The professor is using this method to study human limits, for instance to uncover the true abilities of an athlete. "I am interested in what the mind can do for us, without us knowing it."

But there are wider implications of how it is possible to prime someone to do more for what we mean by free will.

Although bosses may be tempted to try this to spur on their workers, which would be unethical, Prof Aarts says that the effect may be subtle and short lived, only occurring for a couple of minutes.

With his colleagues he shows in the journal Science that people expend more effort on a work out when a little subliminal flash of what he calls "meaningful" words such as "exertion" and "vigorous" is accompanied by the presentation of a positive word such as "good" or "nice".

The researchers tested this idea on 42 people by showing one group of people only positive words and later measuring their performance on a handgrip test. Two other groups had other messages.

A second group saw positive words and exertion words, but these words were not linked to each other. And they did tests on a third group, where the exertion words were linked with the positive ones.

After 50 trials of exposure to the words, they found that people who saw encouraging words such as "good" mixed in with the exertion words were more persistent and put more effort in their grips - doubling the force used - that those who saw exertion words or only positive words.

"We know from other research that people evaluate positive words automatically, and when these positive words are linked with a subliminal flash of physical exertion, their effort is motivated. They keep on squeezing, so they are more persistent."

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