An inactive or 'dummy' form of treatment that has no effect, used in research trials. One group of participants are given a new treatment whilst the other group are given a placebo. This assists researchers by maximising the objectivity of the results - because the participants are unaware of which substance they are taking, they are not able to consciously or unconsciously influence their reaction. Sometimes the researcher is also unaware of the group that has been allocated the placebo treatment- this is called a 'double-blind' experiment. The 'placebo effect' refers to an individual's ability to experience the alleviation or worsening of symptoms despite taking a neutral substance, attributable to the expectation of an effect.