A personality factor that may inhibit grad school performance is the Big Five trait of neuroticism. A 2003 study of 145 undergraduate students at University College London published in the Journal of Research in Personality found that neurotic students tended to perform worse on exams than non-neurotic peers, likely due to feeling increased stress from tests.

The downside of neuroticism goes beyond test-taking, says Tomas Chamorro-Premuzic, PhD, a business psychology professor at University College London. High neuroticism combined with low self-esteem, for instance, can lead students to react poorly to negative feedback from faculty.

Interestingly, though, other neuroticism combinations may work well for grad students. "High ambition and high neuroticism can result in a very high-achieving combination: If you are insecure but ambitious, you will work extra hard and do even better than if you are secure and ambitious," he says.

—Nikhil Swaminathan