In what is now my favorite study in the field of persuasion and attitude change, researchers from the university of Queensland have carried out the following experiment (you can find the paper here, freely available). Participants were taking part in a classical study of persuasion: they were tested on their attitude towards a contentious issue (like abortion), they were given some counter-attitudinal arguments and tested again to see if the arguments had affected their attitude.

However, in these experiments, people were made to drink an orange juice before they proceeded. In one condition, the orange juice contained some added caffeine: in the other it was pure orange juice. When the attitudes were measured after the persuasive message, it was found that the attitudes of participants under caffeine had changed more than that of the control participants.

But coffee doesn't just make you more gullible: quite the contrary. In another experiment, the researchers studied the effect of argument strength. They did exactly the same thing as in the first case, but this time there were two types of arguments: the strong ones and the weak ones. It turns out that people under caffeine are not more convinced by weak arguments, but only by strong ones. So in fact caffeine makes you more suited to understand the strength of good arguments. This fits with what we know of its effects: that's what you would expect of people who are more aware and able to concentrate.

So now we are faced with a dilemma: is an increase understanding of other people's arguments worth the price of being more easily convinced? From a normative point of view, it is surely worth it, especially when taking into account that we can expect people under caffeine not only to be better at understanding good arguments, but also to be better at creating good arguments.