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For years scientists in psychology and neurology have been pushing two rather contradictory ideas:

Psychologists:  Intelligence is like height.  Very stable and genetic, especially after puberty.  Aside from pathological cases like organic dementia or brain damage, the IQ you have in youth, you pretty much die with.  Even when low IQ kids are given the most extreme cultural enrichment imaginable, their real world intelligence doesn’t improve. See comments in below videos from Jordan Peterson:

Neurologists: Intelligence is like a muscle.  The more you exercise it, the stronger it becomes.  The brain is marvellously plastic.  Every time you learn a new skill you alter the chemical and physical structure of the brain.  It’s even possible to largely recover from brain damage.  See video below with Lara Boyd:

How can both be these views be true?

I think overall intelligence (what IQ tests try to measure) is almost as hard to change as height, but all the specific parts of intelligence (mental arithmetic, sense of direction, understanding irony) are like muscles that can be exercised.

Every time you exercise a part of your brain, that region gets bigger, just like every time you exercise a muscle, that muscle gets bigger.  One difference is, muscles are outside the skeleton, so they have room to expand indefinitely, but the brain is inside the skeleton, so its expansion is limited by cranial capacity.

Thus, the only way to make a part of the brain bigger is to make another part smaller.  So while you’re exercising your arithmetic IQ, your sense of direction IQ is slowly atrophying.  Start exercising your sense of direction IQ and your arithmetic IQ decays.

So while very specific parts of intelligence can be greatly improved, overall intelligence is limited by the size of the cranium and many other very finite resources.  So when you get a university degree, learn a new instrument, or acquire a new language, you haven’t actually made yourself much smarter overall, you’ve just reallocated cognitive resources from one ability to another.

So when low IQ children are adopted into extremely enriched environments, their IQs do shoot up but it doesn’t much translate into real world intelligent behavior, because all they have done is invested all their brain power in abilities measured by the test, but they haven’t actually increased the amount of brain power, so when new learning challenges inevitably show up,  they’re right back to where they were before the intervention.

Arthur Jensen referred to such IQ gains as “hollow with respect to g”, the general factor of intelligence.  He found for example that adoption into the upper class would improve the IQs of children from lower class homes, but the degree of improvement was uncorrelated with the g loadings of the specific tests.  It was hollow with respect to g, and he predicted that high IQ upper class adopted kids would not do as well in later life as their equally high IQ non-adopted siblings, because the former high IQ was hollow, while the latter was flowing with genetic g.

On the other hand, James Flynn argued g was irrelevant, citing the example of fetal alcohol syndrome (FAS) where IQ is obviously impaired with real world effects, but the degree of impairment is unrelated to g.  However I’d argue FAS is a pathological case, and thus not relevant to normal biological functioning.