What is psychological regression? How does said person behave?

Regression is a very normal phenomenon which can be fleeting. It happens commonly when people get very upset. This makes it hard to utilize our more mature strategies and ways of understanding the world, and we regress to simpler forms of understanding the world, ourselves and others.

One example of regression would be in terms of the concept of mentalization. Being able to mentalize means that you can acknowledge that other people have rich inner lives which affects their behaviours. This is a mature way of understanding the world. However, especially when we get upset, angry or scared, we tend to lose sight of this complex reality. The breakdown could be evident in a lot of ways, like externalizing, "trait"-like thinking (lazy, tired, short fuse, etc), obsessiveness with rules, denying own involvement, looking for fault in others, excessive confidence in one's judgement of others (she obviously meant to...), etc etc etc. In mentalization theory, you regress to more primitive experiential modes: teleological mode, pretend mode and psychic equivalence.

/r/askpsychology Thread