The reasoning behind authors using pen names (pseudonyms) varies and each author has their own personal reason for choosing one. Some authors do not like their "real" given names so choose one they prefer. Other authors chose a pseudonym for privacy; nothing worse than a crazy book fan who literally believes he is a character from an author’s book suddenly appearing at the author’s door without being invited. There are authors who prefer to use first and middle names instead because publishers prefer only two names on the book jacket front instead of three or more due to strategic marketing and eye appealing front cover issues. According to the Wikipedia website, pseudonyms are picked because this is a perfect method to disguise one's gender like J.K. Rowling (aka Joanne Murray who wrote the "Harry Potter" Fantasy genre book series), or distance an author from his work like Mark Twain (aka Samuel Clemens who is most famous for his fictional characters Huckleberry Finn and Tom Sawyer), and then there is the oldest reason of them all, to protect the author from retribution for what an author has written like Lewis Carroll (aka Charles Lutwidge Dodgson) who was a respected Vicar of the Anglican Church and wrote the book "Alice's Adventures in Wonderland". Good thing Queen Victoria of England liked Lewis Carroll’s book and did not have a problem with the description of the “Queen of Hearts”, a very blood thirsty ruler. Mr. Dodgson could have gone to prison for writing unkind monarch fictional depictions in “Alice’s Adventures in Wonderland.”

The reason behind my choosing a pen name and the pseudonym being my first and middle name was due to having been married several times. My first and middle name never change, have remained a constant; unlike my surname where like most women I legally and traditionally have accepted my husband’s last name. My mother appreciated my using the names she gave me at birth. She has been bragging about this ever since she received her copy of "Just Alex". This was a side benefit.

Another excellent legal reason for choosing one’s first and middle name is that these names are legally given and on one’s birth certificate. No one can force an author who has chosen their legal first and middle name to change their pen name; even if one or more author has the same pen name. This is a protective measure which has saved many authors from being forced by others into altering their pseudonyms.

Granted these are not all of the reasons for having a pen name, but the most common ones. If I were to list all of them the list would go on and on and on and on and on and on. Authors who do choose pen names have their reasons; usually a personal, publishing or legal one which gives validity for having a pseudonym.

The most common reason is because they are writing different books, but it can also be to hide gender.

One of my favorite authors writes under two names to distinguish between his fiction writing ( Iain Banks ) to his science fiction writing ( Iain M Banks ) - the M means its a scifi book.

A very famous nom de plume was "Franklin Dixon". This name was used to hide the identity of female authors of Hardy Boys stories - they didn't think boys would like to read stories written by women...
(source: http://www.wisegeek.com/what-is-a-nom-de-plume.htm )

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Most of the time when I see this done it's because they have switched they're genre completely and therefore have a pen name for each genre. And sometimes they do it for different series as well. I have no idea why. It seems like it would be a lot easier just sticking with one.

The answer depends on the author and the era. J.K. Rowling, for example, has a kinda-sorta pen name - she used her initials because she didn't think that kids would really want to read books written by a girl. (Oh, how wrong she was.)

Stephen King published under Richard Bachman partly because he wanted to see if people would figure it out (they did, obviously), but also because he was afraid of being viewed as being too prolific. The Bachman Books aren't departures from his usual, horror writing, so it didn't really take all that much for a King fan to read a Bachman novella and start to wonder.

In the past, novelists hid behind pseudonyms because writing was not considered an honorable profession. They didn't want to embarrass themselves, their families, and even their friends, so they used pen names.

Writers today might also use pen names when they depart from their usual genres. It's a convenient way of not ticking off the fans who expect a certain "type" of book whenever they pick up a certain author's work. I wouldn't care if I picked up a Stephen King novel and found out that he'd written a straight, pure romance - he has every right to experiment with other forms and genres, after all - but a lot of fans would feel tricked and cheated because their preconceived expectations were not met at all.

So, really, the answer depends on the author.

Some authors choose an alias because they do not want to publish under their real name. For others, I think, it's a matter of marketing. Say that Jayne Krentz only writes urban fantasy. She would present her publisher with a problem if she sent them, say, a crime novel. This is not, according to the publisher, want her readers expect from her. How to market her new book? Well, simply change her name and claim it's someone else! Voilà. You can provide as much details as you like.

However, style may betray a writer. And, does it still matter all that much? Genres mix much more easily. The term "interstitial art" wasn't coined for nothing.