“…Although the spacing effect has been demonstrated in a vast number of studies (e.g., Challis, 1993;Greene, 1989Greene, , 1990Greene & Stillwell, 1995;Hintzman & Block, 1973;Hintzman, Summers, & Block, 1975;Russo, Parkin, Taylor, & Wilks, 1998) it has largely defied a unitary explanation (for reviews on proposed theoretical mechanisms see Crowder, 1976;Dempster, 1996;Hintzman, 1974Hintzman, , 1976Kintsch, 1970). For instance, different mechanisms are proposed to account for spacing effects in cued memory tasks and in free recall.…”