How we change what others think, feel, believe and do
We are generally more optimistic than pessimistic and tend to over-estimate the probability of good things happening as compared to the chance of bad things happening.
The valence of anything is the emotional charge that we feel when we think about it. This can be positive or negative emotion, which indicates positive valence or negative valence.
It is natural for us to want good things and so we think more about them. The reverse is generally true when we think less of bad things. The availability that this creates when we are assigning probability tends to make good things seem more likely.
Of course if people are pessimistic, then they may think bad things more likely. However, most of us, most of the time, find optimism a more effective state as it can create a self-fulfilling prophecy through the motivational effects it causes. It also likely has evolutionary benefits.
Because of the bias towards optimism, being slightly pessimistic is likely to make you more realistic. People whose future is inescapable, such as those with a terminal illness can be more realistic in this way.
FMRi tests have shown that optimism is related to reduced coding of undesirable information about the future in the frontal cortex that has been is sensitive to negative estimation errors.
The 'Polyanna' effect is where a person sees good in all things and is overly optimistic. Whilst some optimism can be helpful, being unremittingly positive is probably not the best survival strategy.
The valence effect is sometimes also called 'wishful thinking' or 'optimism bias'.
Rosenhan and Messnick offered subjects a pack of cards that had an equal number of smiling faces and frowning faces. When asked to predict the likelihood of picking particular cards, the subjects over-estimated the chance of picking a smiling face.
A young person asked about the chance of them being successful in a difficult task will more likely think they can succeed than that they may fail.
Play to the natural tendency of the valence effect and be positive when persuading others.
When accurate forecasting is important, remember this tendency and carefully balance the positive and negative effects.
Rosenhan, D.L. and Messick, S. (1966). Affect and expectation. Journal of Personality and Social Psychology. 3, 38-44.