When I write and speak about abolishing grading, I draw on a balance of anecdotal evidence and scientific research. One of the studies I often use is summarized nicely in Alfie Kohn's article Education's Rotten Apples:
In a study that appeared in the British Journal of Educational Psychology, Ruth Butler took 5th and 6th graders, including both high- and low-achieving students, and asked them to work on some word-construction and creative-thinking tasks. One-third of them then received feedback in narrative form, one-third received grades for their performance, and one-third received both comments and grades.
The first finding: Irrespective of how well they had been doing in school, students were subsequently less successful at the tasks, and also reported less interest in those tasks, if they received a grade rather than narrative feedback. Other research has produced the same result: Grades almost always have a detrimental effect on how well students learn and how interested they are in the topic they're learning.
But because Ms. Butler had thought to include a third experimental condition—grades plus comments—she was able to document that the negative effects of grading, on both performance and interest, were not mitigated by the addition of a comment. In fact, with the task that required more original thinking, the students' performance was highest with comments, lower with grades, and lowest of all with both. These differences were all statistically significant, and they applied to high- and low-achieving students alike.