Students with poorly developed authorial identity may be at risk of unintentional plagiarism. An instructional intervention designed specifically to improve authorial identity was delivered to 364 psychology students at three post-1992 universities in London, UK, and evaluated with before-and-after measures of beliefs and attitudes about academic authorship, using the Student Authorship Questionnaire. Changes in questionnaire scores showed that the intervention led to significantly increased confidence in writing, understanding of authorship, knowledge to avoid plagiarism, and top-down approaches to writing, and significantly decreased bottom-up and pragmatic approaches to writing. For understanding of authorship, knowledge to avoid plagiarism and pragmatic approaches to writing, significant intervention by year of study interaction effects showed that the greatest improvements were among year one undergraduates. Direct evaluative feedback showed that 86% of students believed the intervention helped them avoid plagiarism and 66% believed it helped them write better assignments. Post-intervention focus groups revealed changed student understandings about authorial identity and academic writing. The results show that interventions can help students avoid unintentional plagiarism by adopting more authorial roles in their academic writing. Further research could explore other influences on authorial identity, and examine the impact of authorial identity interventions on other outcome indicators. (Contains 3 tables and 2 figures.)