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Does having schizophrenia cause a loss of intelligence (as measured by IQ tests)? How does present research comment on this connection, if it exists?

Furthermore, is it possible to recover from this decline in IQ? If so, under what conditions?

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Regarding schizophrenia and loss of IQ:

I don't know how much we can say about the 'loss' of IQ in schizophrenia patients. If anything, it appears to me that a decrease in overall IQ is augmented by the cognitive deficits associated with schizophrenia. This isn't to say that you can't increase your IQ with certain interventions, which I will touch on briefly in a second.

But first, understand that IQ depends on a number of neuroanatomical factors. Among them is volume of grey and white matter in the brain, which is notably decreased at an accelerated pace in early-onset schizophrenic patients.

General intelligence (g) is also highly correlated with fluid intelligence and crystallized intelligence. Fluid intelligence generally correlates with measures of abstract reasoning and puzzle solving, whereas crystallized intelligence correlates with abilities that depend on knowledge and experience, such as vocabulary, general information, and analogies (see: Factor structure). Administered IQ tests, such as the Wechsler Adult Intelligence Scale (WAIS) are particularly concerned with measuring fluid intelligence. For this reason, a deficit in one usually mandates a deficit in overall IQ.

Fluid intelligence is more susceptible to brain injury, and in particular appears to be affected by both frontal lobe damage and deficits in working memory. There is evidence that suggests that patients with schizophrenia have dysregulated prefrontal cortices among other frontal lobe abnormalities. Furthermore, schizophrenia patients often have abnormal deficits in working memory. For these reasons, schizophrenia may be associated with impairments in cognitive / executive functioning and fluid intelligence, thus lending towards a lower overall IQ.

That being said, there are studies that have not found conclusive evidence that suggests a direct link between schizophrenia and low IQ in adulthood (though there may be some link in childhood).

Regarding the recovery of lost IQ:

Medications used to treat cognitive deficits (such as those used to treat ADHD) have repeatedly shown an increase in overall IQ score after long-term use, so I am thinking that medication would be the best route for IQ recovery. Note that this effect does not apply to otherwise high-functioning individuals. In other words, those without cognitive impairment tend to react to psychostimulant medication in ways that are wholly negative and harmful.

I also mentioned above that there is no clear link between adulthood schizophrenia and low IQ, though there is a 7-8 average point decrease found in children. This is similarly consistent with other neurodevelopmental disorders, and may be related to the malleability in the brain. In other words, time and dedication (possibly including brain training, though there is controversy surrounding its benefits) can help restore cognitive abilities that may be reduced or damaged due to schizophrenia.

On a related note, it is worth mentioning that there is a modern-day theory going around surrounding the topic of latent inhibition and schizophrenia. The assumption is that schizophrenia is linked to low latent inhibition (with LI referring to the ability to ignorant irrelevant sensory stimuli), and this persistent cognitive overload leads to psychosis, difficulty in focusing on cognitively-intensive tasks, and occasional creative genius. This effect appears to be reduced in patients who seek medical treatment for their schizophrenia.

With human subjects, there is evidence that acute, non-medicated schizophrenics show reduced LI compared to chronic, medicated schizophrenics and to healthy subjects, while there is no difference in the amount of LI in the latter two groups. [....] In summary, the basic LI phenomenon represents some output of a selective attention process that results in learning to ignore irrelevant stimuli. It has become an important tool for understanding information processing in general, as well as attentional dysfunctions in schizophrenia, and it has implications for a variety of practical problems.

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    $\begingroup$ You asked a great question. However, it is discouraged to include personal anecdotes on this site since individual cases can not necessarily be generalized to a population. $\endgroup$ – Vakalate Oct 9 '15 at 19:53
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Just a note to add, or summarise, the above.

Of course, results vary from study to study, and more still patient to patient. The standard iq loss, or developmental deficit, is usually estimated at less than 10 points, which is clincally significant, but not necessarily life changing. Working memory is often effected. But of course the illness / disability is a complex enough entity to make drawing any conclusions on iq loss or cognitive impairment in adulthood, on an individual basis, moot.

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