Employees of a pharmaceutical sponsor are provided the opportunity on a regular basis to provide blood samples for in-house research protocols. Subjects will be paid $10 dollars for each blood draw. Clinically relevant results will be reported to the company employee health service for follow up with the employee. Supervisors are not provided with lists of subordinate employees who participate, but this information is not considered confidential and there is no prohibition on supervisors inquiring about subordinate employee participation in research. The employee subjects fit best into which of the following categories of vulnerability?