Dr. rea is a psychologist who is about to conduct a study using human participants. the first thing dr. rea should have his participants do when they arrive at the lab is

The answer to this question is: Read and sign an informed consent form
If there study caused some relatable negative effects to the subject of study, the participants could sue the researchers because technically the researcher is the one that create those effects.
But, if you mention the possible side effect and make the participants constantly join the study, you will be safe from potential alwsuit.