The full answer to your question is: To stop experimental subjects from trying to confirm a scientist's hypothesis, a psychologist will sometimes...falsify to participants the true nature of an experiment.
It's easy to see why a psychologist might lie to his subjects about why they are taking part in an experiment. If the psychologist announced to the subjects that he/she wanted to prove if x caused y to happen, this would at once influence the subjects into proving x did cause y, or if they felt like making trouble, into trying to prevent x causing y.
People tend to place their trust in authority figures, especially 'scientific' figures such as psychologists. Once told a statement by such a figure, it would influence their actions, when what is really required is a study of normal reactions to a set of stimuli. The psychologist's hypothesis would then be 'proved' by the subjects, when in fact they were knowingly trying to prove the psychologist correct.
A Test of Telling a Lie
I remember watching a psychologist perform a demonstration during a TV show to this effect. It went something like the psychologist telling the group that left-handed people could tolerate pain for longer than right-handed people.
The whole group were asked to immerse their hands in ice water for as long as they could. Of course, the left-handed people kept their hands in the water longer than the right-handed people, but then psychologist told the group he had lied earlier on, and left-handed people had the same pain tolerance as right-handed people.
The left-handed group had kept their hands in the ice water because they felt they 'should', according to the (false) information the psychologist had given them.
For similar questions on Blurtit, take a look at Do scientists consider a hypothesis to be a fact? and Explain The Double-negative Logic Of Hypothesis Testing (eg: A Scientist Fails To Reject The Null Hypothesis)?
Lying to test subjects enables crucial psychology experiments to take place, as we can see in this YouTube film about the famous 'Milgram Experiment'.