The electronic databases searched in the current review included ERIC, SpringerLink, Scopus, IEEE Xplore, Science Direct, ProQuest Dissertations & Theses, and Google Scholar. The search was focused on the research published in the peer-reviewed journals, government reports, PhD dissertations, and conference proceedings relevant to ESL, CALL, and gaming. The search terms used were “digital game-based learning” and “video games” in combination with “second language learning” and “second language acquisition”. To address the main purpose of a synthesis, studies were considered relevant if they (1) contained evidence of video games’ effects on four language skills and vocabulary outcomes, (2) provided sample data (gender and/or age, and second language), (3) provided information on game types and particular skills acquisition, (4) were in English, and (5) were published from January 2005 to January 2015. The synthesis focus is on qualitative, quantitative, and mixed-method studies involving male and female participants that are over 6 years old. Studies were excluded from the analyses if they had no information on game types or genres and if they were focused on affective variables such as motivation and attitudes. Figure 1 shows the screening process of the 26 selected papers.

The 26 papers that met the inclusion criteria were coded according to both video game and CALL-informed parameters [11] . Video game parameters included game types (COTS or Serious & SIEs), game genres based on Peterson’s [10] system, game language, second language focus (four language skills and vocabulary), and device (PC, console, or mobile). CALL-informed parameters involved sample’s demographics (gender, age, second language and gaming proficiency). The following categories were used for samples’ age distribution: “6 - 7 years old”, “11 - 16 years old”, and “16+”. Second language proficiency was categorized according to “0” (no proficiency) “elementary/beginner”, “intermediate”, and “advanced” levels. Gaming proficiency categories

included “gamers” (regularly played video games for over 3 years) and “non-gamers” (limited or no prior gaming experience). Additionally, seven articles [13] - [19] were included in a discussion section since they addressed some of the research questions such as gender and second language proficiency but they were excluded from the main list since they do not touch upon four language skills outcomes.

Table 1 shows the number of papers identified in each database using the specified search terms. The data demonstrates increased interest of scientific community in regards to the issues of second language learning through video games since 8361 papers were published during the last decade. However, only 26 publications met the inclusion criteria. The majority of papers were found in Google Scholar (42%), followed by ERIC (23%), Science Direct (23%), IEEE Xplore (8%), and SpringerLink (4%). The findings of these studies were summarized in Appendix A with respect to game type and title, second language and second language focus, sample data, method, instruments, outcomes, and researchers’ names.

Table 2 shows the number of qualified papers split by the source of publication. Most of the articles were published in peer-reviewed journals (65%), followed by conference or symposium proceedings (31%), and government reports (4%).

In response to this question, literature suggests that in terms of second language variety, serious games and SIEs provide more options compared to COTS games. Since serious games and SIEs allow the targeting of specific educational objectives, theoretically, they can be developed in any language. Although some COTS games support several languages, the majority of them are usually in English or Japanese [13] . In such a way, there are more COTS games available to practice English and Japanese as second language, while serious games and SIEs are suitable for a wide variety of languages enhancement such as German, Arabic, and Chinese.

The synthesis of research identified that both COTS and serious games and SIEs provide opportunities for second language skills acquisition. However, while both types of games were used for reading, writing, listening, speaking skills, and vocabulary enhancement, serious games and SIEs were more introduced than COTS games in such areas of language acquisition as speaking and writing. In other words, serious games and SIEs offer more support for language skills development. Addressing the question of what types of games are more accessible with respect to delivery, literature suggests that COTS games, especially adventure games, offer more options since they can be played either on personal computer, console, and mobile devices or on all three devices. The vast majority of serious games and SIEs can be played on a personal computer only. Thus, both COTS and serious games and SIEs provide opportunities for second language enhancement in terms of language variety, delivery, and language skills practice but specific language objectives and technology availability should determine the type of game, which may better suit the needs of teachers and learners.

In response to the question, the synthesis found some evidence of the relationship between age, gender, and language outcomes. With respect to the relationship between age and video games outcomes, this synthesis uncovered that reviewed studies did not consider this variable at all. However, the findings of a meta-analysis conducted by Wouters, Van Nimwegen, Van Oostendorp and Van Der Spek [20] revealed that learning through video games is more effective for children than adults. It is noteworthy, only 58% of analyzed studies provided information on both participants’ gender and age. Since age variable may affect learning outcomes, this question requires closest attention.

In reference to the relationship between gender and video games language outcomes, research suggests that male participants usually play video games more than female, which may facilitate better vocabulary acquisition for males than females [16] . Literature also identified that male and female learners prefer different game genres. Males enjoy first-person shooter [21] , MMORPGs [19] and adventure games [22] , while females prefer offline single-player simulation games [16] . This may also result in better vocabulary performance of males than females [23] . To the contrary, Hitosugi, Schmidt and Hayashi [22] found no gender effect on vocabulary outcomes regardless of the fact that female participants did not like adventure game. In such a way, even though, the findings revealed some relationship between gender, age and video games outcomes, there is not enough evidence of true relationship between these variables due to a limited number of studies, which also yielded conflicting results.

