Is a debate more productive when the people debating agree on 50% of all things or when they only agree on 15%
From my experience when two people agree only 15% they tend to butt heads where as when they agree on more things they can compromise and see where the other person is coming from.
That depends on your idea of productive. An ancap and a fascist would have a lot of headway to make, but your average Democrat and Republican would work through issues faster.
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Someone in my family has a better chance of changing my mind than a KKK member who is in jail for lynching black people.