Brain growth across the lifespan

On average, young autistic children, mostly boys in these studies, have larger brains than non-autistic children of the same age. The difference shows up on brain scans by about age 2 to 2½.^1,2 In one large group of children followed in California, the average difference came from a subgroup: autistic boys with typical-sized brains did not differ from other boys, and autistic girls' brains were not larger at all.³

Pooling dozens of studies, about 1 in 6 autistic people (15.7%) have an unusually large head, and about 1 in 11 (9.1%) an unusually large brain.⁴ The usual definition of a large head (at or above the 97th percentile, or 2 standard deviations above average) means about 2 to 3% of any reference group counts as having one.⁵ The brain-size difference in this meta-analysis was larger in autistic people it described as "low functioning".⁴

The extra size seems to lie mostly in the area of the cortex rather than in its thickness.^1,8 A third study, of 3-year-old autistic boys, found the same pattern, mainly in the boys whose brains were unusually large for their height.⁹

In babies with an autistic older brother or sister, the 15 who were later diagnosed showed the surface of the cortex spreading faster between 6 and 12 months. Faster growth in total brain size followed between 12 and 24 months.⁸ That faster second-year growth went, weakly, with more marked social differences at age 2.⁸

Which regions grow most is less clear: one study found the biggest differences in frontal, temporal and cingulate cortex; another found temporal white matter enlarged.^2,1 Some studies report faster growth of the amygdala: between 6 and 24 months in infants later diagnosed, and between about 3 and 4 years in autistic boys, with wide variation between children.^10,11 Across all ages, the largest study found the amygdala slightly smaller on average in autistic people, so the early findings and the later averages point in different directions.¹²

In two separate groups of infants with an autistic older sibling, those later diagnosed had more fluid around the brain from about 6 months, and more fluid went with more marked autistic traits.^13,14 A third study from the same research group found the same excess at ages 2 to 4, including in children with no autistic sibling.¹⁵

In one long-running study, about 15% of autistic boys had brains large for their body size at age 3. Their heads were already larger by about age 2, though they had not been at birth.¹⁶ In this subgroup the enlargement lasted through about age 11, with no sign of shrinking back.³

Some studies report bigger brain-size differences, or faster early head growth, in autistic children who also have an intellectual disability.^17,4 The same California study found no difference in early developmental scores between boys with and without enlarged brains.¹⁶

Changes in single genes account for a few cases. In one small study, 3 of 18 autistic children with very large heads carried a change in the PTEN gene.¹⁸ In a study of 3,730 children, those with changes in the CHD8 gene often had a large head.¹⁹

Most brain-growth studies have included mainly males.^3,20 Two long-running studies covering ages 3 to 36 were all male, and one adult study was about 90% men.^21,22,23

Where girls were studied, autistic girls' brains were not larger on average but grew more slowly between about ages 3 and 5½.³ A 2024 study found autistic girls had thicker cortex around age 3 and faster thinning afterwards, across a wider area than in autistic boys. The differences had faded by about age 11½.²⁰ Both of these findings come from the same group of children in California.

Studies of adults and of large multi-site samples also find that some brain differences linked to autism depend on sex.^24,25

Whether early enlargement "evens out" later is unsettled. One study of males aged 6 to 35 found total brain size meeting the typical curve between 10 and 15, though some regions kept changing atypically into adulthood.²¹ Another study found that boys in the large-brain subgroup kept their larger brains through about age 11.³ Its authors argue that studies comparing different age groups may miss people with large brains in their older samples, making it look as if the difference disappears.³

In the largest study, of about 1,600 autistic people aged 2 to 64 scanned once each, the cortex was slightly thicker in frontal regions and slightly thinner in temporal regions. The differences were very small, so autistic and non-autistic people overlapped heavily, and they were largest around adolescence.¹² An analysis of about 1,000 people aged 6 to 35 found only a few small differences in brain anatomy.²⁶

Studies of how the cortex thins disagree. Two that tracked people over time report faster thinning in autistic adolescents: one in just two left-side regions, the other followed by slower-than-typical thinning in early adulthood.^27,22 One study comparing ages 7 to 39 found less thinning with age, and another, of adults, found more.^28,23

When each autistic person's brain is compared with what is typical for their age, studies of children and young adults, mostly male, find large and varied individual differences: two autistic people rarely differ from the typical range in the same places.^29,30 Group-average differences in these studies range from few and small to present in some brain measures.^29,31

A 2026 study made a related point for brain connectivity. Using normative models on resting-state scans from 1,824 people (796 autistic, ages 5 to 58, across 32 sites), it found that no single connection was extremely atypical in more than about 4% of people in either group, so a group difference on any single connection was not typical of the group. The deviations were not random, though. They converged: in the autistic group they overlapped up to twice as much as in the non-autistic group, and they clustered in the same regions and networks. Connectivity was weaker in sensorimotor and attention areas and stronger involving the frontoparietal and default mode networks and their links with the rest of the brain. The deviation patterns went with social and cognitive scores.⁴³ The autistic group was about 82% male, and the method detected deviations less well at the youngest and oldest ages.

One study of 157 autistic adults (ages 18 to 64, about 90% men, median age 26) found a stronger link between older age and thinner cortex than in non-autistic adults, mostly in the left temporal lobe.²³ Each person was scanned once, so the result shows that older participants had thinner cortex. It cannot show anyone's cortex thinning over time.

A study of 51 autistic adults aged 30 to 75, all with an IQ above 80, found no difference in cortical volume, thickness or surface area.³³ A small study of 29 autistic adults aged 40 to 70 found that tracts linking deep brain structures to the cortex changed with age in parallel with non-autistic adults, not faster, though some were smaller in the autistic group.³⁴

As of October 2026, repeated brain scans of autistic people stop at around age 35 to 39. A study of adults aged 40 to 70 repeated memory tests over about two years but scanned the brain only once.^21,22,35 The adult studies included few or no autistic people with an intellectual disability, so their findings may not apply to them.^23,33

It has been proposed that early overgrowth starts before birth, from the brain making too many cells.^36,37 One small preliminary study of the brains of 7 autistic boys who had died (compared with 6 non-autistic boys) reported about two-thirds more neurons in the prefrontal cortex.³⁸

Brain organoids grown from the cells of 10 autistic toddlers were about 40% larger than those from 6 non-autistic toddlers, in two separate batches, and larger size went with more marked social differences. This is an association in 16 donors and has not been repeated by another team.³⁷ Stem cells from 8 autistic people with enlarged brains multiplied faster in the lab,³⁹ and organoids from 4 autistic people with large heads made too many inhibitory neurons.⁴⁰

Not every finding fits. A study of the brains of 9 autistic males aged about 10 to their mid-40s found fewer neurons in the amygdala, not more.⁴¹ At most, this explanation applies to the subgroup with large brains, not to autistic people in general.

A separate proposal concerns pruning, the loss of surplus connections between nerve cells during childhood and adolescence. In one post-mortem study of 10 autistic and 10 non-autistic brains aged about 2 to 20, the number of dendritic spines fell by about 16% between childhood and adolescence in the autistic tissue, against about 41 to 45% in the non-autistic tissue.⁴² Whether this connects to the cortical thickness changes described above is an open question.