Your grading scale may have some influence.
If you have a curve: The students may be trying to manipulate it by trying less, enforcing a large curve for an easier grade.
If you put a large grade impact on mostly your exams (ex. exam 1 = 33%, exam 2 = 33%, quiz = 14%, assignments = 20%): I can pass your class with at best a B- without ever turning in an assignment and showing up only do your quiz and exams.
If you want to motivate your students, ask them at the beginning and end of each semester. Have them write down on paper what they think of the class, what they want/wanted to learn, what they thought was great and wasn't so great. Foster an environment worth coming to class to learn.