“…When autism as an outcome in siblings is considered, twin and sibling studies offer strong support for the genetic hypothesis. The rate of autism in MZ twins is much higher than in DZ twins (36% to 95% versus 0% to 4.8%, respectively) and the recurrence rate of autism in siblings of individuals with autism is 50-to 100-fold the rate of autism in the general population (August et al, 1981;Bailey, Philips, & Rutter, 1996;Bailey et al, 1995;Bolton & Rutter, 1990;Bolton et al, 1994, Folstein & Rutter, 1977Fombonne & Dumazaubrun, 1992;Gillberg et al, 1992;Minton, Campbell, Green, Jennings, & Samit, 1982, Piven et al, 1990Smalley, 1991Smalley, , 1997Smalley, Asarnow, & Spence, 1988;Steffenburg et al, 1989). Although environmental effects cannot be completely ruled out, the findings that families with one child with autism are at risk for having another child with autism are accepted as evidence that genetic factors play an important role in the etiology of autism (Bailey et al, 1996).…”