Abstract: Most traits that are associated with autism spectrum disorder are also associated with males in general. Indeed, ASD individuals display more extreme versions of these traits than typical males. The evidence listed provides strong support for the extreme male brain theory of autism. Furthermore, females with autism display reduced severity of ‘masculine’ traits compared to their autistic male counterparts, possibly explaining the large sex difference in diagnosis. While more research is needed for confirmation, this finding suggests that autism has an overall masculinizing effect that is ultimately limited according to the foundation point along the male-female spectrum.
Early researchers of autism, such as Kanner and Asperger, noticed that autism is far more common in boys. Hans Asperger wrote “the autistic personality is an extreme variant of male intelligence. Even within the normal variation, we find typical sex differences in intelligence… In the autistic individual, the male pattern is exaggerated to the extreme.”1 Thus the extreme male brain theory was born.
Sir Simon Baron-Cohen took up the hypothesis in 1997 and has been a famous proponent since. The theory is somewhat controversial, and Simon is careful how he frames his findings, focusing on differences in ways of thinking rather than biology. He put forward the empathizing-systemizing spectrum theory (E-S), and provides examples of male tendency to systemize, female tendency to empathize, and the heavy skew towards systemizing in autism.
This article is going to re-examine the extreme male brain theory (EMBT). The theory will be extended beyond the brain to include physical characteristics, so that autism can also be compared to the extreme male body. It is important to understand that these traits are generalizations, and there will always be individuals who do not follow the typical profile. A tick (✔) will be used for traits that support the theory, while a (✖) will be used for those that do not.
Physical traits:
Brain size ✔
Autism is associated with a larger brain. More specifically, a brain that is larger than the average of their own sex, as male brains are 10-15% larger than female brains.2
Body mass ✔
People with autism are more likely to be in overweight or obese weight categories, and this finding was not solely due to mental health causes, such as eating disorders.3 However, autistics do suffer from depression and eating disorders at higher rates. Males in general are also more likely to be overweight and to suffer eating disorders compared to females.4
Facial features ✔
Autistic people have more masculine facial features.56 This includes a greater facial-width to height ratio, which is associated with aggression.7 Because of this, autistic people are viewed as less sociable and are less liked upon first impression. Male autistics are viewed less favorably than female autistics. Observers with autism also viewed autistics less favorably, but this did not reduce their social interest for interacting like it did with neurotypicals. Having knowledge of someone's diagnosis resulted in more favorable impressions of them, unless the observer was also autistic.89 Computers have been taught to predict autism in children by facial recognition, to an accuracy of 94.6%.10
Puberty onset ✖
Autism is associated with earlier onset of puberty. Females enter puberty earlier than males on average.11 This does not support the EMBT.
Immunity ✔
Autism is associated with reduced immune function.12 On average, males have weaker immune systems than females,13 so this supports the EMBT.
Nutrient deficiency ✖
Autism is associated with vitamin deficiency (A, B, D) as well as iron deficiency and anemia.1415 Females suffer from all these symptoms more than males,1617 therefore this trait does not support the EMBT.
Fetal testosterone ✔
Autism is associated with higher levels of prenatal testosterone.18 Males have higher levels than females.19 Research into levels of postnatal testosterone in autistics compared to neurotypicals has produced no clear relationship.
Motion sickness ✖
Autism is associated with higher rates of motion sickness than neurotypicals. This may be due to some degree of dysfunction in their vestibular system,20 which is the system that keeps you balanced. Females experience higher rates of motion sickness than males,2122 so this trait does not support the theory.
Joint hypermobility ✖
Autism is associated with joint hypermobility, which is more common in females than males.23
Testosterone-related disorders in female autists ✔
This 2007 study provides strong support for the EMBT:
Compared to controls, significantly more women with ASC reported (a) hirsutism, (b) bisexuality or asexuality, (c) irregular menstrual cycle, (d) dysmenorrhea, (e) polycystic ovary syndrome, (f) severe acne, (g) epilepsy, (h) tomboyism, and (i) family history of ovarian, uterine, and prostate cancers, tumors, or growths. Compared to controls, significantly more mothers of ASC children reported (a) severe acne, (b) breast and uterine cancers, tumors, or growths, and (c) family history of ovarian and uterine cancers, tumors, or growths. These results suggest current hormone abnormalities in women with ASC and their mothers.24
Levels of acne or hairiness in male autists (hirsutism is excessive hairiness in females) have not been studied, but they are likely to be higher than normal given their connection to testosterone.
