Smartphones have become an indispensable part of everyday life. Given the current debate about the use of smartphones in classrooms and schools, it seems appropriate to examine their effects on aspects of cognitive performance in more detail. Ward and colleagues not only demonstrated the negative effect of smartphones on cognitive performance but also showed that the mere presence of these devices can have this effect—this is known as the Brain Drain effect. In the present article, a meta-analytic approach was adopted in order to verify these findings. Here we show a significant overall negative effect of smartphone use and presence. In a database search we identified 22 studies with a total of 43 relevant effects that could be assigned to the categories “memory”, “attention”, and “general cognitive performance”. A subgroup analysis suggests that not all cognitive domains are equally affected by the negative effect of smartphones. The heterogeneity of the effects reinforces this finding. The nationality of the test subjects or the origin of the studies was identified as a further key variable. Our findings also indicate that the distracting effect of smartphones varies on the area studies and further research is necessary. In view of the present research results, it seems important that people in general, and especially children and adolescents in schools and classrooms, learn how to deal with the distracting potential of smartphones.