I once administered the 2-4-6 task to a friend. He made many guesses, and explained why he had made some of them when we were done. One particularly nice hypothesis he had tested was that the sum of the numbers had to be less than a fixed value. He didn't solve the problem and, in fact, despite his search, he never found an example which the rule rejected before he gave up. My friend was looking for disconfirming evidence, he was just really bad at it. So bad he never through to try negative numbers, or numbers in non increasing order, or listing a smiley face instead of an integer to see whether the rule was defined outside the domain I had specified.

Of course I don't deny the existence of positive bias based on one game with one person, but because of that experiment I'm curious: did you provide the group with a chance to share how well their performance was explained by positive bias or did you just...look for confirming evidence?