Sex differences in autism

Males are more frequently diagnosed with autism than females, but it is debated whether this is due to a sex difference in rates of autism spectrum disorders (ASD) or whether females are underdiagnosed. The prevalence ratio is about 4 males for every 1 female diagnosed.[1] Currently, one in every 42 males and one in 189 females in the United States is diagnosed with autism spectrum disorder.[2] There is some evidence that females may also receive diagnoses somewhat later than males, however thus far results have been contradictory.[3]

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Several theories exist to explain the sex-based discrepancy, such as a genetic protective effect,[4][5][6] the extreme male brain theory[7][8] and phenotypic differences in the presentation between sexes,[6][9][10][11] which may all be intertwined. Researchers have also debated whether a diagnostic gender bias has played a role in females being underdiagnosed with autism spectrum disorder.[12] Researchers have also speculated a gender bias in parental reporting due to the expectations and socialization of gender roles in society.[13]

Since autism is a largely genetic and hereditary condition, genetic factors that can affect sex disparity with medical conditions come into play and include imprinting and X-linked mutations, which would have the ability to raise the frequency and severity in males and confer the hypothesised genetic "protective effect" in females, as they do in other conditions. Simon Baron-Cohen's extreme male brain theory suggests that autistic brains show an exaggeration of the features associated with male brains, such as increased size and decreased relative connectivity as well as systematic thinking over empathetic thinking.[14] The imprinted brain theory suggests genomic imprinting is at least partly responsible for the sex differences in autism and points to the evidence for a common genetic cause with schizophrenia,[15][16][17] together with the increased prevalence and severity of schizophrenia in males,[18][19][20] which is also reflective of the variability hypothesis, which states that there is greater variability in traits, especially intelligence, among males.

There is evidence of increased incidence of social anxiety,[10] anorexia nervosa[21][22] and self-harm in autistic females,[23] though the increased rates of anorexia nervosa may be due to confusion or conflation with avoidant/restrictive food intake disorder (ARFID), which is particularly common in autism.[24]

Extreme male brain theory
Main article: Empathizing–systemizing theory

Simon Baron-Cohen's extreme male brain theory states that autistic males have higher doses of pre-natal testosterone and on average have a more systemising brain, as opposed to the more empathising female brain. He suggests that autistic brains show an exaggeration of the features associated with male brains. These are mainly size and connectivity, with males generally having a larger brain,[14] which is seen in an exaggerated form in those with ASD. Individuals with ASD were found to have widespread abnormalities in interconnectivity in specific brain regions.[25] This could explain the different results on empathy tests between men and women[26] as well as the deficiencies in empathy seen in ASD, as empathy requires several brain regions to be activated which need information from many different areas of the brain.[27] Baron-Cohen therefore argues that genetic factors play a role in autism prevalence and that children with technically minded parents are more likely to be diagnosed with autism.[28]

Imprinted brain theory
Main article: Imprinted brain theory

The imprinted brain theory suggests genomic imprinting is at least partly responsible for the sex differences in autism and implicates schizophrenia as well, referencing the fact that the current genetic and physiological evidence suggests the two conditions are on a spectrum in which some mutations in certain genes cause lower social cognition but higher practical cognition (autism) while others in the same genes cause lower practical cognition with higher social cognition (schizophrenia).[15][16][17] It is already known that schizophrenia is more prevalent and more severe in males than females.[18][19][20] This is also reflective of the variability hypothesis, which states that there is greater variability in traits, especially intelligence, among males.

Female protective effect hypothesis

According to the female protective effect hypothesis, more extreme genetic mutations are required for a girl to develop autism than for a boy. In 2012, Harvard researchers published findings suggesting that, on average, more genetic and environmental risk factors are required for girls to develop autism, compared to boys. The researchers analyzed DNA samples of nearly 800 families affected by autism and nearly 16,000 individuals with a variety of neurodevelopmental disorders. They looked for various types of gene mutations. Overall, they found that females diagnosed with autism or another neurodevelopmental disorder had a greater number of harmful mutations throughout the genome than did males with the same disorders.[29]

Hypothesis of female underdiagnosis

Some authors, clinicians and experts like Judith Gould, Tony Attwood, Lorna Wing and Christopher Gillberg[30] have proposed that autism in females may be underdiagnosed due to better natural superficial social mimicry skills in females, partially different set of symptoms and less knowledge about autism in females among experts.[31] In his preword to the book Asperger's and Girls, Tony Attwood writes: "These tentative explanations for the apparent underrepresentation of girls with Asperger's Syndrome have yet to be examined by objective research studies."[32]

Specifically Judith Gould has discussed the idea that a pervasive developmental disorder called pathological demand avoidance, which is not officially included in diagnostic manuals may offer a glimpse about how autism in females may present in some cases.[33]

Autism and differences in gender and sexuality identification

Sexuality is often discussed within the autistic community, with many observations that identities other than cis-hetero seem to be more common than is observed in the neurotypical population. There have not been many formal studies on this to date, however members of the community speculate that autistic individuals generally have different ideals, perceptions and desires than neurotypicals or simply do not comprehend or agree with society's expectation, making them more apt to diverge from the norm.

Gender identity

A study looking at the co-occurrence of ASD in patients with gender dysphoria found 7.8% of patients to be on the autism spectrum.[34] This is much higher than the estimated 1% of people with ASD in the general population, suggesting a link between autism and gender variance.

As of yet, there have been no studies specifically addressing the occurrence of autism in intersex individuals.

Sexual satisfaction

A study conducted by Byers and Nichols (2014), explored the level of sexual satisfaction of high-functioning autistic individuals, with researchers testing the sexual and relationship satisfaction of neurotypical versus high functioning autistic individuals. The results suggest that men with ASD are generally less satisfied with their relationship or marriage compared to neurotypical men and women, and women with ASD.[35]

History

Hans Asperger was one of the first people to study autism, yet all of his four students were male. Another early researcher, Leo Kanner described "autistic disturbances of affective contact" in the group consisting of eight boys and three girls.[36]

See also

References
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