Observer expectancy, also known as the observer effect, is a phenomenon that occurs when the act of observing a system or event influences the outcome of that system or event. This can occur in a variety of situations, including scientific experiments, social interactions, and even in our daily lives.

One example of observer expectancy is the Hawthorne effect, which refers to the phenomenon of individuals changing their behavior simply because they know they are being observed. This can occur in a workplace setting, where employees may be more productive or behave differently if they know their performance is being monitored.

Another example of observer expectancy is the placebo effect, which is a well-known phenomenon in the field of medicine. The placebo effect occurs when an individual's symptoms improve simply because they believe they are receiving a treatment, even if the treatment itself is not actually effective. This can occur because the individual's expectations and beliefs about the treatment influence their perception of their own symptoms.

Observer expectancy can also influence the outcome of scientific experiments. For instance, if a researcher expects a certain result from an experiment, they may unconsciously convey their expectations to the participants, which can influence the participants' behavior and ultimately affect the results of the experiment. This is why it is important for researchers to be aware of and control for observer expectancy in their studies.