Historical Theories of Cause
In the 1940s when autism was formally identified, the dominant philosophy governing psychiatry was psychoanalytic in nature. This was a time before modern psychoactive medications were available, and when talk therapy was the state of the art for treating all mental disturbances, including those which later turned out to be primarily biological in origin. Not surprisingly, early theories concerning the cause of autism drew heavily on psychoanalytic preoccupations.
The analyst Bruno Bettelheim theorized that autism occurred as a result of poor parenting, specifically as a result of neglectful parents who failed to stimulate their children enough. Children with autism failed to develop normal empathy skills, he proposed, because they were not given sufficient affection. He coined the term "refrigerator mother" to describe the supposedly unaffectionate mothers who 'caused' autism in this manner. Though discredited today, Bettleheim's theory was quite influential in its day. It appears misogynistic (woman-hating) in the light of our contemporary understanding of autism, but it is of a piece with other early psychoanalytic theories, and should be regarded in that light.
Contemporary Theories of Cause
It is definitively known today that no amount of abuse can result in autism (although it can result in a severely disturbed child). However, Bettelheim's focus on environmental causes of autism was almost certainly not wholly wrong. Though a definitive explanation for how autism occurs is not yet understood, the best evidence available today appears to support a diathesis-stress model of causation. Diathesis is a fancy work for vulnerability. Diathesis-stress theories of illness basically suggest that it takes two events to cause an illness to occur. First, a diathesis or vulnerability towards getting that illness must be present. Second, some stressful environmental event (or events) must occur that release the vulnerability and get the disease process going. Prior to the onset of the stressful event (or events), the diathesis is not expressed and remains in what is frequently called a "latent" state. As an example, there may be multiple causes of autism. Some individuals may be genetically predisposed to autism while others are not. Some of the predisposed people will never develop symptoms because they do not come into contact with anything that triggers and activates those genetic vulnerabilities. Other vulnerable people may develop symptoms of autism because elements in their environment (such as a vaccine) might interact with and activate their vulnerability. Thus, vaccines in themselves would not cause autism, but vaccines in combination with an individual's genetic predisposition to develop autism might cause autism.
The exact nature of the diathesis involved in causing autism are not clear at this time. It is not known with certainty whether there are multiple pathways that can result in autism or just one, for instance. What is known at this time, is that one of the pathways for vulnerability to autism likely does involve a genetically inherited biological predisposition.
It is known that children who have siblings diagnosed with an autism-spectrum disorder have a greater risk of themselves becoming diagnosed with an autism spectrum disorder than do other similar children who do not have a family history of autism. The reason for this increased vulnerability risk appears to be genetic in nature, involving a heightened risk for chromosomal abnormalities. Specifically, abnormalities noted on chromosomes 2, 3, 5, 7 and 15 are associated with a heightened risk for autism. Other research suggests that mutations on the X sex chromosome (supplied by the mother) are associated with heightened autism risk. If this latter research finding turns out to be true, it would help explain why more males develop autism than females. Genetically, males carry a single X chromosome and a single Y chromosome, while females have two X chromosomes. Genetic errors on one X chromosome can be canceled out in females by the other X chromosome they have present. This is not the case for males, however, whose Y chromosome cannot cancel out X errors. Correspondingly, if errors on the X chromosome were responsible for autism vulnerability, males would be more likely than females to inherit and express those errors as they develop.
The form or mechanism through which genetic contributions might create a vulnerability to autism is not clear at this time. Genetics may set children up to be particularly sensitive to toxins, to have fragile metabolic systems causing them to have difficulty digesting particular foods, to not be able to make particular enzymes necessary for effective digestion, etc.