Overcoming bias : don't tell people to be impartial, instead tell them to consider the opposite interpretation

At school we had debates in which we were forced to advocate for the side we disagreed with, and it was extremely useful.

The motivational instructions told participants to be "as objective and unbiased as possible", to consider themselves "as a judge or juror asked to weigh all of the evidence in a fair and impartial manner".

The alternative, cognition-focused, instructions were silent on the desired outcome of the participants’ consideration, instead focusing only on the strategy to employ: "Ask yourself at each step whether you would have made the same high or low evaluations had exactly the same study produced results on the other side of the issue." So, for example, if presented with a piece of research that suggested the death penalty lowered murder rates, the participants were asked to analyse the study's methodology and imagine the results pointed the opposite way.

They called this the "consider the opposite" strategy, and the results were striking. Instructed to be fair and impartial, participants showed the exact same biases when weighing the evidence as in the original experiment. Pro-death penalty participants thought the evidence supported the death penalty. Anti-death penalty participants thought it supported abolition. Wanting to make unbiased decisions wasn't enough. The "consider the opposite" participants, on the other hand, completely overcame the biased assimilation effect – they weren't driven to rate the studies which agreed with their preconceptions as better than the ones that disagreed, and didn't become more extreme in their views regardless of which evidence they read.

The finding is good news for our faith in human nature. It isn't that we don't want to discover the truth, at least in the microcosm of reasoning tested in the experiment. All people needed was a strategy which helped them overcome the natural human short-sightedness to alternatives.
http://www.bbc.com/future/story/20170131-why-wont-some-people-listen-to-reason