Refrigerator mother hypothesis of autism thaws
The refrigerator mother theory, blaming autism on a cold and uncaring home environment, was beginning to lose traction in the late 1960s. Bernard Rimland, a psychologist with an autistic son, directly challenged this hypothesis in his book, Infantile Autism: The Syndrome and its Implications for a Neural Theory of Behavior. 
Studies of twins
The first study of twins to address the causes of autism was undertaken by Folstein and Rutter in 1977 and added further evidence against environmental causes of autism. 
In their review of the impact of this paper, Angelica Ronald and Robert Plomin summarize Folstein and Rutter’s findings:
“For 10 pairs of fraternal (dizygotic, or DZ) twins in which at least one twin was diagnosed with autism using strict criteria, not a single co-twin was diagnosed with autism. That is, concordance for fraternal twin pairs was zero. … The stunning result from the Folstein and Rutter study came from 11 pairs of identical (monozygotic, or MZ) twins, who are genetically as identical as clones. Of the 11 pairs, 4 (36%) were concordant for strictly diagnosed autism.”
One would think that with such a small sample size, that no conclusions could be drawn. However, the differences between identical (MZ) and fraternal (DZ) twins was found to be statistically significant.
Conclusions confirmed 1995
Ronald and Plomin cite a follow up study nearly 20 years after the 1977 paper, with a larger sample size, which confirmed a strong genetic component contributing to the cause of autism. .
Fast forward to 2011
Joachim Hallmayer, MD and associates revisited the question of which risk factor, genetics or environment, was dominant through a study of twins in 2011. “The results suggest that environmental factors common to twins explain about 55% of the liability to autism. Although genetic factors also play an important role, they are of substantially lower magnitude than estimates from prior twin studies of autism.”
This is a different conclusion from previous autism studies of twins which suggested that the genetic component contributing to autism was stronger than environmental factors.
So, where are we?
The most current autism study on twins, of which I am aware [Hallmayer, et al.], concludes that genetic considerations are less of a factor in autism risk than other studies have shown. Furthermore, it states that environmental factors explain more than half of the autism risk.
Is Hallmayer’s study the definitive one? Surely more studies will follow. Keeping in mind that the population from which twins have been selected has changed over the past 30-40 years, it is understandable that data has changed. This might explain the shift in results and conclusions.
I’m still confused. Although twins studies divide autism risk between genetics and environment, it seems that the dominant factor has not been conclusively determined.
 Folstein S. and M. Rutter J. Child Psychol. Psychiat. 18, 297-321 (1977)
 Bailey A. et al. Psychol. Med. 25, 63-77 (1995)
 Hallmayer, J., et. al Genetic Heritability and Shared Environmental Factors Among Twin Pairs With Autism Arch Gen Psychiatry. 2011;68(11):1095-1102. doi:10.1001/archgenpsychiatry.2011.76