By understanding how people make decisions, we can design to influence their behavior. Check out this article by Smashing Magazine about designing with cognitive bias in mind. Also, make sure to read Dan Ariely’s fascinating (& fun) book Predictably Irrational if you haven’t already.
Persuasive design can certainly be a good thing. Since interaction design often helps users to make decisions that lead to accomplishing their goals, a persuasive nudge can be both helpful and welcome.
But as any design superhero knows, “with great power comes great responsibility.” What happens when you consciously choose to not do right by your users? To trick, mislead, or take advantage of them?
Harry Brignull gave a presentation about Dark Patterns, which are design patterns that take advantage of people in a less-than-positive way. See the Dark Patterns Wiki for a list of malicious design patterns. Unfortunately, they are far too familiar. A few examples: asking trick questions, creating friend spam, and (I love the name of this one) privacy Zuckering.
When you realize that psychology & design enable you to influence people, what will you choose to do with that power?