"Humans only use 10% of their brains." - Freud or Einstein?

I have no idea how anybody could work out a figure like this. Ultra-smiley personal power gits love this psuedo fact. Are there people out there thinking that they could be 90% smarter, with a money back garuntee? Either way, if we clever humans don't use all of our mental abilities at the moment, what bonuses would we expect to receive if we did harness the full power of the grey mush?

Seeing as most people who can read aren't dead, we must be using some proportion of our brain already. 10% or not. Facing these hard facts, we must realise the importance of the following three words:

Psycho Mind Powers

A commonly used term to describe unexplainable weirdness seemingly controlled or sensed by another human and/or thing with a mind. - Webster, Thursday

Everyday examples of P.M.P. reference:

• Social Joe Average: "It was all going cool until she freaked me out with her psycho mind powers man."
  
• Special Secret Agent G-Man: "The individual demonstrated high levels of P.M.P., then I dropped my cellphone."
  
• Badly translated computer game: "Hero! I am killing you now with the psycho mind powers!".

Examples of subtle Psycho Mind Powers:

• knowing someone's star sign by the cut of their gib
• always having a fresh cup of coffee, but never making it yourself
• looking smart, while thinking stupid

Examples of not-so-subtle Psycho Mind Powers:

• Summoning legions of luminous midget toads at will
• Anything involing invisibility
• Catchy pop music

Protecting yourself from the unwanted influence of psyco mind powers is difficult. There are two proposed methods of psycho mind power defence.

• Install a second brain to act as a filtering buffer
• Wear a lead helmet at all times

Unfourtunately, the former of these methods is currently impossible and the latter does not generally indicate the wearer is actively defending themselves from psycho mind powers, more so, just actively psycho.