First, let me remind you of this old trick.
You know that the circles are the same size, but you can see that visually because of the size of the surrounding circles, the image on the right makes the center circle look larger, even when you know that both center circles are the same.
System 1, which is on autopilot, automatically makes you perceive the circle on the right as larger. That is our intuitive system, which happens in milliseconds. We process information extremely quickly, which saves our brain the effort of having to consciously process every single decision.
System 2 is manual pilot and is highly rational. In the case of the circles, System 2 overrides System 1 and tells you that it's just an illusion, that both center circles are the same size. We know from research in behavioral science that most of our daily decisions are actually made by System 1—our more intuitive system—and this makes sense because we would be paralyzed if we had to make every single decision in our lives rationally. We would never get out of the house in the morning. Instead, you don't need to think about how to leave your house, get in the car, insert the keys, drive down the street, etc... We don't think it as much as we just do it. Every moment is a decision and your brain is making hundreds of decisions on your drive to work. but you're on autopilot because you know how to do it and your brain fills in the gaps instead of making you consciously process it every time.
When we ask someone a question we get an answer from System 2, even if the decision was made by System 1 (the auto pilot). In market research, we often rely on the ability of System 2, such as in a focus group or in a survey where we ask someone a direct question and get a response. The problem is that the answer System 2 is providing is the rationalization of what System 1 did, and not necessarily the motivation behind the decision.
To understand the subconscious drivers of human decision making, click here.