A 2019 meta-analysis of thousands of studies from almost two dozens countries suggests that while as a whole, there is no association between screen time and academic performance, when the relation between individual screen-time activity and academic performance is examined, negative associations are found. Watching television is negatively correlated with overall school grades, language fluency, and mathematical ability while playing video games was negatively associated with overall school grades only. According to previous research, screen activities not only take away the time that could be spent on homework, physical activities, verbal communication, and sleep (the time-displacement hypothesis) but also diminish mental activities (the passivity hypothesis). Furthermore, excessive television viewing is known for harming the ability to pay attention as well as other cognitive functions; it also causes behavioral disorders, such as having unhealthy diets, which could damage academic performance. Excessive video gaming, on the other hand, is known for impairing social skills and mental health, and as such could also damage academic performance. However, depending on the nature of the game, playing it could be beneficial for the child; for instance, the child could be motivated to learn the language of the game in order to play it better. Among adolescents, excessive Internet surfing is well known for being negatively associated with school grades, though previous research does not distinguish between the various devices used. Nevertheless, one study indicates that Internet access, if used for schoolwork, is positively associated with school grades but if used for leisure, is negatively associated with it. Overall, the effects of screen time are stronger among adolescents than children.