In the 1940s, psychiatrist Leo Kanner blamed the environment of poor parenting and “refrigerator mothers,” so-called because of their lack of parental warmth, for their children’s development delays. His theory was overturned as studies in recent decades shed light on heritable factors causing autism. And a substantial percentage of autism research looks at genetics these days, but the hunt for environmental factors is not over, as evidenced by a new paper out yesterday.

The authors of the study looked at 192 pairs of twins — the largest study of its kind for autism — and, based on comparisons of identical and fraternal twins, estimated that environmental factors explained 55% of the variance in autism while genetics were at work in 37%. The remaining 8% was due to random environmental factors, a blanket term to describe any slight differences not shared by a twin pair, such as an infection as an infant.

Philip Landrigan, a pediatrician at the Mount Sinai Medical Center in New York who was not involved in the research, speculates that these environmental factors might be chemicals or other agents that the fetus is exposed to early in development.

Most media coverage of the paper has highlighted the dichotomy of environment versus genetics. The Los Angeles Times article, “Autism study downplays role of genetics,” lays it on thick, and points out that the study’s conclusions are out of step with the numerous studies of late that have focused on genes. Landrigan, who is also an epidemiologist, counters that although “elegant research” has identified mutations associated with autism, no single mutation accounts for more than 2-3% of cases. “In the aggregate, these mutations seem to account for 35 or 40%” of autism cases, says Landrigan, which is “pretty much exactly what the people from this twin study found.”

The researchers behind the new paper emphasize that the cause of autism is not a battle between environment and genetics, but rather a combination of the two. “I hope my study can contribute to the fact that we can get away from this nature-nurture discussion,” says Joachim Hallmayer, an author of the study and a psychiatrist from Stanford University School of Medicine. “I think there is no doubt that they really have an interaction together.”

This interplay between nature and nurture is probably the crux of autism, says Hallmayer: “A gene without an environment is nothing, and so is the environment without a gene.”