Characteristics of the studies included in the review by type of intervention and study design.

ABA = applied behaviour analysis; AIT = auditory integration training; ASD = autism spectrum disorders; CBT = cognitive behaviour therapy; CCT = controlled clinical trial; CFI = communication-focused intervention; DCI = developmental contingency interaction; DIT = developmental incidental teaching; DIR = developmental individual-difference relationship-based intervention; DT = discrete trial training; EM = environmental modification; ERP = event-related potential; IBT = intensive behaviour analytic treatment; IQ = Intellectual quotient; IT = incidental teaching; LEAP = learning experiences, an alternative program for preschoolers and their parents; LT = language therapy; LUFAP = Lancashire under fives autism programme; M = number of males; mo = month(s); MR = mental retardation; ND = not described; NR = not reported; NT = no treatment; OCD = obsessive compulsive disorder; PCS = prospective cohort study; PDD = pervasive developmental disorder; PECS = picture exchange communication system; PRT = pivotal response training; RCS = retrospective cohort study; RCT = randomized controlled trial; REST = restricted environmental stimulation therapy; RI = regular instruction; RPMT = responsive education and prelinguistic milieu teaching; SC = standard care; SE = special education ; SLT = speech language therapy; TEACCH = treatment and education of autistic and related communication-handicapped children; UCLA = University of California, Los Angeles wk = week(s); WL = wait-list; yr = year(s).