In complex systems, many causes can result in the same effect. This is true in social systems, neural systems, and biological systems. And we can see this in the fact that there are at least 400 distinct autisms, at least from the perspective of causes.
The above linked article also notes that one of the causes of autism is also a known cause of ADHD. It has been suggested by a friend with ADD that our daughter, Melina, might have it as well. If there is in fact a connection between ADD and certain kinds of autism, that would make sense, given my (obviously) heritable autism. There is a known protective effect from being female when it comes to autism, and it may be that ADD is what peeked out with Melina.
This points, too, to the fact that when it comes to multiple causes, we have to understand that those causes are all interacting with other causes, affecting effects. This is true in biological systems, neural systems, and social systems, equally. In other words, there could be and likely are a number of environmental factors that affect genetic expression, including degree of expression and thus severity of one’s autism.
We are fortunate that, as the above article as well as this one both point out, we are discovering just how heterogeneous autism is. What is equally interesting, though, is the degree to which that heterogeneity gives rise to similar enough outcomes that we would call them all “autism.” It will be interesting to see how the genetic differences cluster into varying behavioral groups and neural structures.