... In this paper, we use the term 'hard drugs' to refer to the use of cocaine powder, crack or heroin; these drugs that can cause dependence, have been used widely since the 1960s, and have been the focus of much public policy concern. This regularity of the gateway sequence has been confirmed in numerous studies most of which have used samples drawn either from household or school populations (Andrews, Hops, Ary, Lichtenstein, & Tildesley, 1991;Blaze-Temple & Lo, 1992;Brook, Whiteman, & Gordon, 1982;Brook, Whiteman, Balka, & Hamburg, 1992;Cohen & Sas, 1997;Donovan & Jessor, 1983;Elliott, Huizinga, & Menard, 1989;Fergusson & Horwood, 2000;Fleming, Leventhal, Glynn & Ershler, 1989;Golub, Johnson, & Labouvie, 2000a;Golub, Labouvie, & Johnson, 2000b;Hays & Ellickson, 1991;Kandel, Yamaguchi, & Chen, 1992;Mills & Noyes, 1984;Welte & Barnes, 1985). ...