Behavioural traits
Systemizing ✔
Autism is associated with a tendency to systemize. This means that individuals with ASD are, on average, more attuned to noticing systems or patterns than neurotypicals. People with ASD score higher on systemizing quotients,25 and play with construction or mechanical toys more than average.2627 The high frequency of sensory intolerance (hypersensitivity) in autism reinforces this association, as more information is absorbed to be used for analysis. This greater sensory input can sometimes become overwhelming. It is believed that systemizing can help one to exploit the environment for resources.28
Males play with mechanical toys more than females, and it is understood that sex-related biology plays a large role in this behaviour, as male monkeys play with mechanical toys more than female monkeys.29 Systemizing may explain why people with ASD prefer routine, and are resistant to change.30 Systems can be understood when only one variable at a time is changed. To test whether this trait of ‘resistance to change’ supports the EMBT, research can be done on which gender is more ‘spontaneous’ and less likely to follow a routine.
Empathizing ✔
People with autism score lower on standard empathy quotients. Females score higher than males on average, so this supports the EMBT. Sex differences in empathy reflect differences in the personality trait of agreeableness. People with autism are less agreeable, while females are more agreeable than males.3132 However, the topic of empathy is complicated, and some people with autism take issue with being labelled as low in empathy. For example, research has found that:
in both psychopathy and autism, though individuals may exhibit less automatic empathy, when explicitly instructed to be empathetic (i.e., under effortful control), they are capable of exhibiting significantly higher levels of empathy (Gillespie et al., 2014; Meffert et al., 2013; Senju et al., 2009)33
This is difficult to draw concrete conclusions from, as it could be argued that a defining feature of empathy is the independent nature of use; it should not need to be coaxed. Nevertheless, Baron-Cohen and other researchers believe that when someone with autism understands that another person is suffering, they do feel upset and want to help.34 It is likely that IQ plays an important role in the ability to process and express empathy. Studies show that individuals with a higher IQ have a increased capacity for sensitivity, emotional intelligence, empathic concern.353637 However, people with Asperger's or high functioning autism still display lower scores than neurotypical controls.38 It has been suggested that lower scores on the empathy quotient could be due a lack of social interest or awareness in ASD, but not a deficit in sympathy or compassion. This is difficult to determine, as the two different components of empathy cannot be easily separated.39
Another possibility is that people with autism are more selective with their empathy. It is common to feel more empathy for a close family member than a stranger, and for a child than for an adult. The genetic similarity theory40 states that individuals tend to be more altruistic to others who are genetically similar to themselves even if they are not kin. People with autism may be more genetically distinct from the average than most people due to high mutational load, leaving them feeling isolated and unmotivated to empathize with non family members. This could explain the 'double empathy problem'41 where typical people also have difficulty reading and empathizing with those on the spectrum.
Light sensitivity ✔
Sensitivity to bright lights is the most commonly reported sensory overstimulation in autistic people.42 Enhanced vision may also be a trait of autism.43 Males have higher brightness perception and a stronger response to white light than females.44 Males also have better eyesight than females,45464748 so this trait supports the EMBT.
Auditory sensitivity ✖
Autism is associated with sensitivity to sounds.49 However, females are more sensitive to sound than males.50 This trait runs counter to the EMBT, possibly for similar reasons as motion sickness in that auditory dysfunction may be related to vestibular dysfunction.51
Heat sensitivity ✔
People with autism report less tolerance to heat.5253 Females prefer higher temperatures than males,54 partly because they have lower metabolisms and less muscles to generate heat, and partly because a higher core temperature is required for ovulation and egg fertilization.5556
Touch sensitivity ✖
Tactile sensitivity is common in individuals with ASD.57 Females are more sensitive to touch than males,5859 so the EMBT is not supported.
