Having a spectator freely select one out of four objects gives you a chance of 1:4 or 25% that each single object is picked. By knowing which face-down card in a row of four is selected more often than others, you can increase the chances of having your “favorite” card picked to 60%. (You could call this a force, even though it’s not surefire.) This is the essence of a new piece of academic research by Gustav Kuhn et. al., which has just been published. You can read the abstract online here.
According to the paper, out of four cards, 60% of the participants in an experiment freely chose the third card from their left (or the second from the right–but I’m sure that you’ve already calculated that). Please enjoy my masterful visualization of this key finding:
Yet these participants felt that their choice was extremely free. They also underestimated the actual proportion of people who selected the target card.
(My guess would be that this result is partly due to the overwhelming majority of right-handed people in the world.)
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By the way, Matt Tompkins from the SOMA Committee keeps a constantly updated “Science of Magic Bibliography” with academic papers published in English since 1887 (which means that some important early German and French research on the psychology of magic is missing there.) And here’s a special on the topic from “Frontiers in Psychology” in free PDF format.
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Read more about the upcoming SOMA conference in London here.
ZZZAUBER 
Very cool!
Thank you for sharing this!
It’s similar to the fact that people usually choose the center card between three options.
However, in this case, it turns out to be more deceptive because of being four options!