A further example: walking down the street our attention is drawn to a verbal altercation between two men. The disagreement seems on the verge of boiling over into physical confrontation. We become aware that one of the men is holding something and gesturing with it to the other man – it glints in the sunlight – we see that it is a knife and brace ourselves for the worst. Then as suddenly as it began the heated quarrel ends when the second man breaks off discussion and leaves. At this point, to our surprise, we notice that what the “knife-wielder” is holding is actually a steel tradesman’s rule. The demand characteristics and our expectations “tricked” us into ‘seeing’ a knife.

We sometimes perceive what we expect to see even if no such thing is present, and if we expect not to see a certain stimulus we may not notice something which is there. We may also mistake one image or object for another – especially in cases where the stimulus is vague or ambiguous, or when clear observation is difficult. Expectations, together with schema, help us to (often) successfully negotiate, operate in, and interpret the world ‘at large’. On occasions, though, expectations and prior beliefs obstruct clear perception (and thinking), such as being significant in whether people believe they have observed ‘Unidentified’ Flying Alien Spacecraft and may provoke us to generate ‘meaningful’ narratives to “explain” such impressions and to cling tenaciously to their assumed validity.