The players are anonymous so no obligations can be introduced. Taking the money is the most rational. (Well I guess the group was so small that egoistic behaviour could cause you to lose longterm because of reduced willingness to cooperate even though no one knew it was you).

There are plenty of examples of irrational behavior in psychological economics exeperiments, many are given in

http://en.wikipedia.org/wiki/Thinking,_Fast_and_Slow

I that if you're told that the other player is a machine you won't feel the need to refuse.