*This research was supported in part by research grant no. 410-83-0413 from the Social Sciences and Humanities Research Council of Canada, and by the Fonds de la Recherche en Santé du Québec. The comments by anonymous reviewers were helpful in revising a preceding version of this manuscript. The text was edited by Louise Valois. The manuscript was received in April, 1985 and accepted in July, 1986.