CONTEXT: Autism spectrum disorders (ASDs) have been suggested to represent the extreme end of a normal distribution of autistic like traits (ALTs). However, the evidence of this notion is inconclusive. OBJECTIVE: To study whether there are similar genetic and/or environmental etiologies behind ASDs and ALTs. DESIGN: A nationwide twin study. PARTICIPANTS: Consenting parents of all Swedish twins aged 9 and 12 years, born between July 1, 1992, and December 31, 2001 (n = 19 208), were interviewed by telephone to screen for child psychiatric conditions, including ASDs. MAIN OUTCOME MEASURES: Two validated cutoffs for ASDs, 2 cutoffs encompassing the normal variation, and 1 continuous measure of ALTs were used with DeFries-Fulker extreme-end analyses and standard twin study methods. RESULTS: We discerned a strong correlation between the 4 cutoffs and the full variation of ALTs. The correlation was primarily affected by genes. We also found that the heritability for the 4 cutoffs was similar. CONCLUSION: We demonstrate an etiological similarity between ASDs and ALTs in the normal variation and, with results from previous studies, our data suggest that ASDs and ALTs are etiologically linked.
CONTEXT: Autism spectrum disorders (ASDs) have been suggested to represent the extreme end of a normal distribution of autistic like traits (ALTs). However, the evidence of this notion is inconclusive. OBJECTIVE: To study whether there are similar genetic and/or environmental etiologies behind ASDs and ALTs. DESIGN: A nationwide twin study. PARTICIPANTS: Consenting parents of all Swedish twins aged 9 and 12 years, born between July 1, 1992, and December 31, 2001 (n = 19 208), were interviewed by telephone to screen for childpsychiatric conditions, including ASDs. MAIN OUTCOME MEASURES: Two validated cutoffs for ASDs, 2 cutoffs encompassing the normal variation, and 1 continuous measure of ALTs were used with DeFries-Fulker extreme-end analyses and standard twin study methods. RESULTS: We discerned a strong correlation between the 4 cutoffs and the full variation of ALTs. The correlation was primarily affected by genes. We also found that the heritability for the 4 cutoffs was similar. CONCLUSION: We demonstrate an etiological similarity between ASDs and ALTs in the normal variation and, with results from previous studies, our data suggest that ASDs and ALTs are etiologically linked.
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