) [73], the players not only improve mathematical skills but also stay physically active. Similarly, ExerCube [34] provides training for body and mind. Furthermore, recent evidence on exergame-based therapy for Parkinson disease shows that exergames can enhance cognitive skills and are at least as effective as traditional therapies [74,75].

Thus, serious games should always focus on achieving the characterizing goal and should support the player in achieving this goal. Learning or training content must remain in focus during gameplay and game elements should not interfere with the learning or training process.

Similar to entertainment games, in which the game goals should be clear [13,14], the characterizing goal of a serious game should also be transparent so that players can work towards achieving this goal. In particular, a serious game should ensure that players always know what to do to complete the tasks or exercises; otherwise, a tutorial is required (see also the “Intuitive Game Mechanics and Natural Mapping” section). For example, in the ExerCube, a virtual avatar demonstrates required movements in a short tutorial to make sure that players know how to execute them [3]. The educational games Meister Cody – Talasia and Meister Cody – Namagi [60] show how each exercise should be solved before players start to solve the tasks.

Engaging in the serious part of the game should be mandatory for playing the game. Otherwise, players may avoid the serious part to get to the fun part more quickly [76], preventing the characterizing goal from being reached. In particular, the training and learning tasks should not be a hurdle while playing a serious game. In other words, the characterizing goal should be embedded in the gameplay (see also the “Integrated Serious Part With Gameplay” section).

The most evident requirement for serious games is that they must not contain any errors with respect to their subject matter, such as erroneous mathematical equations, incorrect information on historical events, or inadequate information on physical exercise. If typing or presenting errors occur, these errors must not mislead players. Furthermore, conveyed information must not only be factually correct but also be imparted using appropriate technical language.

Moreover, even though there is usually no connection between neutrality and the correctness of a given opinion or issue, serious games should remain neutral, especially on political and social issues. For example, the games Orwell: Keeping an Eye on You [9] and Orwell: Ignorance is Strength [58] do not convey a specific political opinion to the player. Instead, they only show the effects of the player’s actions without judging them. The player then has the opportunity to assess and question their own decisions. In particular, serious games should be appropriate for the target group, depending on religion, culture, and traditions.

A serious game should provide appropriate feedback to players so that they can assess their progress. Thus, to enhance player performance, effects should be visible and recognizable (eg, through a progress bar). Continuous feedback on progress is essential in all serious games as the players work towards achieving the characterizing goal [77]. Moreover, multimodal feedback (eg, visual, audio, or haptic feedback) can be beneficial [14]. For example, in the ExerCube, players receive immediate visual and audio feedback to enhance their movements [3].

Apart from in-game feedback, Ravyse et al [78] furthermore show that postgame feedback also improves learning. Similarly, the level number can indicate the player’s progress (ie, the higher the level, the more skills the player has developed). A higher in-game level seems to have a more significant effect on motivation (desire to practice) than individual rewards (eg, achievements) [79]. Game statistics additionally show the player’s progress after ending the game or a level and are not only advantageous for players themselves, but also useful for their therapists (especially in games for health [20]) or their parents and educators (in educational games [60]).

Games should provide positive reinforcement and in-game awards [1] to immerse players more deeply in the game [14]. For example, in the educational game VocabiCar, players who accomplish a learning task or complete a challenge gain points and can access their progress in a high-score table [57]. High-score tables allow players to compare their performance against other players. In addition to points, in-game awards consist of virtual badges, achievements, power-ups, and desirable objects. Fancy animations or a possibility to change their avatar (eg, new clothes, hair color, or equipment) as a reward for accomplishing a task can further motivate players. However, there is ample evidence that “rewards or feedback delivered in a controlling manner undermine intrinsic motivation and deeper forms of learning” [80]. Therefore, rewards should be deliberately deployed in serious games.

A serious game is effective when players achieve the characterizing goal and the learning or training effects are sustainable. Researchers often validate effectiveness with a study, such as a scientific, clinical, or empirical evaluation, by monitoring heart rate (eg, ExerCube [3] and ErgoActive [37]), number of steps (eg, Pokémon GO [4]), or aerobic fitness (eg, Dance Dance Revolution [30]). However, potential aversions to video games, certain game genres, or specific interaction technologies among players have to be considered when designing and evaluating a study. In educational games, the results of a group exposed to a serious game and a group exposed to traditional methods can be compared (eg, Meister Cody – Talasia [2] and ViPOL [59]). However, empirical studies often suffer from numerous sources of error (eg, bias) [81]. In this regard, randomized controlled trials with a sufficient number of participants are the gold standard for empirical proof of effects.

In addition to scientific studies that evaluate usability and player experience, game awards (eg, German Computer Games Award, European Innovative Game Award, and International Educational Games Competition) are also an important aspect of identifying high-quality serious games. Further quality criteria include professional or user ratings, the number of players or downloads (eg, Google Play, App Store [Apple Inc], and Steam [Valve Corp]), and recommendations by domain experts and game reviews (eg, IGN, GameSpot, and PC Gamer).

Player engagement is tightly associated with enjoyment. Koster, the author of A Theory of Fun for Game Design, addresses the importance of a game being engaging, enjoyable, and fun [82]. The GameFlow approach proposed by Sweetser and Wyeth [13] includes 8 dimensions with numerous criteria and recommendations to ensure player enjoyment in games. Calvillo-Gámez et al [87] furthermore present the core elements of the gaming experience to provide a positive experience while playing video games. Moreover, Desurvire and Wiberg [14] propose heuristics of playability for game developers to develop better games.

However, due to different kinds of players, not every player will find all components equally important. Therefore, the game should provide different fun components to provide an engaging experience for different player types (eg, Bartle’s player types [88]).

