Back story is a fine thing to use in novel-writing. However, it is important to use it in the correct way and not slow the story down. This article provides insight on using back story correctly.
I know some there are published authors and professors out there who when asked how much backstory is OK to use, will answer with an emphatic: NONE. I get where they’re coming from. It’s important not to break the flow of the story by taking us off to a different time and place, not to mention a different character, really. Who the character is now is not who the character was then. And if you haven’t yet “seated” your reader firmly into the present story, you risk losing them by taking them back. If you have hooked them firmly enough into the story, then why take them back? Learn more.