Studying cognitive psychology is hard because we have no direct access to people’s minds. As a result, we often have to rely on studies of behaviour and infer what people must be thinking.
But the behaviour may not be authentic. People may not behave as they normally would during an experiment, but instead behave the way they think the experiments are expecting. There is often as subtle desire to try to please the experimenter and give them what they want. This idea is called “demand characteristics” and the this article describes new research that uses this idea to question a common body part illusion.
In psychological experiments the study design must take into consideration the fact that subjects subconsciously try to figure out what the experimenter wants and then gives it to them. Any subtle cue that one response is more desired than another can affect the outcome. So the influence of demand characteristics must be carefully controlled for.