Women have fewer cognitive disorders than men dobecause their bodies are better at ignoring themutations which cause them
與男性相比,患有認知障礙的女性較少,因為她們自身的身體能更好的忽略導致認知障礙的基因突變
AUTISM is a strange condition. Sometimes its symptoms of “social blindness” (an inability toread or comprehend the emotions of others) occur alone. This is dubbed high-functioningautism, or Asperger's syndrome. Though their fellow men and women may regard them as abit odd, high-functioning autists are often successful (sometimes very successful) members ofsociety. On other occasions, though, autism manifests as part of a range of cognitiveproblems. Then, the condition is debilitating. What is common to those on all parts of the so-called autistic spectrum is that they are more often men than women—so much more oftenthat one school of thought suggests autism is an extreme manifestation of what it means,mentally, to be male. Boys are four times more likely to be diagnosed with autism than girls are.For high-functioning autism, the ratio is seven to one.
Moreover, what is true of autism is true, to a lesser extent, of a lot of other neurological andcognitive disorders. Attention deficit hyperactivity disorder (ADHD) is diagnosed aroundthree times more often in boys than in girls. “Intellectual disability”, a catch-all term forcongenital low IQ, is 30-50% more common in boys, as is epilepsy. In fact, these disordersfrequently show up in combination. For instance, children diagnosed with an autistic-spectrumdisorder[1] often also receive a diagnosis of ADHD.
Autism's precise causes are unclear, but genes are important. Though no mutation which, byitself, causes autism has yet been discovered, well over 100 are known that make someone withthem more vulnerable to the condition.
Most of these mutations are as common in women as in men, so one explanation for thedivergent incidence is that male brains are more vulnerable than female ones to equivalentlevels of genetic disruption. This is called the female-protective model. The other broadexplanation, social-bias theory, is that the difference is illusory. Girls are being under-diagnosed because of differences either in the ways they are assessed, or in the ways they copewith the condition, rather than because they actually have it less. Some researchers claim, forexample, that girls are better able to hide their symptoms.
To investigate this question, Sebastien Jacquemont of the University Hospital of Lausanne andhis colleagues analysed genetic data from two groups of children with cognitive abnormalities.Those in one group, 800 strong, were specifically autistic. Those in the other, 16,000 strong,had a range of problems.
Dr Jacquemont has just published his results in the American Journal of Human Genetics. Hiscrucial finding was that girls in both groups more often had mutations of the sort associatedwith abnormal neural development than boys did. This was true both for copy-number variants(CNVs, which are variations in the number of copies in a chromosome of particular sections ofDNA), and single-nucleotide variants (SNVs, which are alterations to single genetic letters inthe DNA message).
On the face of it, this seems compelling evidence for the female-protective model. Since allthe children whose data Dr Jacquemont examined had been diagnosed with problems, if thegirls had more serious mutations than the boys did, that suggests other aspects of theirphysiology were covering up the consequences. Females are thus, if this interpretation iscorrect, better protected from developing symptoms than males are. And, as furtherconfirmation, Dr Jacquemont's findings tally with a study published three years ago, whichfound that CNVs in autistic girls spanned more genes (and were thus, presumably, moredamaging), than those in autistic boys.
The counter-argument is that if girls are better at hiding their symptoms, only the moreextreme female cases might turn up in the diagnosed groups. If that were true, a greaterdegree of mutation might be expected in symptomatic girls as a consequence. However, DrJacquemont and his colleagues also found that damaging CNVs were more likely to be inheritedfrom a child's mother than from his or her father. They interpret this as further evidence offemale-protectedness. Autistic symptoms make people of either sex less likely to becomeparents. If mothers are the source of the majority of autism-inducing genes in children, itsuggests they are less affected by them.
None of this, though, explains the exact mechanism that makes boys more susceptible thangirls. On this question, too, there are two predominant theories. The first is that males aremore sensitive because they have only one X-chromosome. This makes them vulnerable tomutations on that chromosome, because any damaged genes have no twin to cover for them.One cognitive disorder, fragile-X syndrome, is indeed much more common in men for thisreason. Dr Jacquemont's study, however, found only a limited role for X-chromosomemutations. That suggests the genetic basis of the difference is distributed across the wholegenome.
The other kind of explanation is anatomical. It is based on brain-imaging studies whichsuggest differences between the patterns of internal connection in male and female brains.Male brains have stronger local connections, and weaker long-range ones, than do femalebrains. That is similar to a difference seen between the brains of autistic people and of thosewho are not. The suggestion here is that the male-type connection pattern is somehow morevulnerable to disruption by the factors which trigger autism and other cognitive problems.Why that should be, however, remains opaque.