Smell hyposensitivity ✔
Studies have reported different findings on olfactory perception in ASD. Some have found that autistics are hypersensitive to smell,60 some have found typical levels of sensitivity,61 but most recent studies have found that individuals with autism have lower sensitivity and poorer sense of smell than is typical.626364 Females outperform males on smell tests,65 so this supports the EMBT.
Taste ✔
Children with ASD are described as picky eaters, however research finds less taste sensitivity overall in people with ASD.66 Females have greater taste sensitivity and more taste buds than males.676869
Socializing ✔
The etymology of the word autism is from the Greek autos meaning self, and the English suffix ism, combining to produce selfism, meaning solitude, self-reliance or self-absorption.
People with autism are less responsive to voices and faces.70 A study found that if a toddler spent more than 69% of his or her time fixating on a non-social stimulus, then the predictive value for accurately classifying ASD was 100%.71 Furthermore:
Overall, almost one-third of young adults with ASD were socially isolated, with no contact with friends, no phone calls, and no invitations to activities…One-half to two-thirds have no close friendships… When friendships do occur, they appear to be less close and supportive than in the general population (Baron-Cohen & Wheelwright, 2003).72
We can recognise this as partly a lack of interest in socializing, and partly because people with ASD, as 1% of the population,73 have trouble finding people like them. Between the genders, females are more social than males, with larger networks of friends and a greater amount of social support.74
Impulse control ✔
Autism is associated with lower impulse control.75 This association holds for other neurodevelopmental disorders that have similarities, such as ADHD. Males show lower levels of impulse control, although the findings are not always clear.7677 Yet males also display greater levels of addictive behaviours.7879 This impulsion is likely the reason why people with autism are known to have narrow interests. They struggle with motivation for activities that do not interest them. There is a link between impulse control and empathy, and impulsivity may reflect a more self-absorbed pattern of thinking.8081 We can see similarities in toddlers who are in the preoperational stage of development within Piaget's theory of cognitive development. Egocentrism is a defining feature of the preoperational stage; toddlers are impulsive as they do not fully understand the agency of others and are unable to see their point of view.
Vengefulness ✔
People with autism are described as having periods of intense anger. However, males and females do not differ in overall levels of anger, only in ways of expression. This discrepancy can be resolved with the trait of vengefulness. Vengefulness has not been directly studied, however people with autism have longer periods of 'anger rumination',82 within which is it likely that thoughts of revenge arise. Autistics are higher in irritability,83 lower in frustration tolerance and are often described as vindictive within the framework of oppositional defiance disorder.84 Males are more vengeful than females,85 who are more forgiving, so this trait supports the EMBT.
Aggression ✔
People with autism exhibit higher rates of aggression on average than neurotypicals, although results vary.86 Aggression is related to impulse control, vengefulness, and testosterone, and males display more aggression than females.87 Just as with many traits associated with autism, it is likely that IQ plays a dynamic and possibly mediating role regarding aggression.8889
Spatial ability ✔
Individuals with autism score higher on tasks involving spatial ability than neurotypicals.9091 There is a wealth of material on the subject, with studies on many tests such as mental rotation, block design, navigation tasks and embedded figures. These findings have been explained with theories such as weak central coherence, and increased visual pattern recognition skills. The explanation is a simple one, and can be understood in terms of the extreme male brain theory. Males display superior performance on spatial tasks compared to females, it is the only area of intelligence that shows a gap.92 Furthermore, rats with testosterone injections also perform better on spatial navigation tasks.93 Given that autism is associated with higher levels of prenatal testosterone, an advantage in spatial tasks is no surprise.
Spatial ability is useful for physical tasks that can help one exploit the environment for the acquirement of resources. Hunting and fighting are prime examples, and relate to systemizing and the solitary forager hypothesis.
