A person who avoids at all costs any possible contact with society's lunatic fringe. A berther intentionally establishes a wide 'berth' between him- or herself and groups like the birthers and other crazies.
The reasons for becoming a berther may include maintaining one's sanity, protecting one's self and others, including the lunatic fringe, and preserving one's faith in mankind.
Person 1: *eyes glazed over* "uuuggghhhh....uuughhhh.... I've just been reading the reader's comments on foxnews.com...." *collapses to the floor, twitching*
Person 2: "Man, you really should become a berther...that shit'll kill ya."
Nearby Scientologist: "Dude, you said it. What a bunch of crazies." *Sips the kool-aid*
Person 2: *coldcocks him*