Can people disguise the way they write?

Can people deliberately fake their writing style?

By way of background…

Perhaps 25% of the cases I handle involve the authorship of anonymous, disputed, or forged documents.  The client wants to know who’s writing those nasty, threatening emails or letters.  I typically ask the client for writing samples from the suspected author.  Sometimes there’s more than one suspect, and I have to decide which of them may be the author of the anonymous document(s).