Because red pens are closely associated with error-marking and poor performance, the use of red pens when correcting student work can activate these concepts. People using red pens to complete a word-stem task completed more words related to errors and poor performance than did people using black pens (Study 1), suggesting relatively greater accessibility of these concepts. Moreover, people using red pens to correct essays marked more errors (Study 2) and awarded lower grades (Study 3) than people using blue pens. Thus, despite teachers’ efforts to free themselves from extraneous influences when grading, the very act of picking up a red pen can bias their evaluations.
Source: “The pen is mightier than the word: Object priming of evaluative standards”