What phenomenon occurs when an experimenter's expectations influence the outcome of a study?

The phenomenon in which an experimenter's expectations influence the outcome of a study is known as the experimenter effect. This effect occurs when the researcher’s beliefs or expectations regarding the expected results of the experiment inadvertently impact their behavior or the way they interact with participants, which can lead to biased results. For instance, if an experimenter anticipates a particular outcome, they may unintentionally give cues to participants or interpret data in a way that confirms their hypothesis.

Understanding the experimenter effect is crucial in psychology research, as it underscores the importance of maintaining objectivity during the research process. Researchers often use techniques like double-blind studies to minimize this effect, where neither the participants nor the experimenters know which treatment is being administered to avoid any unconscious biases affecting the results. The other options reflect different types of effects that are related but distinct; for example, the observer effect refers to changes in behavior caused by the awareness of being observed, while participant effects involve how individual characteristics of the participants may influence study outcomes.