As the name implies, Autism Spectrum Disorder (ASD) presents in a variety of ways. No two people with autism are exactly alike and neither are any two cases of ASD. One of the biggest dividing lines among people with ASD is sex; in many ways, autism differs for boys and girls. From diagnosis rates to symptoms, here are some of the most important things to know about how autism differs among the sexes.

Autism and Gender Statistics The most notable difference between males and females with ASD is the rate of diagnosis. Since autism was first identified in the 1940s, the sex ratio has been skewed, with men being diagnosed significantly more often than women. Studies have found ratios as high as 9 to 1 (meaning a ratio of nine males with ASD for everyone one female), but, for years the general consensus has been a ratio of 4:1, suggesting that males are four times as likely as females to be diagnosed with ASD. In recent years, as research into ASD improves, that number has been shrinking. A 2017 analysis of 54 studies found that the ratio to be closer to 3:1, and some experts suspect that number may still be inflated due to a number of factors, including research participant pools’ tendency to skew male.

Why are boys diagnosed with autism more than girls?

Several factors could play a role in ASD’s skewed sex ratio:

1. Diagnostic Bias One of the biggest factors impacting the autism sex ratio is the diagnostic bias. Since autism studies have historically included more male participants than females, much of what we know about autism is really about how autism presents in males—which isn’t necessarily applicable to females or nonbinary individuals with autism. For example, there’s growing evidence that girls with ASD experience different and/or less severe symptoms than their male counterparts. As a result, many girls with autism are overlooked and either diagnosed later in life than boys or misdiagnosed completely.

In addition to presenting with different symptoms, girls with autism may be overlooked because of biases held by teachers and doctors, many of whom have been conditioned to see ASD as a “male” issue. There’s a large body of anecdotal evidence that suggests girls have a harder time obtaining ASD diagnoses, even when their parents express concerns about development.

2. Autism Masking In addition to having different symptoms (boys are more likely to be hyperactive, make loud and/or inappropriate comments in public and to engage in some repetitive behaviors strongly associated with autism, like lining up objects or obsessively spinning parts of objects), girls with ASD may actually take steps to hide their condition, a behavior known as “masking.” A 2016 study found that masking, also known as “camouflaging,” was commonly reported among adult women with autism who were not diagnosed until their late teens as a tool for fitting in. Masking involves mimicking or learning socially acceptable behaviors, but women who discussed their experiences with it said it often led to side effects like constant exhaustion or a loss of their own sense of identity.

3. Biology There is also some evidence to suggest that biology may play a role in the autism sex ratio and the differences in how ASD presents for males and females. A 2013 report found that “several sex-differential genetic and hormonal factors may contribute” to the lower prevalence of ASD among females as compared to males. Furthermore, a 2014 study found that it takes more mutations to trigger autism in women than in men and a 2016 study pointed to differences in gene expression between the sexes that could be linked to different rates of autism.

Autism and Gender-Diversity

As we learn more about autism’s relationship to sex and gender, researchers have (rightly) begun to study the how ASD affects people across the gender spectrum, including non-binary, transgender, and other gender-diverse individuals. In 2020, the largest study to date of autism and gender-diversity found that people who don’t identify with the sex they were assigned at birth are 3-6 times as likely to be diagnosed with ASD as are cisgender individuals. Even when they aren’t diagnosed with autism, gender-diverse people are more likely to suspect they are autistic and to report having traits associated with autism. More research needs to be done into gender-diversity and autism specifically and researchers will need to make an active effort to include more girls and gender-diverse individuals in general autism studies going forward in order to build a more accurate profile of what ASD looks like in people who aren’t cisgender males.