The causes of autism aren’t fully known. There is variety in those with autism, from high-functioning, to low-functioning. As it is something people are born with it is clearly genetic, but how it expresses differently is currently unknown. It could be that genes associated with autism are expressed differently in the presence of other genes, or it could be they are expressed differently in the presence of certain levels of certain hormones during birth, and some of the differences in how it is expressed could be environmental, like how the child is raised. It is likely to be a mixture of all of these elements. One question is why does autism continue to persist? This may be because it has benefits for society when someone is on the high-functioning end of the spectrum. Someone with high-functioning autism is likely to be very good at focusing on one task and obsessing about that one task and that one subject, so they manage to achieve things others don’t have the time, focus or patience to achieve, like maybe becoming obsessed with how arrows travel through the air, and wanting to know why an arrow travels as it does, and what would make it travel further and more accurately. Someone with high-functioning autism may focus on this thought for years until they develop an arrow which is more efficient and better than other tribes arrows, which helps the tribe survive and gives the autistic person an advantage to the tribe. It could be that the downside is some people are born with low-functioning autism which means they require more care, but may not contribute to the tribe. This is a similar argument to one I remember learning about schizophrenia when I worked in mental health homes. That schizophrenia is genetic, and at it’s worst (and without medication etc) it isn’t particularly helpful, but for some people before it develops into schizophrenia they have incredible creativity and connections of thought which others generally don’t have. With schizophrenia it is something which some people have a pre-disposition for it, and often a life event or situation triggers the schizophrenia, this could be puberty or a traumatic or stressful experience etc., it is something the person has the genes for, but to turn on the epigenetic expression for the person to have schizophrenia usually takes a trigger.
There are questions about why more people have autism nowadays. The currently thought reason for this is that there aren’t more people with autism, just less stigma, so more people are comfortable seeking diagnosis, and better ability to diagnose autism. There are also more females being diagnosed with autism than their used to be. It used to be thought that autism was more of a male issue, and it may have a slightly more male bias – although with time it may turn out this is incorrect, but now it is recognised that females and males can have autism, it is just expressed differently in females to males. For example I think females may find emotional sensory overload more of an issue than males, but this isn’t so easily picked up because people seem to think those with autism don’t have feelings and don’t get emotional, so in the past if a female was getting emotionally overwhelmed psychologists and psychiatrists are likely to assume the problem is something else, not autism.