People on the autism spectrum are conceptualized here as ecologically competent individuals that could have been adept at learning and implementing hunting and gathering skills in the ancestral environment. Upon independence from their mothers, individuals on the autism spectrum may have been psychologically predisposed toward a different life-history strategy, common among mammals and even some primates, to hunt and gather primarily on their own. Many of the behavioral and cognitive tendencies that autistic individuals exhibit are viewed here as adaptations that would have complemented a solitary lifestyle. For example, the obsessive, repetitive and systemizing tendencies in autism, which can be mistakenly applied toward activities such as block stacking today, may have been focused by hunger and thirst toward successful food procurement in the ancestral past. Both solitary mammals and autistic individuals are low on measures of gregariousness, socialization, direct gazing, eye contact, facial expression, facial recognition, emotional engagement, affiliative need and other social behaviors. The evolution of the neurological tendencies in solitary species that predispose them toward being introverted and reclusive may hold important clues for the evolution of the autism spectrum and the natural selection of autism genes. Solitary animals are thought to eschew unnecessary social contact as part of a foraging strategy often due to scarcity and wide dispersal of food in their native environments.94
Sex drive ✔
People with autism are found to have a stronger desire for sexual activities, both solitary and dyadic.9596 Males have a greater sex drive than women, due to higher levels of testosterone.97 Given that the average person with autism has lower impulse control, a higher sex drive and is socially isolated, they present increased vulnerability to addictions involving pornography and masturbation.98 Furthermore, this vulnerability may be just as great in higher functioning forms of autism. Although higher intelligence improves impulse control, social isolation also is also increased.
Verbal ability ✔
A delay in developing speech and language skills is one of the earliest signs of autism.99 Females talk earlier than males, a sign of their greater tendency toward socializing.100 In fact, a 2019 study states that:
Almost all developmental disorders primarily affecting communication, speech, and language skills are more frequent in boys. … We argue that the functional organization of the female brain gives women an inherent advantage in the acquisition of communication and language system over men.101
Ambition ✔
Males are more career ambitious than females,102 because females sexually select for status and resources, while males select for health and fertility.103 Therefore, according to the EMBT, we should see greater levels of ambition in people with ASD. It has been found that autistic traits, such as dominant behaviours, social insensitivity and problem solving, can "help people gain and sustain power, and [have] probably done so throughout history."104 It is possible that individuals with ASD have difficulty reconciling their higher ambitions with their lower status (most autistics are unemployed).105
Eye contact ✔
People with autism make less eye contact than neurotypicals in early and later life. Males make less eye contact than females.106 This follows the theory that females are more social and look for more emotional cues from others, and supports the EMBT. People with autism consciously avoid eye contact as it causes hyperarousal of the amygdala and results in negative feelings ranging from “uncomfortable” to “burning”.107108 Researchers are unsure exactly why this occurs, but some theories have been suggested. As eyes contain large amounts of emotional information, this would mean that eye contact requires extra processing for emotional reading, which is already difficult for individuals with ASD. However, eye contact is only difficult when the gaze is returned.109 This author suggests that eye contact may be interpreted by an extreme male brain as either sexually suggestive or defiant and challenging, in females and males, respectively. However, this does not easily explain gaze aversion in children. It is likely that there are several factors at play, including a degree of social indifference.
Focus ✔
Adults with autism display greater ability to focus and block out distractions than typical adults.110 Males also possess greater ability to focus than females, who are superior at multitasking.111112 Since females have been the primary carer of children for most of human history, multitasking may have been adapative for keeping an eye on children, while greater ability to focus in males is helpful for goal-attainment and physical activities like hunting.
Hygiene ✔
In general, people with autism maintain poorer levels of personal care and hygiene. This trait is associated with sensory issues, for example from water or toothbrushes, however there is evidence to suggest that an equally important reason for poorer self maintenance may be a lack of interest.113114 Across the globe, males have looser hygiene norms and less interest in self maintenance115116 likely due to the differences in sexual selection preferences mentioned earlier.
Fight or flight ✖
A 2017 study found that the ‘smell of fear’ (skydiver sweat) increased physiological arousal (sympathetic nervous system response) in typically developing individuals, but had a calming effect on autistic participants.117 Furthermore, hexadecanal was administered, which again had opposing effects. We can speculate that the calm response to a smell of fear could reflect a riskier approach to stressful situations that may be found in hypermasculine profiles.118 However, autistic participants exhibited a response more similar to that of females for the hexadecanal experiment. Since hexadecanal has been found to reduce aggression in men but to trigger aggression in women,119 this suggests "social chemosignaling as a sensory substrate of social impairment in ASD" and does not support the EMBT.
