The American Psychological Association and British Psychological Society have developed ethical principles urging investigators to: increase the difficulty level of research endeavors while maintaining validity. increase the difficulty level of research endeavors while maintaining validity., explain the research to the participants after the study has been completed. explain the research to the participants after the study has been completed., avoid the use of monetary incentives in recruiting people to participate in research. avoid the use of monetary incentives in recruiting people to participate in research., forewarn potential research participants of the exact hypotheses that the research will test. forewarn potential research participants of the exact hypotheses that the research will test., avoid the manipulation of independent variables in research involving human participants.