Are the most effective psychological studies often rooted in deception?

Not so sure.
The questions on Hare’s psychopath test are pretty straightforward.

I am no expert, but it’s my understanding that the purpose of the study must be completely revealed up front to all participants in full. It would be unethical to lie to the participants about the subject of the study.

I don’t think so. How could any effective psychological study be rooted in a fallacy?

Many clinical trials are conducted using a double-blind to account for the placebo effect.

Sorry, but, based on my own research, I have to disappoint you. Take the Asch Conformity study: College students thought they were participating in a simple perceptual task with other students when, in actuality, they were placed one at a time in a group along with actors. The students were shown a card with a vertical line on it, then another card with 3 more lines.

When asked to say out loud which lines matched in length, the students consistently provided the same answers as the actors, even when the actors picked one of the clearly incorrect lines. The student subjects believed they were being tested on perception, but what was actually being assessed was adherence to conformity.

If you want more information, check on wikipedia:
https://en.m.wikipedia.org/wiki/Asch_conformity_experiments

Because of the Hawthorne effect, which you previously mentioned, subjects of psychological experiments are sometimes misled as to what they are being tested for. I don’t know the extent to which this occurs

No. Rigorously developed research may not require deception. Using deception with human subjects in research is VERY difficult to do and requires major justification and full board approval.

You have to really understand what type of bias you think you are preventing from using deception and determine whether you can avoid that bias using other approaches.

Ultimately, it may not matter if deception-involved studies are “most effective”, they cause psychological harm and distrust amongst the participants. You would clearly have to define “most effective” and weight it against the participants reasonable assumption of honesty from the researchers.

Deception-based research is largely considered unethical, but researchers can obtain approval to use deception. They will have to jump through MANY hoops to do so.

The purpose of the decoy in not to deceive, but to keep the person from thinking about the effect you want to study. That way, they don’t second guess what you want. e.g. if you wanted to study the effect of noise on studying, you would not tell your subjects that because they would think about the noise while studying. You would tell them to study, then vary the noise without telling them.