Dan Levin and Daniel Simons conducted a person-change study, and found that about 50% of people didn’t notice they were talking to a different person. See the clip here:
This sort of person-change rarely (if ever) happens in the world. You might assume, without doing the study, that people actually keep track of all of the details of the people they interact with. Only by making a change can you reveal the extent of their change blindness. In fact, people who missed the change would never have known anything was amiss had we not asked them. This effect reveals the “Illusion of Memory” — we think we remember far more than we actually do.
This sort of cognitive, everyday illusion is akin to a visual illusion. When you view a visual illusion, you are seeing the world as it isn’t — the illusion capitalizes on one of the shortcuts your brain takes when processing visual information, with the result that you see the world the way you assume it to be rather than the way it actually is. With cognitive illusions like change blindness, we think we see and remember far more than we actually do because we are unaware of the shortcuts our brain takes when representing the world. For the most part, we simply assume the world to be unchanging, and typically we’re right. We just don’t realize we’re making that assumption.
Here’s another clip from Derren Brown showing that the change blindness is indeed a powerful visual illusion:
Read more on the phenomenon here:
Simons, D. J., & Levin, D. T. (1998). Failure to detect changes to people during a real-world interaction Psychonomic Bulletin and Review, 5, 644-649