Final answer:
The main reason researchers may choose to disguise the presence of an observer in a study is to prevent the Hawthorne effect and control for experimenter bias.
Step-by-step explanation:
The main reason researchers may choose to disguise the presence of an observer in a study is to prevent the Hawthorne effect, where participants change their behavior because they know they are being observed. By disguising the presence of an observer, researchers can ensure that participants behave naturally and provide more accurate data. Furthermore, researchers may also choose to disguise the observer to control for experimenter bias, which refers to the possibility that the researcher's expectations might bias the results. This can be achieved through single-blind or double-blind studies, where either the participants or both the participants and researchers are blind to group assignments.