I did not have sexual relations with that woman

I don’t know about you, but I always get nervous when I hear this:
I want you to know…
To tell you the truth…
Let me be perfectly clear…
As far as I know…
I want to be perfectly honest…

Performatives
Language experts have textbook names for these guys—”performatives”. Harmless constructions, even polite, right? Unfortunately they may often signal that bad news, or even some dishonesty on the part of the speaker, will follow. 

Deceit
James W. Pennebaker, Professor and Chair of Psychology at the University of Texas at Austin, has done an in-depth study of the linguistics of deceit as well as that of truth. When people tell the truth, they usually use I-words at high rates. The one big exception is the performative case, such as when people start a sentence with something like “I want you to know that…” or “Let me be perfectly clear…” Anything that follows can’t be judged as false or truthful. Performatives are a delightful way to deceive while technically telling the truth.

Frankness
Social media make things even more complicated, because they combine the definiteness of print with the casualness of online communication. Now more than ever before, we need clear positions, frank feedback, honest opinions. Politeness vs. deception, it’s a thin line online.

Read more on presentations.