Here is another well researched article made available from Oikos and originally from Energy Source Builder.
Since a picture is worth 1K or more words, use these images to decide if you want to read further:
(http://oikos.com/library/moisture/moisture1.jpg)
This is a great chart - one that will make you want to check your own indoor humidity.
(http://oikos.com/library/moisture/moisture3.jpg)
This is why indoor humidity goes down when you bring in winter air.
(http://oikos.com/library/moisture/moisture2.jpg)
The article: http://oikos.com/library/moisture/index.html