The US health secretary, Robert F Kennedy Jr, has long been consumed by the neurological condition autism – what causes it, and whether there’s a treatment. This week, Donald Trump took on the cause, making claims about acetaminophen, also known as Tylenol and paracetamol, that were dismissed outright by medical experts around the world. Jonathan Freedland speaks to Carter Sherman, the reproductive health and justice reporter at Guardian US, about when and why the obsession with autism became political