How we change what others think, feel, believe and do
Principles > Completion principle
We seek to complete that which is incomplete.
Certainty and uncertainty
When something is certain and known then we feel comfortable and in control. When something is not complete, we cannot close that item in our mind as we have to keep thinking about it. This maintenance activity adds effort and leads to predictions that might give us cause for concern.
This is the basis for the need for completion, and we will hence seek to close off things that we do so we can forget them and move on to the next item of interest.
Sentences which are not
The title above probably creates a bit of irritation or tension in you as you wonder what the end of the sentence should be.
If you start a sentence and leave the rest undone (even for a few moments, other people will guest what the completed sentence might be. In this way, you can lead their thoughts.
Some people have a particular need for completion and in teams will be the person who makes sure all jobs are done (often doing the jobs themselves). Meredith Belbin identified such a position in his Team Roles.
People who compulsively tidy up are also acting as completer-finishers as they see untidiness as a step before the completion of tidiness.
Note that this also implies that there are some people who are not completer-finishers and who will happily start something but will be unlikely to see things through to the end.
The Zeigarnik Effect
Russian psychologist Bluma Zeigarnik found that waiters remembered orders only as long as the order was in the process of being served. This extends into the Zeigarnik Effect, whereby we keep thinking about those things about which we have not achieved closure.
So leave things for people to complete. Start a sentence and see if they will complete it for you -- if they do, you have put the other person into the complete-finisher position. This can be a powerful tool in changing minds.
Even if they do not verbally complete the sentence, they will do so in their minds. Watch their body language for signs of what they might be thinking.
Likewise you can use completion in physical tasks. Start something and give it to another to complete.
Give rewards for completion, particularly if you have no completer-finishers who will end the job for you.