The danger with praise

Last week I gave back my students their first papers, which they had written about the Viking Age in the North Atlantic. I actually gave them their papers one day with my comments on it, but without a grade, and let them tell me what grade they thought the paper deserved. Most students gave themselves a B+, which actually is about what I was going to give them. One student gave himself a C, which was too low; his paper was not that bad. 

Another one of my students came up to me all freaked out about my comments, and wanted to argue every single point I had made. He tried to verbally explain things he had not explained well in the paper. After about 5 minutes of this, I finally put my arm around his shoulders and asked him if he thought his paper was really terrible or something? "Yes! You said my thesis contradicted my conclusion, that makes a bad paper!" I simply shook my head and took the paper away from him. Today I am giving him back his paper and he'll see his final grade. It is an A-.

I am concerned of course that now he will think he does not have to try very hard on his next paper, since he did not get a failing grade on this paper. That would of course be disastrous, if he were to decide that a paper with that sort of flaw was indeed good enough. But instead I am keeping my fingers crossed that he will know next time not to write something so incomprehensible and vague in his introduction as to even appear to contradict with his conclusion. 

I am hoping, in other words, that he is self-motivated to write well, rather than being motivated by some sort of arbitrary grade.