Definition of "overreactor"

The overreactor develops poor focus while the underreactor has trouble developing empathy. Either pattern will affect how others interact with that person, and thus will affect the neurochemical profile.

2005, Katherine M. Ramsland, Inside the Minds of Mass Murderers: Why They Kill

You are going to go from being a toxic-thought overreactor to a toxic-thought zapper.

2009, Jeffrey Bernstein, Liking the Child You Love