Deliberation is usually treated as a single event: people arrive with one view, leave with another, and the bit in between is a black box. Ask participants what changed their minds and they'll often tell you their view shifted - but rarely what shifted it. The arguments of other participants? The expert evidence? Simply having time to think?

We think commissioners of deliberation should be able to make decisions grounded in evidence. That’s why we wanted to open the box. Because our Zeitgeist platform combines small-group discussion with polling throughout a workshop (not just at the start and end) we can watch opinion move as it happens, and see which part of the process moved it.

We found that when a deliberation on a contested reform gets stuck on whether something could work, a well-chosen example from elsewhere - so perhaps drawn from how another country approaches the issue - can help break the deadlock,  turning an abstract "it can't be done" into a concrete "here's how it has been done, and what we might learn from it."

In February, working with the Electoral Commission and New Citizen Project, we brought together 72 members of the public from all four UK nations to weigh up six possible reforms to the way political parties are funded - and to find out whether any of them could help rebuild trust in our democratic system. Using a method called deliberative polling, we asked participants to allocate 10 points across the six policy options at three moments: after learning how the current system works; after watching expert videos on the reform options and discussing them in small groups; and after learning how three other countries - Ireland, South Australia and the USA - fund their political parties.

One option moved far more than the rest: increasing public funding for political parties. And one stage of the process did almost all the work.

Public funding started as relatively unpopular. Just 13% of the points were allocated to this option at the first vote. Perhaps this is no surprise given that the Electoral Commission's own 2003 research found only 6% of the public backed state funding. At the second vote, after the expert videos explaining the options and small group discussions, it barely shifted, with still only 14% of total points allocated. But after participants saw how other democracies fund their political parties, support for public funding jumped - with 22% of all points allocated to this option, and giving it level-pegging with introducing donation limits as the public's top recommendation for rebuilding trust.

That contrast is the key finding. Expert input and discussion moved support for this option by a single point; but the much shorter segment on international examples moved it by an additional eight.

The international examples didn't just inform people - they changed what participants thought was possible at home. And two of the comparators did opposite jobs. The US system, with its minimal public funding, was interpreted by people as a cautionary tale: it hardened what participants found unacceptable and the direction they didn't want the UK to go in. South Australia, which bans private donations and funds parties per vote, worked as a proof of concept, making an option that had felt radical to feel feasible (though many participants also wanted evidence this new system was actually working before fully committing).

But the lesson for commissioners and practitioners of public deliberation travels well beyond party political finance. Academic research has shown that citizens instinctively use other countries as a yardstick for judging their own system, and we look to others to decide what is acceptable. So when a deliberation on a contested reform gets stuck on whether something could work, a well-chosen example (or better - contrasting ones) from elsewhere can help break the deadlock - turning an abstract "it can't be done" into a concrete "here's how it has been done, and what we might learn from it."

If you're commissioning public deliberation and want to talk about how to engage people constructively in contested policy choices, we'd love to hear from you. Please email us at [email protected].

Part of a series of experiments from Nesta's Centre for Collective Intelligence.