Objectivity ✔
People with autism are less likely to lie in situations where they might be socially advantaged if they do so.120 Also, they are less likely to deceive themselves in order to conform to group standards.121 Furthermore, people with ASD display higher levels of rationality and logical decision-making than neurotypicals.122 According to Stanovich, West and Toplak (2016), males display higher levels of rationality than females by approximately half a standard deviation.123 The reason for these findings can again be explained by the solitary forager hypothesis, where there would be less need for maintaining reputation and 'playing the social game'. Higher levels of honesty, combined with poor social awareness might explain why people with autism are often victims of manipulation, maltreatment and crime.124125126
Innovation ✔
Autism has been linked to increased ability to invent or innovate. People working in science, technology, engineering or mathematical fields (STEM) score higher on the autism quotient,127 and rates of childhood autism are significantly higher in the Eindhoven region in Netherlands compared to control regions. Eindhoven is known for its schools and businesses based in IT and technology. Between genders, males significantly outnumber females in STEM.128
Meta traits:
Male Variability ✔
The male variability hypothesis states that males vary more than females, as an evolutionary tool to ‘test out’ new genes that can be rapidly spread across the population (males have a higher reproductive threshold). There are more genetic mutations on the Y chromosome, and so we see a greater number of extreme case profiles in males across many aspects of life, including neurological disorders. So it is to be expected that rates of autism are higher in males. However, the sex ratio is higher in autism than most neurological disorders. Autism has a 4:1 ratio, ADHD is 3:1, Psychopathy is 2.7:1, while schizophrenia is unclear.129130 Left-handedness is associated with genetic mutation, is more common in males and more so in people with autism.131132 Mutational load can explain why autism has high levels of co-occurring conditions.133
Older fathers ✖
The chance of having a child with autism increases with age for males, while for females the results are unclear.134 This may be because people with autism are delayed in finding a partner, and they simply pass on their autistic traits later in life. Regardless, older parents are more likely to have girls,135 therefore this does not support the EMBT.
Sex-specific mitigation ✔
Those with genetic predispositions towards disorders like schizophrenia are less likely to suffer the disorder if they have higher IQ.136 Thus it can be said that IQ 'protects' against some neurological disorders. There is also evidence that IQ protects against autism and mental health.137 This could explain why autism is characterised overall by low IQ,138 as the more intelligent go undiagnosed. There are well-established links between intelligence and impulse control,139140 face narrowness141, empathy (as noted earlier) and possibly immune health.142 With these effects, we can start to see how autism can be offset by higher intelligence. Given this protection phenomenon, there is a possibility that females are protected against autism by virtue of their sex. If IQ can mitigate physiological dysregulation, then perhaps the female sex chromosome can mitigate hormone dysregulation (autism is associated with higher levels of prenatal testosterone).143
While autistic brains are larger than average brains, female autistic brains are only larger than female typical brains, but not male typical brains or autistic male brains.144 We also see that autistic females score lower on the empathy quotient than typical females, and even slightly lower than typical males, but still higher than autistic males.145 These results suggests that lack of a Y chromosome can mitigate autism to some degree. This would support the EMBT as it provides confirmation that autism seems to 'push' individuals towards the male extreme, however it can only push so far. It also suggests that the heavily skewed sex ratio is not from under diagnosis of females as a result of cultural bias. Females receive diagnoses of autism later in life than boys, and this leads people to think there is a diagnostic bias. However, the reason may be because autism is harder to spot in girls, as the symptoms are generally less severe due to sex-specific mitigation of autistic traits.146
Trivers-Willard effect ✔
The Trivers-Willard hypothesis states that parental traits can adjust the sex ratio of their offspring to maximise their reproductive success. One effect of this hypothesis is that traits that are likely to benefit one sex more than the other help select for a child of the beneficiary sex. This effect is the reason that more attractive parents have more daughters,147 and taller parents have more sons.148149 This effect may help explain the heavily skewed sex ratio in autism. It is possible that parents with autistic traits are more likely to have sons, as males benefit more from autistic traits than females.150
Conclusion
