I agree that the experiment as described does not prove that conclusion: first, Kathy really describes a metaphor, rather than a quantitative or realised model. Second, there are lots of potential confounding factors: you gave a potential alternative explanation, and when I read the post, I thought that possibly people are more likely dissuaded by the propsect of appearing greedy, so they will only take the cake if they think they appear to have deserved it. Possibly, considerations of calorie control or significant for a small proportion of the test subjects.

But this kind of discussion really needs to address the original article, not kathy's summary: the PDF can be downloaded from a link at http://www.jstor.org/stable/10.1086/209563 - from my quick glance, the paper does deal with the suggestion aboutit being just social context (the student-drawn subject pool did indicate that they thought one should generally avoid cake), and the background for the study did cite research into neurological underpinnings of the model.