Disproven vaccine hypothesis
Parents may first become aware of autistic symptoms in their child around the time of a routine vaccination. This has led to unsupported theories blaming vaccine "overload", a vaccine preservative, or the MMR vaccine for causing autism. In 1998 Andrew Wakefield led a fraudulent, litigation-funded study that suggested that the MMR vaccine may cause autism. This conjecture suggested that autism results from brain damage caused either by the MMR vaccine itself, or by thimerosal, a vaccine preservative. No convincing scientific evidence supports these claims, they are biologically implausible, and further evidence continues to refute them, including the observation that the rate of autism continues to climb despite elimination of thimerosal from routine childhood vaccines. A 2014 meta-analysis examined ten major studies on autism and vaccines involving 1.25 million children worldwide; it concluded that neither the MMR vaccine, which has never contained thimerosal, nor the vaccine components thimerosal or mercury, lead to the development of ASDs. Despite this, misplaced parental concern has led to lower rates of childhood immunizations, outbreaks of previously controlled childhood diseases in some countries, and the preventable deaths of several children. Etiological hypotheses
Several hypotheses have been presented that try to explain how and why autism develops by integrating known causes (genetic and environmental effects) and findings (neurobiological and somatic).
Causes
It had long been presumed that there is a common cause at the genetic, cognitive, and neural levels for the social and non-social components of autisms symptoms, described as a triad in the classic autism criteria. However, there is increasing suspicion that autism is instead a complex disorder whose core aspects have distinct causes that often co-occur. While it is unlikely that a single cause for ASD exists, many risk factors identified in the research literature may contribute to ASD development. These risk factors include genetics, prenatal and perinatal factors (meaning factors during pregnancy or very early infancy), neuroanatomical abnormalities, and environmental factors. It is possible to identify general risk factors, but much more difficult to pinpoint specific factors. Given the current state of knowledge, prediction can only be of a global nature and therefore requires the use of general markers. Biological subgroups
Research into causes has been hampered by the inability to identify biologically meaningful subgroups within the autistic population and by the traditional boundaries between the disciplines of psychiatry, psychology, neurology and pediatrics. Newer technologies such as fMRI and diffusion tensor imaging can help identify biologically relevant phenotypes (observable traits) that can be viewed on brain scans, to help further neurogenetic studies of autism; one example is lowered activity in the fusiform face area of the brain, which is associated with impaired perception of people versus objects. It has been proposed to classify autism using genetics as well as behavior.