What predicts achievement? Self-discipline or IQ?

An article published in the December 2005 issue of Psychological Science reports on a study of 140 8th grade students and predictors of final grades. The authors, from the Positive Psychology Center at U Penn, discuss their findings from the study that suggest that the "degree to which students practiced self-discipline" was twice as likely as IQ to predict final grades. The article, titled Self-Discipline Outdoes IQ in Predicting Academic Performance of Adolescents, by Angela L. Duckworth and Martin E.P. Seligman, is available online thorough WSU in Psychological Science, v. 26, issue 12, page 939.
(Reported in AFT On Campus, May/June 2006.)