Oklahoma-based Hobby Lobby CEO David Green and his wife, Barbara, became one of seventeen families committed to donate more than half of their wealth to charity in the second group taking the Giving Pledge. The first group consisted of 40 families, including project spearheads Warren Buffett and Bill and Melinda Gates, taking the pledge in August.

"The Giving Pledge is about changing the culture of giving," said Peter Singer, professor of bioethics in the University Center for Human Values at Princeton University and author of "The Life You Can Save." "Research shows that when people know that others are giving, they are themselves more likely to give. So publicly pledging to give will encourage others to give."

George Kaiser, chairman of BOK Financial Corporation, is the only other Oklahoman so far to take the pledge.

"Like my parents and their parents before them, you don't have to be wealthy to give," David Green wrote in his pledge letter. "You can give your time, your talents and your passion. We congratulate those who have taken on this wonderful pledge."

A full list of those taking the pledge and personal letters by many of these are available online at www.givingpledge.org.