Autism spectrum disorders and autistic like traits: similar etiology in the extreme end and the normal variation.

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.