Most of the research into contract cheating focuses on this problem as every academic discipline is the same. But essay mills are savvy and are marketing to students based on their background.
These slides, presented at Plagiarism Across Europe And Beyond, review the demand for contract cheating in different discipline groups, as well as the marketing reach of contract cheating services to discipline groups.
The slides are available on the SlideShare account for Thomas Lancaster. They are also included below.