Some relationship can be also established between learners’ second language proficiency level and video games language outcomes. Findings in the research involving COTS games uncover that commercial games, particularly, MMORPGs [24] , adventure [25] [26] , and rhythm [27] may be challenging for the lower-level second language proficiency learners and inexperienced gamers because difficult game language, game missions, and game mechanics of these genres may cause cognitive overload. The findings also indicate that learners with elementary or beginner second language competency show little evidence of willingness to communicate in a target language [14] and are not able to significantly enhance vocabulary [24] through MMORPGs. By contrast, findings in the serious games and SIEs section suggest that learners with relatively low levels of second language proficiency may improve vocabulary [28] -[31] , listening [28] [30] [32] , writing [30] , speaking [28] , and reading [32] skills. ESL learners with a higher level of proficiency also may improve their academic writing skills [33] and vocabulary [22] but not significantly enhance listening and reading [32] . In such a way, COTS games may be challenging for second language learners with lower-language proficiency, while serious games and SIEs may facilitate better language outcomes for them. Thus, there is some evidence of the relationship between second language proficiency and video games language outcomes; however, the lack of studies focusing on this issue as well as the lack of information on participants’ demographics leave the question inconclusive.

In regards to the question whether there is a relationship between gaming experience and video games language outcomes, findings reveal little attention to this issue since only 15% of the reviewed studies provide information on learners’ gaming experience. At the same time, literature suggests that novice players may experience cognitive overload [19] ; therefore, they usually need more time to get accustomed to the game’s mechanics. Consequently, this can decrease the students’ ability to communicate in a target language [15] during playing. In addition, unexperienced gamers with advanced second language proficiency may limit the participation in gaming sessions because of unfamiliarity with the game mechanics and in-game language. Meanwhile, experienced gamers with a high-beginner second language proficiency that act as mentors for less experienced players may improve their communicative competence [17] . In such a way, there is some evidence of the relationship between gaming experience and language outcomes but these findings should be treated with caution since only few studies discussed this question.

In response to this question, the research suggests that both types of games are promising in terms of second language skills and vocabulary acquisition. In spite of the challenges with the game mechanics the majority of the ESL learners are able to succeed in vocabulary learning through MMORPG [24] [34] [35] , simulation [36] -[38] , and adventure [23] [25] [26] [39] -[41] games. At the same time, synthesis revealed that vocabulary of MMORPGs, sports, first-person-shooter, and strategy games is limited by specific terminology such as sports, military, history, and geography [13] [18] [21] ; therefore, these game genres may suit better for specific lexicon learning. To the contrary, simulation games such as The Sims are helpful for the expanding the 2000 most frequent English words list [36] -[38] . In addition, although COTS facilitate vocabulary acquisition, there is not enough evidence that gaming is better than traditional classroom instructions [34] [35] . Some learners may still prefer conventional drill-and-practice approach [37] .

Furthermore, findings disclose that different game genres may differently affect language skills outcomes. Sports and adventure games are found to be helpful for learners’ listening and reading comprehension [25] [26] [40] [42] but less effective for writing and speaking [26] [40] . First-person shooter games [21] show little evidence of listening comprehension improvements and rhythm games [27] are less effective for vocabulary acquisition. In such a way, COTS games offer a variety of opportunities to enhance certain language skills, primarily, vocabulary but they may better suit the needs of more advanced second language learners who would like to practice language skills and interact with people who share common interests beyond the classroom.

Similarly to COTS, serious games and SIEs may be also challenging for learners since this type of games may contain difficult vocabulary [22] , require immediate response and more advance game mechanics skills [43] ; therefore, more scaffolding is necessary [22] [44] . At the same time, MUVEs [28] , VirtUAM [30] , AHSs [29] , manipulation [31] [43] , and adventure [22] [44] games are found beneficial for vocabulary acquisition; manipulation [33] and VirtUAM [30] for writing; and MMORPG for listening, reading, and writing [45] . Similarly to COTS, different types of serious games and SIEs may also differently influence the language outcomes. For instance, ubiquitous games or ARGs [46] and MUVEs [28] may increase gains in speaking and listening comprehension while MMORPG and adventure show little evidence of speaking [45] and listening [32] improvement. In addition, some of the participants may have less confidence in serious video gaming approach for second language acquisition in comparison to traditional classroom activities [44] [46] . In such a way, depending on the educational objectives the use of one or the other game genre may result in better language outcomes.