From a Geneva University Neurocenter release:
…the optimal strategy when faced with two propositions is to sum up the values associated with the memories you have of each choice, then calculate the difference between these two sums (do I have more positive memories linked to chocolate eclairs or macaroons?). The decision is made when this difference reaches a threshold value, fixed in advance, which determines the time taken in making the decision. This model leads to rapid decision-making when the values of the two possibilities are very far apart. But when two choices have almost the same value, we need more time, because we need to draw on more memories so that this difference reaches the decision threshold.
Our brains are constantly faced with different choices: Should I have a chocolate éclair or macaroon? Should I take the bus or go by car? What should I wear: a woollen sweater or one made of cashmere? When the difference in quality between two choices is great, the choice is made very quickly. But when this difference is negligible, we can get stuck for minutes at a time – or even longer – before we’re capable of making a decision. Why is it so difficult to make up our mind when faced with two or more choices? Is it because our brains are not optimised for taking decisions? In an attempt to answer these questions, neuroscientists from the University of Geneva (UNIGE), Switzerland, – in partnership with Harvard Medical School – developed a mathematical model of the optimal choice strategy. They demonstrated that optimal decisions must be based not on the true value of the possible choices but on the difference in value between them. The results, which you can read all about in the journal Nature Neuroscience, show that this decision-making strategy maximises the amount of reward received.