Include underscore in word

This seems like a trivial question but a lot of searching has not produced an answer. I'm trying to make the change from viper to evil. One thing that surprises me is the underscores are not considered part of a work for various word commands (e.g, w, cw, etc). I tried vim and they are considered part of the word and in viper they are as well. I find the choice in evil strange. Is there a way to make underscores part of words in evil? Thanks, Bob