Do check on omnilibrium too. It calculates a reverse to onesidedness called non-partisanscore. You get non-partisan score when people that do not vote like you do, vote for your posts (ie those outside of your political group althoguht there is slight difference between political cluster and voting block).
What if some policies correlate with kinds of arguments people find appealing? An argument from natural law against the legality of homosexuality is unlikely to convince anyone who doesn't love St. Thomas Aquinas, while the liberty principle won't convince a single Dominican. Then again this is more a problem of ethical foundations than factual arguments, so perhaps by separating values from beliefs you can finesse this difficulty.
Follow-up to Reverse Engineering of Belief Structures
Pro-con-lists of arguments such as ProCon.org and BalancedPolitics.org fill a useful purpose. They give an overview over complex debates, and arguably foster nuance. My network for evidence-based policy is currently in the process of constructing a similar site in Swedish.
I'm thinking it might be interesting to add more features to such a site. You could let people create a profile on the site. Then you would let them fill in whether they agree or disagree with the theses under discussion (cannabis legalization, GM foods legalization, etc), and also whether they agree or disagree with the different argument for and against these theses (alternatively, you could let them rate the arguments from 1-5).
Once you have this data, you could use them to give people different kinds of statistics. The most straightforward statistic would be their degree of "onesidedness". If you think that all of the arguments for the theses you believe in are good, and all the arguments against them are bad, then you're defined as onesided. If you, on the other hand, believe that some of your own side's arguments are bad, whereas some of the opponents' arguments are good, you're defined as not being onesided. (The exact mathematical function you would choose could be discussed.)
Once you've told people how one-sided they are, according to the test, you would discuss what might explain onesidedness. My hunch is that the most plausible explanation normally is different kinds of bias. Instead of reviewing new arguments impartially, people treat arguments for their views more leniently than arguments against their views. Hence they end up being onesided, according to the test.
There are other possible explanations, though. One is that all of the arguments against the thesis in question actually are bad. That might happen occassionally, but I don't think that's very common. As Eliezer Yudkowsky says in "Policy Debates Should Not Appear One-sided":
Instead, the reason why people end up with one-sided beliefs is bias, Yudkowsky argues:
Especially if you're consistently one-sided in lots of different debates, it's hard to see that any other hypothesis besides bias is plausible. It depends a bit on what kinds of arguments you include in the list, though. In our lists we haven't really checked the quality of the arguments (our purpose is to summarize the debate, rather than to judge it), but you could also do that, of course.
My hope is that such a test would make people more aware both of their own biases, and of the problem of political bias in general. I'm thinking that is the first step towards debiasing. I've also constructed a political bias test with similar methods and purposes together with ClearerThinking, which should be released soon.
You could also add other features to a pro-con-list. For instance, you could classify arguments in different ways: ad hominem-arguments, consequentialist arguments, rights-based arguments, etc. (Some arguments might be hard to classify, and then you just wouldn't do that. You wouldn't necessarily have to classify every argument.) Using this info, you could give people a profile: e.g., what kinds of arguments do they find most persuasive? That could make them reflect more on what kinds of arguments really are valid.
You could also combine these two features. For instance, some people might accept ad hominem-arguments when they support their views, but not when they contradict them. That would make your use of ad hominem-arguments onesided.
Yet another feature that could be added is a standard political compass. Since people fill in what theses they believe in (cannabis legalization, GM goods legalization, etc) you could calcluate what party is closest to them, based on the parties' stances on these issues. That could potentially make the test more attractive to take.
Suggestions of more possible features are welcome, as well as general comments - especially about implementation.