Autism is a highly heritable condition151 that can also be affected by prenatal hormone levels. Autism is higher in Caucasian and Asian populations, with lower rates in African and central American populations.152153 This has led some to make a connection between Neanderthal admixture and autism. Neanderthals seem to have possessed many autistic traits; they were larger, hairier, less social, skilled strategists, innovators with possibly a lower preference for sunlight.154155 However, a more likely scenario is that autistic traits are more advantageous in a colder, food scarce environment, and that Neanderthals evolved these traits over time as Homo-sapiens did. Some neurodivergence is adaptive for the group at low to medium rates, as it allows the population to specialise and fill social and ecological niches.156
Individuals with ASD may exhibit, on average, higher levels of physical or strategical competence, and lower levels of social competence. It can be said that autistic individuals are more attuned to a solitary lifestyle, but may also suffer periodically from excessive levels of isolation and the struggle to fit in. A 2014 study found that 66% of people with Asperger's have suicidal ideations and 35% have plans for or have made attempts at suicide.157
Higher levels of sensory input can be overwhelming. A study found that people with autism have a greater number of neurons on average,158 which can lead to faster rates of mental fatigue. Combined with a set of narrow interests, this can cause people with autism to find it difficult to maintain a typical 9am-5pm occupation. Low frustration tolerance occurs at higher rates in autism, because their sense of self is stronger, while empathy is lower. This is why people with autism report accentuated responses to 'goal blockage'.159
The role of neurotransmitters may be different in ASD. There may be lower base levels of oxytocin, higher numbers of dopamine receptors, and greater rates of serotonin synthesis, just as there are in males.160161 This different dynamic regarding neurotransmission may be the cause of repetitive movements or ‘stimming’. Autistics can struggle with anger at times. Higher levels of impulsiveness and vengefulness may lead to conflict with friends and family who may be more forgiving of mistakes and wrongdoing. People with autism may also struggle with the higher levels of dishonesty (often subconscious) and social competition in neurotypicals. It is not uncommon for people with autism to cut contact with family for extended periods.
A 2021 study involving language analysis found that rational words have been declining while emotional words have been increasing since 1980.162
All in all, our results suggest that over the past decades, there has been a marked shift in public interest from the collective to the individual, and from rationality toward emotion.
There have also been social changes coinciding with the rise of feminism. The large majority of teachers are females, whereas 100 years ago boys would have often learned a trade from their father. Schools are more likely to favour passive, acquiescent behaviours that are more typically found in girls,163 which might partly explain the high rates of oppositional defiance disorder co-occurring with autism. Furthermore, research has shown that spatial ability is neglected in educational and occupational settings. As spatial ability is a strength of autism, this can lead to missed opportunities and adverse effects on the development and psychological well-being of people with ASD.
It is difficult for students whose intellectual pattern is dominated by spatial ability to reach their full potential with traditional school curricula, where reasoning with numerical and linguistic symbols rather than forms and shapes is the norm...When placed in environments that are aligned with their preferred intellectual–substantive medium, spatially talented students can thrive and even outperform students with commensurate general intelligence but with strengths in other areas. However, due to suboptimal educational provisions and a general lack of recognition of what spatially gifted students can accomplish, a large number of potential engineers and physical scientists are likely not earning undergraduate or advanced degrees commensurate with their overall intellectual abilities. Spatially talented individuals may find themselves undereducated and under-employed...A great loss occurs at talent searches that identify intellectually precocious young adolescents. Current talent search procedures focus on the assessment of mathematical and verbal ability164
All in all, the balance of evidence is in favor of the extreme male brain theory by a count of 28 to 8. Furthermore, the traits that counter the theory are not the typical traits associated with autism, such as impulsiveness, systemizing and empathizing. These traits are not comprehensive of all the traits observed in autism, however can be used as an informal checklist for a self-diagnosis of autism. The ASD profile shows a strong tendency towards the extreme male side of the spectrum, and the theory remains valid.
With regard to motion sickness, cerebellar developmental disorders are way more common in men. With a lot of these things it also depends on the etiology to classify them are more male or more female