The main function of System 1 is maintaining and updating a model of your personal world, which represents normality. This model is constructed by associations that connect ideas of events, circumstances, outcomes and actions that regularly occur. The formed connections become a pattern of associated ideas, which represents the structure of events in your life. It determines how you interpret the present and your future expectations.

Surprises are crucial elements of our mental life, they are the most sensitive indication of our understanding of the world and our expectations from it. Surprises can be divided into two varieties: conscious and active surprises, and passive surprises. Around dinner time, you expect your partner to return home from work. When you hear car doors slamming, you expect to see his face. It would be a surprise if someone else walks in. In case of a passive event you do not wait for it, but you are also not surprised when it happens. Although not actively expected, it is normal in that situation.

One event can make a recurrence less surprising. Imagine you run into your old friend Kim while holidaying far away from home. A few weeks later, you see her at a concert in Amsterdam. The second meeting is a more remarkable coincidence, but you are less surprised to meet her. The first one changed the idea of Kim in your mind. While System 2 knows it is an absurd idea, System 1 made it seem normal to meet Kim in unusual places. You would have been more surprised if you met another old friend at the concert, even though meeting Kim was statistically no more likely than any other old friend.

Passive expectations sometimes turn active. Imagine you are driving on the highway and spot a car with a flat tire. Exactly one week later, you see another person stranded with a flat tire, at the same location. You will be less surprised the second time, because you consider that location as the ‘place where people strand with flat tires’. Because the circumstances were similar, the second event evoked an active expectation: you will be reminded of stranded cars every time you pass that location for a long time and are prepared to spot another unlucky driver.

How does our mind assess normality?

How incidents come to be perceived as abnormal or normal can be explained by the ‘norm theory’. If you witness two abnormal events, the second event will retrieve the first one from memory and together they will make sense. An example is the ‘Moses illusion’: “How many animals of each kind did Moses take into the ark?” Very few people realize that it was Noah who took them into the ark. The thought of animals in an ark creates a biblical context, in which Moses is normal. Hearing or reading his name did not come as a surprise. The (unconscious) associative coherence makes you accept the question. Replace Moses with Bill Gates and there would have been no illusion, because his name is abnormal in the context. The brain quickly detects deviations of normality. It immediately responds when you hear a guy say “My belly hurts, it must be menstrual cramps”. Our world knowledge instantly recognizes the abnormality and is why we can communicate with each other: we use the same words. We have ‘norms’ for lots of categories, which provide the background for the instant detection of abnormalities (such as menstruating guys).

The role of norms in communication can be explained by the following example. “The big ant climbed on the neck of the tiny giraffe”. We are likely to have similar norms for the size of giraffes and ants. We both will not picture the ant larger than the giraffe. System 1 has access to norms of categories, specifying the range of possible values and the most typical cases.

How does our mind find causal connections?

“Jane’s husband was late. His parents could arrive any minute. Jane was irritated.” Irritation (effect) and being late (possible cause) are linked in your network of associations. You instantly found the cause of Jane’s irritation, it was not linked to his parents coming over. Searching for causal connections, an automatic operation of System 1, is a component of understanding a story. Your (conscious) System 2 accepted the causal interpretation it was offered. When there is little information about what happened, System 1 starts searching for a coherent causal story that brings the fragments of information together.

People have impressions of causality from birth. Our minds are from an early age ready to identify agents, assign them personality traits and certain intentions. Before the age of one, we are prepared to identify victims and bullies. People have the tendency to apply causal thinking when the situation actually requires statistical thinking. Statistical reasoning derives conclusions about individual cases from ensembles and categories, which System 1 is not capable of. System 2 is able to reason statistically, but this requires training (which most people do not receive).