But research over the last 70 years has shown that this is not the case. This knowledge gap has tried to fill all sorts of strange and crazy theories about the cause of autism: Television, overhead lines, vaccinations and the position during sexual intercourse during conception. None of these are scientifically tenable, but they have repeatedly fed the mystery about the cause of autism.
The myth of the refrigerator mothers
During the 1950s and 1960s, there was a widespread belief that autism was triggered by parents being emotionally cold towards their child. The mothers of autistic children were often referred to as refrigerator mothers.
Leo Kanner, who was one of the first to describe autistic behavior, saw a lack of maternal warmth as the cause of autism. This misconception caused the parents of autistic children shame and guilt for at least two decades. Families were torn apart, autistic children locked up in psychiatric hospitals.
Several respected scientists were finally able to eradicate this myth. Some of them pointed to a major weakness of this theory: parents who corresponded to the refrigerator stereotype also had non-autistic children.
Others examined twins in order to estimate how big the influence of genes is in relation to the environment.
In Search of the Autism Genes
To understand these studies, one must know that there are two types of twins. Identical twins share all their DNA and, if they grow up in the same household, their entire environment. Fraternal twins also share their entire environment, but only half their DNA, as do siblings who are not twins.
Twin studies begin by determining which population to study, for example, the inhabitants of a particular city. Then the researchers search for as many twins as possible in this city that have the same trait, in this case autism.
The researchers then investigate the concordance rate, i.e. the percentage probability that if one twin is autistic, the other is autistic. If the concordance rate for identical twins is higher than for fraternal twins, this indicates genetic causes.
A large meta-study examined the heredity of autism using twin data. This study included data from 14,921 pairs of twins, of which one or both were autistic (including Asperger’s syndrome) or had autistic traits.
The result of the study: heredity has a much greater impact than the environment. If one identical twin is autistic, the other is it with 98 percent probability too. For fraternal twins this probability is much lower: in the study it is 53-67%, depending on where you draw the line between autism and non-autism.
Other studies came to similar results.
But that doesn’t mean that autistic children are only born in families where there are other autistic family members. The inheritance of autism is more complex.
The autism gene doesn’t exist.
After finding that genes are a cause of autism, the researchers next tried to identify the exact genes that might play a role. But even after decades of intensive research, they could not find a single gene variant that all people in the autism spectrum had in common, and non-autistic people could not.
Perhaps several genes together could cause autism? But the researchers could not find a combination of gene variants that all autistic people examined had.
This finding (or lack of finding) made scientists think in a different direction: they stopped looking at autism as a single finding with a single cause and began to see it as many different findings with similar symptoms.
There are probably a variety of genetic factors that cause autism. These, alone or in combination with environmental factors, can cause the brain to develop differently and the person to behave in what we call autistic ways.
How many genes are we talking about here?
A few years ago we were talking about 100 genes associated with autism – not necessarily as a single gene, but in different combinations. And more and more are being found. Neurogenetics researcher Daniel Geschwind said at the time that he would not be surprised if the number reached 500 in the foreseeable future. We are now talking about up to 1000 genes.
In order to put these numbers into context: The human genome has about 25,000 genes. A gene consists of many so-called base pairs, similar to a word with many letters. And every 25th gene has a variant that is associated with autism.
This is a considerable part of the human genome.
For me, these findings suggest that autism has been an essential aspect of human neurodiversity for a very long time.
Possible environmental factors
There are many crude theories about environmental factors that are not supported by anything.
Nevertheless, if we look at the figures from the twin study above, there are twins where one is autistic and the other is not.
In 98% of cases, both identical twins were autistic.
In 2% of cases, one identical twin was autistic and the other was not.
I have said above that my presentation of twin studies was simplified. One complication is this: There are genetic differences between identical twins.