Teachers' grades could be biased towards students with more "agreeable" personalities - rather than actual ability, according to this international study from Queen’s University Belfast, Goldsmiths, University of London and Sirius University of Science and Technology in Russia.
It found that those with personality traits seen as more positive - such as more sociable, well-behaved and "agreeable" - received a boost in teachers' grades, which were not then borne out by performance in anonymous exams.
Teachers also seemed to be more sympathetic in grading towards students who were stressed or anxious, showing more "leniency" in grades for those seen as more "emotionally fragile".
This pattern worked against those who were perceived as having more negative, anti-social or annoying traits.
The findings, although based on a sample of 14 to 18 year olds in Russia, have a relevance in all education systems.