From Dan Amira at New York Magazine here:
George Stephanopoulos brought up a controversial passage from Santorum's 2005 book, It Takes a Family, in which Santorum contends, "The radical feminists succeeded in undermining the traditional family and convincing women that professional accomplishments are the key to happiness." Santorum insisted to Stephanopoulos that he isn't saying women shouldn't work, only that there's nothing wrong with being a stay-at-home mom, if that's what they choose. The explanation wasn't new; Santorum has been asked about that quote many times. What was new was that this time, Santorum added that, oh, by the way, "that section of the book was co-written, if you want to be honest about it, by my wife." ...
You also have to wonder why Santorum is only now bringing up his wife's co-authorship of that controversial passage. When the book initially came out, in 2005, Santorum was constantly on TV defending this very same "radical feminists" quote. But, curiously, he never mentioned that his wife helped to write it, according to a search of Nexis transcripts — not in a July 25 interview on Hannity and Colmes, or in a Today show interview that same morning, or a July 27 interview on Hardball, or a July 28 interview on CNN's American Morning, or a July 31 interview once with ABC's This Week (deja vu!), or an August 5 interview with Tucker Carlson on MSNBC.
More recently, Santorum was asked about this very same passage during a May 2011 Fox News presidential debate in South Carolina, and again, he neglected to credit his wife.
A failure to provide proper attribution for someone else's words in order to make them appear to be your own is called plagiarism.