A partial explanation of this phenomenon is that the experimenter becomes part of the field the influence of which he or her is attempting to measure.
By way of analogy,consider the problem of making critical temperature measurements.
A thermometer inserted into a substance not only reacts to the temperature of the substance but also to its own temperature.
In a similar way,an experimental psychologist is not only an observer of behavior,but also a part of the environment to which the subject reacts.