Cognitive Bias: The Law of Small Numbers

A study carried out by an eminent institution in a poor, tropical country reveals that 20% of the inhabitants of a small rural village have a psychological disease, whose average prevalence in the country is 1%. 

This region is very dry so their inhabitants lack of vitamin C. Moreover, the level of endogamy is particularly high in this village. 

What do you think are the most likely reasons for the psychological disease?

In fact, you cannot say much since the population is very small.

Moreover, you can wonder if the method of evaluating a psychological disease in a poor country is reliable. 

However, we tend to draw fast conclusions from this study due to different cognitive biases:

  1. Authority argument: the newspaper is ‘’eminent’’ so we tend to trust the data it provides
  2. Hypercausality and story-telling: we tend to make correlations for everything whereas most events that occur in our daily life are random, independent and uncorrelated. 
  3. The laziness of System 1: we prefer trust than doubt.


Thinking fast and slow, D.Kahneman

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