Although it seems obvious that there is a single "you" inside your head, research from several subdisciplines of psychology suggests that this is an illusion. The "you" who makes a seemingly rational and "self-interested" decision to discontinue a relationship with a friend who fails to return your phone calls, borrows thousands of dollars he doesn't pay back, and lets you pick up the tab in the restaurant is not the same "you" who makes very different calculations about a son, about a lover, or about a business partner.
Three decades ago cognitive scientist Colin Martindale advanced the idea that each of us has several subselves, and he connected his idea to emerging ideas in cognitive science. Central to Martindale's thesis were a few fairly simple ideas, such as selective attention, lateral inhibition, state-dependent memory, and cognitive dissociation. Although all the neurons in our brains are firing all the time, we'd never be able to put one foot in front of the other if we were unable to consciously ignore almost all of that hyperabundant parallel processing going on in the background. When you walk down the street there are thousands of stimuli to stimulate your already overtaxed brain — hundreds of different people of different ages with different accents, different hair colors, different clothes, different ways of walking and gesturing, not to mention all the flashing advertisements, curbs to avoid tripping over, and automobiles running yellow lights as you try to cross at the intersection. Hence, attention is highly selective. The nervous system accomplishes some of that selectiveness by relying on the powerful principle of lateral inhibition — in which one group of neurons suppresses the activity of other neurons that might interfere with an important message getting up to the next level of processing. In the eye, lateral inhibition helps us notice potentially dangerous holes in the ground, as the retinal cells stimulated by light areas send messages suppressing the activity of neighboring neurons, producing a perceived bump in brightness and valley of darkness near any edge. Several of these local "edge detector" style mechanisms combine at a higher level to produce "shape detectors" — allowing us to discriminate a "b" from a "d" and a "p." Higher up in the nervous system, several shape detectors combine to allow us to discriminate words, and at a higher level, to discriminate sentences, and at a still higher level, place those sentences in context (thereby discriminating whether the statement "Hi, how are you today?" is a romantic pass or a prelude to a sales pitch).
State dependent memory helps sort out all that incoming information for later use, by categorizing new info according to context — if you learn a stranger's name after drinking a doppio espresso at the local java house, it will be easier to remember that name if you meet again at Starbucks than if the next encounter is at a local pub after a martini. For several months after I returned from Italy, I would start speaking Italian and making expansive hand gestures every time I drank a glass of wine.
At the highest level, Martindale argued that all of those processes of inhibition and dissociation lead us to suffer from an everyday version of dissociative disorder. In other words, we all have a number of executive subselves, and the only way we manage to accomplish anything in life is to allow only one subself to take the conscious driver's seat at any given time.
Martindale developed his notion of executive subselves before modern evolutionary approaches to psychology had become prominent, but the idea becomes especially powerful if you combine Martindale's cognitive model with the idea of functional modularity. Building on findings that animals and humans use multiple — and very different — mental processes to learn different things, evolutionarily informed psychologists have suggested that there is not a single information-processing organ inside our heads, but instead multiple systems dedicated to solving different adaptive problems. Thus, instead of having a random and idiosyncratic assortment of subselves inside my head, different from the assortment inside your head, each of us has a set of functional subselves — one dedicated to getting along with our friends, one dedicated to self-protection (protecting us from the bad guys), one dedicated to winning status, another to finding mates, a distinct one for keeping mates (which is a very different set of problems, as some of us have learned), and yet another to caring for our offspring.
Thinking of the mind as composed of several functionally independent adaptive subselves helps us understand many seeming inconsistencies and irrationalities in human behavior, such as why a decision that seems "rational" when it involves one's son seems eminently irrational when it involves a friend or a lover, for example.