Science is based in the scientific process, the study and integration of information, the theory or hypothesis that stands up to the test of time. Scientists may study blood or diseases or slices of brain (to name a few), or look at brain scans to come up with theories. They study human behavior as well, through trials, and long years of research and sometimes 'hands on' with patients. They come up with theories together or separately after long years of this kind of work.
They accrue information in experiments and tests, to come to conclusions, they offer theories on why something is the way it is. They provide concrete evidence through long years of research, and look at previous research and utilize that or sometimes reject it. Which changes the theory that was offered up in the past. Theory is a system of ideas intended to explain something.
Now, when they do that they might come up with something like the DSM.
The diagnostic and statistical manual for mental disorders. Something that many people have read and understand, many members here for example. If you have any background and/or understanding of psychiatry and or science in general it's not difficult to comprehend, it lists behavior, symptoms, traits, of many kinds of neurological disorders. Although in the last few years, it's been scrapped for use by psychologists when it comes to autism. Autism has been removed from the DSM, as it's not considered as a mental illness. It's now termed a pervasive developmental disorder or PDD.
This is a description of autism in children by the American Psychiatric Association, although very basic it's not difficult to understand.
Autism differs from person to person in severity and combinations of symptoms. There is a great range of abilities and characteristics of children with autism spectrum disorders — no two children appear or behave the same way. Symptoms can range from mild to severe and often change over time.
There is no medical test for autism. It is diagnosed based on observing how the child talks and acts in comparison to other children of the same age. Trained professionals typically diagnose autism by talking with the child and asking questions of parents and other caregivers.
Scientists do not clearly understand what causes autism spectrum disorder. Several factors probably contribute to autism, including genes a child is born with or environmental factors. A child is at greater risk of autism if there is a family member with autism. Research has shown that it is not caused by bad parenting, and it is not caused by vaccines.
https://www.psychiatry.org/patients-families/autism/what-is-autism-spectrum-disorder