Douglas Murray is a modern exemplar of how simplistic framing of IQ is problematic. He and Sam Harris defending his views, show a real lack of critical thinking about terms and methodology around intelligence. They are still reacting like they can't see any problem with what they've been saying, despite very widespread criticism.
IQ is quite good for measuring impairment, eg. from poor nutrition, cognitive damage etc. However it's very problematic at quantifying high 'intelligence', because of general problems with the tails of distributions, and dispute about what high intelligence is. Murray glosses over high IQ people who have unsuccessful lives, saying because they are clever they choose a wide range of lifestyles, which just side-steps the weak correlation at the extreme. One of Murray's arguments for the validity of IQ as measure of intelligence is it's correlation with earnings. But what about the lack of correlation at the high end of IQ, with deep insight, or significant intellectual achievements?
There's a really interesting paper: A dynamical model of general intelligence: the positive manifold of intelligence by mutualism. This outlines how we should think of intelligence as like the health of an ecosystem. We might take the concrete measurable 'biomass' as a proxy for abstract 'health of ecosystem', analogously to taking IQ as proxy for intelligence. It would be good at picking up the main kinds of unhealthy ecosystems: where there are few niches, few biological strategies active. But it would rate some kinds of unhealthy fragile & unresilient ecosystems very highly - like where algae has choked out everything else, or where a small number of species (strategies, equivalent to types of mental algorithm in this analogy) dominate. Truly healthy resilient ecologies are diverse, self-stabilising, have many niches and a wide range of strategies - usually including ones unique to that ecosystem, but also often convergently evolved strategies.
So not just biomass but biodiversity compares to true intelligence, while IQ only captures the equivalent of the former, in terms of the capacity to solve unique problems rather than just a narrow set used as proxies for all (practicing IQ tests improves scores, so does motivation like rewards for doing well & local cultural regard for high scores). Generating fittedness of a species to a niche, requires an ecology that developed against the right mix of stability & challenge to result in the most effective creatures/insights/algorithms. Equivalent to a good varied challenging but supportive learning environment during an individuals development.
Growing up in a house with at least 80 books has lasting impacts. Not only plentiful resources though, but also challenge are required to develop a resilient ecosystem: it's interesting to note success in competitive debating is a better predictor of career success than IQ or grades across a wide range of jobs and in management, but having such debate clubs has typically been limited to elite schools. Cultural priorities and values affect developmental environments.
If there were no consequences to accepting a simplistic definition of intelligence fine, leave the details for academia. But there are many problems, especially denying people's ability to grow and flourish and develop their intelligence by essentialising it as fixed - this is captured with ideas around the benefits of a growth mindset, where research has shown attitude is more important than IQ for many tasks.
It seems obvious also, that extreme intelligence requires a culture of respect for it, and special dispensations toward odd people for communities to fully benefit - and modern society is typically bad at that, and historically has failed to recognise a problem.
I'm thinking of the mathematician Ramanujan who had a cultural framework and support to totally dedicate himself to mathematics, as a form of religious devotion. Or Einstein (who's developmental language delay could see him diagnosed as on the autistic-spectrum now) as coming from a Jewish heritage, where rich families considering it a religious service to marry exceptional scholars into their family, despite lack of fiscal prospects (see the focus of the modern Haredi community on prioritising full-time religious scholarship for men over paid work). Intelligence occurs within a social ecology, as well as being manifested through a kind of personal mental ecology. And we would benefit more as a society through valuing neurodiversity, the equivalent of biodiversity, than by valuing a narrow picture of the mechanism and function of intelligence.
Essentialising people as having fundamental unchanging differences is intrinsically problematic, it goes against have a growth-mindset, and acts contrary to intersubjectivity which supports intelligence (discussed here: How would you apply John Rawls "Theory of justice" to everyday decisions?). Being good at putting ourselves into the mindset of others is the basis for most uniquely human intelligence, and picturing minds as fixed undermines that.
IQ is good at picking up people who underperform, usually because of developmental problems. And for picking up population change - the Flynn Effect is thought to be principally linked to improved diets and more highly enriched environments. But the human genome is far narrower than most species, because of repeated population bottle-necks. And intelligence has been useful in every environment. But, we evolved our intelligence focused primarily on our complex social environment (see the Dunbar Number linking neocortex size in apes to social group size), and that may be changing.
There is evidence autism-spectrum conditions are increasing. Children on the spectrum are substantially more likely to have parents who are engineers, accountants, in medicine or science ("risk of having a child with ASD was almost two times greater for mothers in highly technical occupations" from here), and similar correlations between having close family on the spectrum & success in STEM fields. This may indicate a trend of our species reallocating our intelligence from primarily social challenges, towards prioritising technologically useful skills. So not so much increasing intelligence in technological societies, but repurposing it. And, IQ clearly values technologically useful intelligence over social (I linked the change in values to almost abandonment in modern philosophy of considering wisdom here Wisdom and John Vervaeke's awakening from the meaning crises?). That is the result of a cultural judgement on what intelligence is useful.
Inter-group variation in IQ is almost invariably far smaller than in-group variation (given good nutrition & other base-line factors like not growing up in a war zone). While IQ weakly correlates with earnings, I suggest a society's success has a lot more to do with the occasional person of exceptional insight than it does to population averages. And the success of people like that, depends crucially on being in societies that support neurodiversity - I'd argue over millennia, Sephardi Jewish communities and Hindu communities have been better at valuing these (eg early Buddhism had over 10,000 sutras all memorised not written, and special sirnames permanently honoured families with an exceptional scholar of the Vedas). Valuing and supporting unusual minds, is what we should prioritise culturally, not IQ.
The computer coding world is leading the way in this, and their practices and adaptations should be learned from for schools, other employment sectors, and society as a whole. Biologist Robert Sapolsky has interesting ideas on the origins of OCD & schizotypy disorders as extreme cases of mindsets that have historically benefitted societies. Intelligence is a landscape to be explored, not an unambiguous direction. And we should pay attention to positively shaping the landscape in so far as we can, rather than pushing people in one direction through it.
Additional relevant discussions:
What is intelligence?
Are Life and Intelligence analogous?
Free Will and Intelligence
Are levels of intelligence different levels of consciousness?
AI singularity, and the transmission of intelligence