For those of you who just happen to come across my blog, welcome. I am interested in autism because it has directly affected my family. My older brother is autistic, nonverbal and low functioning. I am trying to look at autism from all directions. During the past couple of days I am looking at Autism and Genetics.
Usually I mull things over quite a bit in private before talking or writing about them. Maybe it would be a good idea to use my blog to “think out loud”. This post is an attempt to puzzle out several confusing issues about autism research: 1) twin studies and their significance; 2) heritability of autism; 3) environment and autism.
That is a lot to tackle. Don’t know how far I’ll get today.
Researchers have been studying twins for years. Since identical twins (monozygotic – MZ, from the same egg) have identical genetic traits, any differences between them cannot be genetic; they must be attributed to differing experiences. Fraternal twins (dizygotic – DZ, from different eggs) share ~50% of genetic traits. Since they were born at the same time, raised by the same parents and exposed to the same environment, differences are attributed to genetics (taking into account a random likelihood that occasionally one twin would experience something different than the other).
Of course, one cannot study only one family of twins. They might not be typical. Scientists, therefore study a large number of MZ and DZ twins to get an idea of the effects of genetic makeup and environment. The larger the number of the families studied, the more statistically significant the result. Of course, underlying assumptions must also be valid.
The confusing part
“There are three kinds of lies: lies, damned lies, and statistics.” Mark Twain
Wikipedia documents some of the common objections to twin studies:
1) “[H]eritability estimates used for most twin studies rest on restrictive assumptions which are usually not tested, and if they are, can often found to be violated by the data.”
2) “Twins are not a random sample of the population, and they differ in their developmental environment. In this sense they are not representative.”
Also documented in the same Wikipedia article are answers to these objections:
1) “[T]he newer [statistical] methods allow for explicit testing of the role of different pathways and incorporation and testing of complex effects.”
2) “Twins differ very little from non-twin siblings. Measured studies on the personality and intelligence of twins suggest that they have scores on these traits very similar to those of non-twins.”
It is not good to be confused at this point. I haven’t even touched on autism twin studies.