Do you think It is ethical for a researcher to deceive a participant? When and why?

It will depend a lot on whether the deception will cause any harm, physical or psychological. If it will cause harm, the deception is not ethical. Studies can be done with placebos where some deception is involved, but it has to be approved by a research board or committee to ensure that the welfare of research particpants are the first consideration. Take the instance of using positive suggestions when using hypnosis. It is absolutely critical that the therapist frame a suggestion in a positive sense in order to achieve the desired result, because the result depends on the degree to which the client "believes" the suggestion - some people will call this deception. So its just important when using deception that sufficient consideration is given to participant interests, that no harm is caused, and that participants are voluntary and provide informed consent.

There is no doubt that the practice of deception is unethical, for reasons that cannot necessarily be explained other than attributing the judgment to social perceptions against deception, although it is certainly necessary in some instances. Placebo effect in its very nature requires deception to occur, so without it there would be no reliable results. Certain fields of Psychology, such as Social Psychology, also lend themselves to the practice of deception because of the nature of the theories they profess; conformity and social normalities in a group are hard to manipulate reliably without deception.

I am personally not bothered by the use of deception in research, as long as the practice is absolutely crucial to the production of reliable results and that the practice does not cause harm. Many experiments that involve deception also involve a 'de-briefing' that can clarify the deception, so that no psychological harm of any sort is caused.