Does Beyonce write her own songs?

For a long time now there have been rumours that pop star Beyonce does not write her own tracks.  Despite the fact that B's name appears in the credits of many of her hit songs including 'Crazy in Love', 'Irreplaceable' and 'Single Ladies' the rumours have persisted.  Along the way there have been various forms of supposed 'proof', including an early demo of the aforementioned 'Irreplaceable' by Ne-yo that is virtually word-for-word identical to the one later recorded by Beyonce, yet somehow she has gained a co-writer credit.  More recently producer Bangladesh stated, when asked about the topic "It doesn’t matter if Beyonce is there physically writing the song. I mean even if she’s not, it doesn’t mean that she can’t. It might be a case of her not having time or whatever".  It's hardly a denial.
It's certainly an interesting topic.  There's a lot to be gained (financially as well as in terms of credibility) from having a song writing credit on your own track, and the lines between genuine songwriter and slightly changing the words of a song are very much blurred.  Famously, Leona Lewis admitted the only reason Simon Cowell received a song writing credit on her single 'Footprints in the Sand' is because he came up with the idea for the track, despite the fact he didn't write a word of it.  By the same token then, should Britney Spears have been listed as a co-writer on her track 'Piece of Me'?  She didn't write a single word of the lyrics, but she is clearly the inspiration behind them.
Whether Beyonce has been totally honest about her song writing or not, what is undeniable is her talent - not only vocally but in her stage presence - she is clearly a force to be reckoned with and continues to be one of the most commended and rewarded artists of her generation.  Although it's a sticky subject, we will probably never know for sure either way..... perhaps it's just easier to believe what we're told?