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Most cases of sex chromosomal abnormalities in humans have a case of learning disability or less developed cognitive abilities. Wikipedia says:

When additional X and/or Y chromosomes are present in 48,XXXY, 48,XXYY, or 49,XXXXY, developmental delays and cognitive difficulties can be more severe and mild intellectual disability may be present

So, the question is simply this: How is cognitive ability linked to the presence of sex chromosomes?

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Cognitive impairment does not have to do anything in particular with the sex chromosomes. Downs syndrome results because of trisomy of an autosome and it also results in cognitive impairment.

So, cognitive impairment is associated with incorrect gene dosage. As mentioned in the other answer, most aneuploidies are lethal.

Persons affected by aneuploidies do not just suffer cognitive impairment but also some other physiological problems. It is just very easy for us to notice the cognitive defects. Moreover, brain is a complex organ and neuro-development has a complex regulatory network. So dysregulation of common pathways is likely to affect brain development in a noticeable manner.

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    $\begingroup$ There's also the family of en.wikipedia.org/wiki/X-linked_intellectual_disability which I think is fair to say is less about the sex chromosomes being important for cognitive ability and more that, like other X-linked genetic disorders, males with a single X chromosome are more likely to show symptoms/stronger symptoms because they have a single copy. $\endgroup$ – Bryan Krause Nov 3 '20 at 15:54
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If you are thinking that, say, a chromosome 1 triploidy will not result in cognitive impairment... that's not quite right. Almost all triploidies result in death. Same with chr2, chr3, chr4...just about every chromosome is lethal if there are extras.

It's not so much that having extra sex chromosomes is bad for cognitive function, it's that having extra chromosomes is usually lethal, only a few triploidies are even survivable, and they all cause lots and lots of problems, including cognitive impairment. Having extra sex chromosomes is the mildest one, and it results in lots of comparatively mild problems.

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