Chances are, you will be a less reliable indicator of your own behavior than a brain scan will...... [A] research team, led by Matthew Lieberman, a psychology professor at UCLA, had subjects watch a public service announcement about the benefits of sunscreen while in an fMRI machine. The researchers looked for an increase in activity of the medial prefrontal cortex, a part of the brain associated with values, preferences, and self-reflection. Then, the researchers asked the subjects how likely they were to use more sunscreen during the upcoming week. After one week, the subjects were asked how often they ended up using sunscreen.What the researchers found was that the subjects who showed an increase in medial prefrontal cortex activity were 75% more likely to use sunscreen, whereas the subjects who self-reported the intention to use more sunscreen were only about 50% more likely to do so. Thus, the researchers had better information about how the subjects would behave during the upcoming weeks than the subjects themselves.
Here is a link to the original research (also found in the Philosopher's Eye page, of course.)