How we change what others think, feel, believe and do
Principles > Uncertainty principle
When a person is not sure about something, they will seek to reduce their uncertainty.
We all have a need for a sense of control and certainty. When we are not sure, when we cannot predict, we feel tension and will grasp at straws that seem to be more certain, even if they might have a longer-term cost. This means we will also become more suggestible, seeking more information and believing more of what we are told.
A sale in a shop works by suggesting that availability of a product in the future is not certain. Other people hovering over the last available item make you want to grab it quickly!
A sales person might indicate that prices are going up next month, but not say exactly by how much ('It's the exchange rate on imports, and things are not looking good').
When a person behaves erratically, then others cannot predict what they will do and so become nervous around them. This can range from inconsistent parenting to bad management or fairweather friends.
If you say that something is unknown, then if the other person knows they may fill that gap for you, or if they do not know, they (now also unknowing) will seek to find out. This can be useful both in teaching and in getting others to discover things, either for your information or to get them engaged.
When people are confident then have their confidence shaken, for example when someone challenges them or provides disconfirming evidence, then the person becomes less certain, not just about the current decision but even about their ability to decide (this is often increased if the criticism is personal). In fact when a person fails at anything, their confidence may be knocked, again reducing their certainty about themselves.
We also become less certain about the ability of other people when they do not meet our expectations of them. This can range from simple honesty to physical dexterity.
Threats and opportunities
A threat is often seen in terms of uncertain harm. It thus becomes a focus of attention and people will act to reduce uncertainty around it.
Opportunities are the opposite and people will move towards the uncertain gain, seeking to secure it.
So create situations of uncertainty where the other person has to act in the ways you want to reduce the uncertainty. Get them to doubt themselves and others. Even giving them more information that does not contradict what they know opens up the possibility that there is further information that they have not yet considered.
For example if you say 'I don't know if John is coming', the other person may well be persuaded to go and ask John (saving you the bother of doing so, and particularly if John is more likely to come if the other person asks him).