Reactance is unpleasant motivational arousal that emerges when people experience a threat to or loss of their free behaviors. It serves as a motivator to restore one's freedom. The amount of reactance depends on the importance of the threatened freedom and the perceived magnitude of the threat.

When you tell people they cannot do something, the more likely that they will do it. This psychological trick can be used to make people do what you want by making it seem like they cannot.

For example, If you write an email with the subject line: “ don’t read this email till 11 pm”, the chances are people will open it as soon as they get it. You can use reactance in your marketing to get more reaction.