How We Decide, by Jonah Lehrer

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I picked this up from the library, as yet another in the recent series of books I’ve been reading that reinforce my own biases. Overall, I liked it – I knew most of the patterns in cognition that the book describes, but it summarized them nicely with good anecdotes.

One standard model of decision-making is that we are rational beings. We examine all of our options, we think through the consequences of making a decision, we weigh the costs and benefits, and then we decide. Philosophers like Descartes think that this rational mind is what separates us from other species (“I think, therefore I am”).

Another model is that of the unconscious mind, as popularized in recent books like Sources of Power and Blink. The theory here is that our brains have evolved over millenia to have an enormous amount of processing power that is not consciously accessible, and sometimes we have to trust the “intuition” that the unconscious mind is giving us.

Lehrer’s book reviews the strengths and weaknesses of each of these cognition models to help people understand when it’s appropriate to use each model.

The rational conscious mind is limited in power – we’ve all heard the idea that we can only keep 7 information nuggets in our brain at a time. It’s a bandwidth-limited single processor (one estimate is that it processes at 20 bits/second). Its strengths are that it can logically process new situations, override our kneejerk impulses that may not be appropriate to the situation, and come up with responses that have not been tried before. Also, decisions made using the rational path are easy to explain, as they are based in logic. Its weaknesses are that it is slow and has limited capacity (check out his anecdote on self-control when trying to remember too many things), and therefore works best on well-defined problems with only a few dimensions to consider.


However, the unconscious brain does not deal well with novel situations, as it may seize on an already-trained, but inappropriate, response. It is also unreliable in situations where previously seen inputs have different outcomes because the training doesn’t work – Lehrer cites slot machines as an example of the unconscious brain desperately trying to find patterns when none exist. One final weakness is that the decisions made by the unconscious brain are difficult to explain, as they are expressed through emotions we feel and so we can’t analyze the decisions rationally.

Lehrer describes many situations when the two minds are used inappropriately. For instance, complex multivariable problems can not be answered by pure reason (Lehrer cites the example of a man who lost his emotional capacity after a brain tumor was cut out, and was completely unable to make normal life decisions). In fact, if we try to attack such problems with the rational brain, we make poorer choices because we seize on variables that are easy to explain rationally rather than considering all of the possible benefits (Lehrer cites an amusing study where undergrads had to choose a poster to take home; those that had to give a reason for choosing a poster ended up choosing posters they were less happy with compared to the ones that just chose a poster). Lehrer suggests that the best strategy when confronting a complex decision with many variables is to study it carefully to load all of the information into our unconscious brain, and then go do something else (take a walk, go for a driver) while the unconscious brain processes that information. This idea is reflected in the standard trope that the best ideas come in the shower.

However, the unconscious brain only works well in repeatable situations where it can try out different responses to the same set of inputs and encode what works into the neural pathways. In novel situations, we can’t trust our instincts and have to slow down and engage the conscious brain. Lehrer tells the story of a team fighting a forest fire when the wind shifted unexpectedly and came towards them. The leader realized the fire was going to overtake him before he could get to safety, stopped running, thought for a second, and then set his own fire to create an already burned spot, which he then stepped into so that the forest fire would go around him. Most of his team was lost because they were only listening to their emotional brains telling them to run from the fire.

I liked the book’s balance between the “Blink” theory of trust your instincts and the “Descartes” theory of following reason. Both methods of cognition have advantages and disadvantages, and the best decisions will be made by taking those strengths and weaknesses into account. In some sense, the two brains are mental tools, and it’s up to us to understand when it’s appropriate to use each tool.

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