There are both genetic and non-genetic influences along with environmental factors that are thought to be involved in the development of autism, but, none of them is clearly understood.
To review the incidence of Autism in 2018: 1:59 in the US (CDC) and 1:66 in Canada (Autism Speaks).
I found it fascinating that in our pediatric practice in Roswell, NM, with a population of about 15,000 patients, we had a very low incidence of Autism overall in the first decade of this century. About 70-75 % of our practice was Hispanic. At that time the non-Hispanic whites had a much higher incidence than Hispanics or Blacks. However this is rapidly changing. The 1 in 59 rate in the general US population in 2018 was an overall 15% increase from the previous 2 years, largely due to better outreach and diagnosis in the minority populations. But minority rates are now exceeding white rates, especially among Blacks, and Hispanics are paralleling Whites, implying that there are differences in risk factors (either greater exposure to environmental influences, or perhaps another trigger.). I'll have to follow this one.