Houjou's Iris and Labeling Theory
Houjou's Iris and the effect it's had on his personality has foundations in a sociological concept called "labeling theory." At its simplest, if you label a person (or group of people) as something, then that person or group is more likely to act according to the label. The reasons are manifold.
Because humans are social creatures, the expectations others have for us matter in how we behave. When a person gets labeled as something negative or lesser, their own self-expectations are lowered or altered in reaction. At the same time, others react to the label they have placed and limit a person's avenues of expression by their expectations. Label someone as "violent" and any attempts to behave otherwise are seen merely as deception, waiting for their "true nature" to come out. This is a classic problem for anyone convicted of a serious crime, many of whom cannot find solid jobs after serving time - good luck getting hired when everyone already thinks you'll assault them or rob them blind, no matter the circumstances of the crime.
The decision has already been made: others will think of them as something bad without regard to what they do, so they don't see the point in making an effort to rise above that label (or they start thinking that the label may be accurate, internalizing it). Applied more broadly to groups, individual members subject to that group's label have been pre-judged unfairly for something completely beyond their control to change, and accept the label as a guideline for their own conduct. When it comes to labels, one is guilty until proven innocent (which may never happen).