There have been demonstrated effects of teachers' verbal approval on pupils' behavior (e.g. Swinson and Harrop, Educ Psychol Pract 17(2):157--167, 2001) and academic achievement (e.g. Weinstein et al., Coll Stud J 43(2):452--453, 2009). Although much of the teachers' time is spent on marking pupils' work, not too much is known about the effect of written approval on pupils' academic and social behavior. Therefore, the aim of this exploratory study was to investigate the effect of written approval on pupils' academic performance and on-task behavior. No effects have been demonstrated of written approval on pupils' academic performance, probably because of an overlap between approval and numerical grade functions. The effects of written approval on the increase of pupil's time on-task confirm the results of the literature on verbal approval.
Rollo, D., Sulla, F., Massarini, M. A., & Perini, S. (2016). The Effect of Written Approval on Pupils’ Academic and Social Behavior: An Exploratory Study in a Northern Italian Middle School (pp. 251–256). https://doi.org/10.1007/978-3-319-24391-7_26