Jul 16 2008

More Evidence that Autism is Genetic

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Much of the controversy surrounding autism in recent years is based upon the premise that there is an autism epidemic – that autism rates are climbing. However, the evidence strongly suggests that this is not the case and that autism diagnoses are increasing through a combination of broadened diagnosis and increased surveillance and awareness. However, proponents of various discredited theories, such as those anti-vaccinationists who claim there is a link between vaccines and autism (there isn’t) want to believe there is a true epidemic because that in turn implies an environment factor (like the vaccines they despise).

But the environmental hypothesis of autism, while impossible to completely eliminate, has not been fairing well. On the other hand, the theory that autism is largely (if not entirely) genetic has been very fruitful. Researchers are finding more and more genes that correlate strongly with autism. Also, studies have shown that the risk of autism rises with paternal age – suggesting that aging sperm may play a role. It is also true that as our ability to diagnoses autism more reliably, and based upon earlier and more subtle signs, we are finding that infants show early signs of autism – before they receive most of their vaccines or environment has much of a chance to play a role.

Essentially, multiple independent lines of evidence are converging on the conclusion that autism is dominantly genetic. Well, now there is yet another line of evidence supporting this conclusion

Researchers at Caltech and UNC studied the manner in which those with autism and those without pay attention to the faces of other people and how they discern emotion. Non-autistics generally look to the eyes, while autistics prefer to focus on the mouth.  The researchers emphasize that one way is not necessarily better than the other or “normal” – they are just different.

Their hypothesis was that if autism is genetic than perhaps the parents of children with autism may also share this tendency to favor the mouth over the eyes.  From the press release:

The parents participated in an experiment that measured how they make use of the face to judge emotions. The subjects were shown images depicting facial expressions of emotion that were digitally filtered so that only certain regions of the face were discernible–the left eye, for example, or the mouth. The subjects were then asked to decide as quickly as possible if the emotion depicted was “happy” or “fear.” The part of the face shown, and the size of the revealed area, randomly varied from trial to trial.

An analysis of the subjects’ correct responses revealed that “aloof” parents relied much more heavily on the mouth to recognize emotion than they did on the eyes, as compared to nonaloof parents and, to a greater extent, to a group of parents of children without autism. Prior studies by Adolphs and his colleagues have shown that humans normally evaluate emotions by looking at the eyes–but studies by Adolphs and Piven have shown that individuals with autism do not.

Very interesting.  Perhaps, then, parents have some genes which predispose to autism and bestow some of its features. Of course, the alternate hypothesis is that aloof parents raise aloof children through their parenting style, rather than their genes. In order to eliminate this possibility follow up studies looking at adopted children or separated twins would be helpful. But this hypothesis – that aloof parents (once referred to as “refrigerator moms”) cause autism has already been rejected by autism researchers and is therefore an unlikely explanation. But confirmation would be reassuring, as always.

Promoters of the environmental hypothesis of autism don’t really have much to say about all the evidence for genetic causes. Often they say, correctly, that a genetic cause does not rule out an environment trigger. This is true enough – but the real question is how strong of a genetic influence is there. The evidence so far suggests that it is very strong, leaving little room for environmental factors. Also, no one has established that vaccines, toxins, or other specific environmental factors are truly correlated with autism. The science simply does not support those claims, as is evidenced by the revolving door of claimed environmental trigger. First it was MMR, then thimerosal, now a host of dubious toxins.

Meanwhile a genetic picture (although very complex – autism is not a single entity caused by a single gene) is slowly and consistently emerging.

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