Warm hands make you nice?

In a study from Yale University, researchers found that people who held something warm in their hands were nicer towards others than those who held something cold. The lead researcher says that subtle clues from our environment can impact our psychological disposition; warm hands help you have a "warm" heart. But...this study seems to overlook something. Something icy in your hand is uncomfortable; something warm is close to your natural body temperature and is therefore more "normal" than warm. It seems to me that merely the discomfort caused by the chill in your hand probably explains why people might be more irritable towards others, not some connection between physical warmth and psychological "warmth." In order to show that, people would need to hold something uncomfortably hot in their hands and still have the same friendliness towards others. If the comfort level is different between the two groups, then we don't know if it's really the temperature that makes the difference or the discomfort that makes the difference. Since more than one variable is involved, the results of the study are suspect. Thoughts?