Studies confirm that sometimes our decisions are driven by irrational patterns, nevertheless there is some logic that we can study and apply in our design solutions.
The human brain tends to simplify when the information obtained is too large to be managed efficiently. Our brain receives millions of bits of information per second, signals from our senses at every moment, and it has to decide which ones are urgent and necessary to analyse in order to do the task we are carrying out at that moment. For example, it is curious when we work for the first time in noisy environments, it seems impossible to develop a work day. However, usual operators who are used to that can do their jobs without even notice. The brain believes this information is useless for the activity they perform and allows them to ignore it. In terms of usability, we will have to take these factors into account if we have to alert these people by a sound, looking for those that are as much different as possible from the background noise to which they are accustomed or even better if we could use lights or other signals.
Another situation, in which our brain is irrational, is when testing a new consumer product. Like it or not, our opinion of the product will be influenced by external factors such as packaging or price, and also by subjective factors like friends opinions. Two cups filled with the same wine, but presented differently, for example, displaying the price in each, will make that people react differently to each cup, appreciating more that one with the highest price. Actually, we could think that it is a mental process that happens apart from the testing, but our brain really thinks that this cup has better taste. In some studies, with normal people and sommeliers, fMRI scan shows certain parts of our brain associated with pleasure activated when this occurs.
A priori, it seems a bug of our brain, but perhaps this should be seen as an evolution advantage. Our brain is able to manage huge amount of stimuli, although sometimes it has to reject some important information.
Another example is the purchase price, as mentioned by Dan Ariely in his book Predictably Irrational. Prices seem more or less expensive according to the prices we see in the environment. If we have two washing machines, the most expensive usually seems too expensive, and it is hard to lead to complete the purchase. Whereas if we add a third washing machine even more expensive than the other two ones, the one that was too expensive before, now seems a normal price. Our brand loves to optimize, and now is easy to sell the washing machine. This effect is commonly used in restaurants, where some dishes or bottles of wine are usually a lot higher than the medium price of the menu.