Brenner and Spaeth (1988) examined the docket books of justices of the United States Supreme Court and found that in 86 percent of the Warren Court's minimum winning original decision coalitions that broke up the marginal justice (i.e., the justice ideologically closest to the dissenters) defected. We attempt to explain why the marginal justice defects. Use of a difference of means test and the maxmum likelikhood estimation technique probit indicate ideological closeness and the lack of ability of the marginal justice to be the most important factors contributing to the defection of the marginal justice. Analysis of the behavior of the five justices who most often were marginal shows each of them also responded ideosyncratically to one or more identified case characteristic variables.