Autism has historically been viewed as a condition that affects men and boys more frequently than women and girls. But a massive new study based on data from millions of people suggests this isn't actually the case, at least in Sweden.

While boys are diagnosed with the condition at higher rates during childhood, by adulthood, the ratio is roughly 1:1. This suggests it's not a case of fewer women having autism: it's just that they aren't diagnosed until later in life.

Autism spectrum disorder (ASD) diagnosis rates have been climbing since the 1990s, but diagnoses are not equal between males and females. The DSM-5, which defines mental health conditions and recommended treatments, states that for every four males diagnosed with autism, only one female receives a diagnosis.

But the recent study, led by medical epidemiologist Caroline Fyfe of the Karolinska Institute in Sweden, paints a very different picture, adding to a growing body of research that suggests autistic women and girls are being let down by current systems of diagnosis and treatment.