A new study by Joachim Hallmayer and colleagues at Stanford, Autism Speaks, Kaiser Permanente and the MIND Inst at UC Davis (Archiv Gen Psychiat, on line July 4, 2011) has riled up autism geneticists. The primary new finding is a 31% concordance between dizygotic (non-identical) twins (31% chance for ASD in second twin if one is ASD). Amazingly, this is identical to the 31% found by Rosenberg et al. in 2009 in a huge study of 277 twins. Hallmayer’s study had the advantage of use of gold-standard diagnostic tools, close attention to possible ascertainment bias, and a large sample size. The crucial point about the 31% figure is that it is much higher than the concordance between siblings (3-14%). Since dizygotic twins and siblings have the same degree of genetic relatedness, the higher rate for the former indicates an important influence of the fetal environment. This is consistent with the maternal infection risk factor for ASD, which also points to the maternal-fetal environment as being critical for ASD risk. What got geneticists really upset, however, (see for instance, http://sfari.org/news-and-opinion/news/2011/experts-critique-statistics-conclusion-of-autism-twin-study) was the calculation by Hallmayer that showed that the environmental factors accounted for 58% of ASD while genes accounted for only 38% of ASD. since genetics has dominated the field of autism research for many years, it is clear why they went on the attack. Regardless of the argument over the 58% vs 38% numbers, what is undeniable is that we now have two large twin studies showing that dizygotics have a much higher concordance than siblings. This supports the importance of the fetal environment in developing autism.