For the optimal player experience, the game has to establish a satisfying balance between challenges and skills. Csikszentmihalyi’s well-known flow theory describes the feeling of enjoyment when the task difficulty and skill levels are in balance [83]. This theory is complemented by Bushnell’s theorem of “easy to learn, difficult to master” [84]. Games that are easy to learn enable flow because they are not overwhelming, whereas games that are hard to master keep players from dropping out because of boredom. Thus, as the player’s performance improves, the complexity or difficulty of the game should also increase. In other words, serious games should adapt to the current performance level of the player (ability vs skills).

To avoid boredom, game developers should also ensure that the gameplay varies. As proposed by Desurvire and Wiberg [14], any fatigue or boredom should be minimized by varying activities and pacing during the gameplay. Furthermore, research by Scoresby and Shelton [89] identified that content, emotion, motivation, and engagement associated with the game are necessary criteria for flow.

Moreover, serious games should automatically adapt to the players to motivate them to keep learning/training and to increase the effectiveness. One of the primary advantages of educational games is their ability to engage the learner so that they voluntarily complete sufficient repetitions of activities, ensuring that learning takes place [90]. For example, both of the Meister Cody educational games adapt their difficulty depending on the player’s skills [60]. The results show that, due to adaptivity, game-based learning is particularly promising for children who want to learn in a home environment or do not have access to individual reading support [53]. Similarly, exergames should ensure that the intensity matches the player’s fitness level in order to motivate players to repeat the exercises continuously and regularly [91]. For example, ExerCube [3] or ErgoActive [37] identify the individual’s optimal strain to adapt the game difficulty and complexity gradually based on the player’s heart rate.

Additionally, players should get emotionally involved in a serious game. Dillon [85] has drawn attention to the fact that emotions and instincts increase players’ engagement to continue playing the game. The game designer and developer proposed the 6-11 framework, which contains 6 basic emotions and 11 instincts. For example, various serious games use an instinct to survive and thus to fight (eg, Re-Mission [38]) or to collect something (eg, VocabiCar [57] and Debugger 3.16: Hack’n’run [54]). Furthermore, LeBlanc’s theory of 8 kinds of fun describes the desirable emotional responses evoked in players when they interact with the game [86].

Players should feel in control over their actions in the game world [14]. In particular, players should have control and influence on the game world. For example, in the serious game PlayForward: Elm City Story, players can see how different actions lead to different outcomes [48]. Similarly, Escape from Diab allows players to influence the storyline and the characters [51,52]. As a result, the ability to influence the game world and in particular the story progress can motivate players to keep playing the game. Furthermore, serious games should support an optimal relationship between the player’s actions and the game’s reactions. For example, increased pedaling frequency in ErgoActive will always cause the character to rise [35].

Bond and Beale [15] identified that good games offer some form of social interaction (see also the self-determination theory [92]). Social interactions in games are important for players who perform better depending on the game mode (eg, playing with friends or against them). For example, Pokémon GO [27] lets friends feed the player’s creatures, and friends and family playing Dr Kawashima’s Brain Training [49] can compete against each other. Vorderer et al [93] suggest that competitive elements are important for enjoyment. On the contrary, the work of Staiano et al [31] reveals that playing a cooperative version of the Nintendo Wii Active game is more effective than playing a competitive version. Thus, different game modes motivate players more or less. Especially if the players perform better in groups, collaborative and competitive multiplayer settings can contribute to motivating players [3]. For specific players, playing in a group (ie, multiplayer games) is more motivating than playing alone (ie, single-player games) [86]. Depending on the game purpose and target group, the game developers and designers should try to include different game modes so that a serious game is enjoyable for a broad player base.

Immersion in virtual environments can be increased by stimulating different human senses, especially by including appropriate audiovisual elements in the game. The game should use visceral, audio, and visual content to immerse players more deeply in the game [14]. Slater and Wilbur [94] describe immersion as the extent to which the computer system can deliver an illusion of a virtual environment to players. Thus, an immersive virtual environment should accommodate a wide range of appropriate synchronized sensory modalities. Recent studies already provide evidence that multimodal sensory stimulation improves the sense of presence (ie, the sense of “being there”) and immersion [95]. For example, SnowWorld [50] successfully distracts players during rehabilitation (wound treatment) by immersing them in a virtual world. For a fully immersive virtual reality experience, serious games should include visual (eg, current-generation head-mounted displays), audio (eg, noise-canceling headphones), and haptic (eg, data gloves with force feedback or vibrations) feedback, as well as sense of smell (eg, smell dispenser) [96].

The included graphics should look attractive and engaging, as well as appropriate for the game purpose, application area, and the target group. For example, a game should have different designs for children, adults, and people with disabilities. The game designer should ensure clear interfaces without unnecessary information. Ravyse et al [78] furthermore suggest creating games that are high in realism; however, it should not be overloaded with unnecessary objects so that players do not get distracted from a specific task. For effective training, particularly for firefighters [97] or police training [59,98], the simulations should provide realistic virtual environments. However, in contrast to high-end graphics, reduced graphics can also be appropriate for some game types. For example, Minecraft (Mojang Studios) [99] is one of the most successful video games and it contains a world of blocks.

Serious games should not only be visually appealing but should also include appropriate background music and sound effects. Previous studies have shown that audio influences the sense of presence, particularly in immersive virtual reality applications [100,101]. Martin-Niedecken et al [3] have expressed a similar view. The researchers show that music increases the motivation and immersion of the test subjects while playing the ExerCube. However, due to the players’ varying music preferences, the choice of music could also be an important factor in motivation [102]. For example, the game Beat Saber allows players to create levels with custom songs [33].