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GUY RAZ, host: As many as one in 80 American children are thought to be autistic, and most scientists trace the main risk factor to genetics. But now, researchers at Stanford believe that environmental factors could play as big a role as genetics. The Stanford group just completed the largest study of autism in twins ever conducted. And to their surprise, they found an unusually high number of fraternal twins with autism.

Now, unlike identical twins, fraternal twins only share half their genes. So the researchers believe there must be something else going on. Dr. Joachim Hallmayer is a psychiatrist at Stanford School of Medicine, and he led the study.

Dr. JOACHIM HALLMAYER: Roughly, about half of what we see is due to environmental factors, and half of what we see is due to a genetic factor.

RAZ: My understanding says scientists have long suspected that there's an environmental contribution to autism risk, and that may occur in the early stages of pregnancy. Is that what you suspect as well?

HALLMAYER: Yes. I must emphasize here, we did not look for specific environmental factors. And that's not the right study to do this, also. There are a number of studies which have been carried out, so we know, for example, that adversities during pregnancies increase the risk for a child to be later diagnosed with autism.

We also know that extreme prematurity also increases the risk to develop autism. So we have some hints, but none of these has such a strong effect which explains the data we found very easily. So we need, really, to look much more carefully at some of these factors much more specifically.

RAZ: The assumption must be that there is something out there. I mean, as most psychiatrists will say, the rate of autism has increased. I mean, that is a fact, right? I mean, there are more children diagnosed with autism today than ever before.

HALLMAYER: Yes. There's no doubt that there are many, many more children diagnosed with autism than ever before. There's a lot of discussion how much this is due, basically, to a better detection. There is also a lot of discussion how much we have broadened our view about the symptoms of autism. Whether this explains all of the increase, I have doubts. But it's - again, it's a very, very complicated issue, and a lot of research is going on, also, in this area.

RAZ: I know that this is a little bit beyond your research here specifically, but there had been some theories that age may have to do with it, the age of a mother.

HALLMAYER: Yes. There are two factors here. One is the age of the mother and the age of the father. It looks like the older the mother, the higher is the chance that the child receives a diagnosis of autism. And the same applies, also, to the father. There's a - fairly large studies - has been conducted here in California, where they show that both of these factors play a role.

RAZ: And boys more than girls.

HALLMAYER: Yes. I mean, from all the risk factors we know clearly being made, it's the most significant risk factor we know of.

RAZ: It's such a mystery. And it's amazing because you think back to the polio vaccine, which was such a miracle in the 1950s. And before that time, nobody really knew what was happening, and why it was developing. I suppose autism is also confounding, in quite a similar way.

HALLMAYER: Yes. Now, autism is, I think, also very complicated because everything we have to say has to be seen on the backgrounds since there are probably multiple different forms of autism. And I think we have to really, really be very careful to look at both environmental and genetic factors, look at the interaction - and then really drill down, so to speak, piece after piece after piece, to really then get into the individual subgroups of autism and find out what is causing these groups. And I'm very convinced that there will be no one size fits all.

RAZ: That's Dr. Joachim Hallmayer. He's an associate professor at the Stanford School of Medicine. His study will be published in the November issue of the Archives of General Psychiatry. You can find it there online now. Dr. Hallmayer, thank you so much.

HALLMAYER: Thank you.

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