How we change what others think, feel, believe and do
'Duper's delight' is the delight that people feel when they deceive others.
Duping is another word for deception, for tricking people into some kind of action or misunderstanding.
The delight of power and control
People who deceive often find a secret pleasure in their success. It is similar to the pleasure that people get when they exercise power. It is as if the brain rewards us for achieving control over other people.
The delight of secrecy
Related to power is having something that others do not have. We thus tend to take pleasure in the secret nature of deception, of not only knowing that we have gained something, but also that the other person does not know this.
People who deceive others, and hence feel duper's delight, often cannot hide their feelings. If you can spot the signals by which they give themselves away, you can avoid being deceived.
There is common a transient pleasure in deception that appears, typically as pleasure-signaling body language, such as:
The body language may also be combined with attempts, conscious or otherwise, to conceal this, which will lead to the signals appearing very briefly before they are quashed and suppressed. For example a liar may flash smile very briefly before returning to the mask of emotions they are wearing.
So watch other people for the signs of delight when they might be trying to persuade you of something or otherwise deceive you. When you know this, then you will have reversed the situation: you have the power of knowledge that they do not have -- so beware of sending them back another 'duper's delight' signal!
Ekman, P. (2001). Telling lies: Clues to deceit in the marketplace, marriage, and politics (3rd ed.). New York: Norton.