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It's free and available to everyone online. Check them out at Give Weblog. Welcome to, rationally speaking, the podcast, where we explore the borderlands between reason and nonsense. I'm your host, Julia Gillard, and my guests today is Stuart Richi. Stuart is an expert on human intelligence and cognitive ageing. And maybe to introduce him, the best thing is just to read you his Twitter bio. Stuart is a postdoctoral fellow at the psychology department of the University of Edinburgh and looks like a cartoonish, startled hedgehog.

Stuart, welcome to the show. Hi, how it doing?

I just have to tell you that what I love about your Twitter, I mean, I love your Twitter presence overall. Thank you.

But one particular thing that I love is how the photo of you in the photo that you posted on Twitter, you're sort of staring horrifyingly down at the lower right hand corner of your screen, which makes it look with every tweet you post, it makes it look like you're horrified by what you've just written because you're staring at it in dismay.

Yeah, well, that often is often something, you know, annoying or depressing picking up there. You know, someone who's interpreted a paper or there's a rubbish scientific paper or some. Yeah, some of that.

And you're like covering half your face almost in a oh God expression. Yeah.

I also like I couldn't bear to put up a serious picture because I just can't I just can't bear the thought of posing for a serious picture and putting on that one.

So you definitely avoided that until you're married. Okay. So, oh, I should also add that Stewart recently published a great book with a somewhat misleading title. The title is Intelligence All That Matters. Now, the reason that's misleading, I learned, is that when you first read the title, you assume it's saying that intelligence is the only factor that matters in determining someone's success or a society of success. But in fact, what I discovered is this book is part of a series of short explainer books called All That Matters.

So they have titles like Bioethics, All That Matters or the Future, All That Matters. And so in context, it's clear that all that matters refers to, you know, we're going to tell you everything important there is to know about this topic and not this topic is the only thing that matters.

It's extremely unfortunate. And I've had to start with many conversations. Get out of the way. Yes, I'm glad you did. Yeah. In this case. Yeah.

Yeah, I was surprised at first because I don't think that Stuart Richey believes that intelligence is the only important thing determining success. Just based on what I've seen him post on Twitter. I'm surprised that he chose this title for his book.

I have a say in the book. It's like all the matters. I don't know, actually not right. It's an unfortunate aspect.

I know it's just especially unfortunate for for intelligence just because. Yeah, people are so sensitive to that. Miss, like, a lot of people think that that's something proponent's like like researchers of IQ science actually believe that intelligence is, in fact, all that matters. And so it's kind of a, you know, a strawman that the title inadvertently pops up. Anyway, so the there's a couple of things I wanted to talk to you about. The first is conceptual objections to the idea of IQ testing as opposed to, for example, ethical objections to the idea of IQ testing.

And the first and maybe most basic objection that many listeners will probably have heard before is we can't even define intelligence. So so it doesn't make sense to have a test that's claiming to measure it. What do you say to that?

Yeah, well, I think I think the standard objection to that is that you can have something which is useful and a useful tool without actually having a definition of exactly what it measures. And what kind of classic example is is the example of of temperature. Right. So we knew that the mercury rises and the thermometer when it's hotter and falls when it's when it's colder. Long before we had the kinetic theory of gases, that that that tells us exactly why that's happening and the movement of molecules.

This is what underlies temperature. And so thermometers were extremely useful. We before we had that theory. And so having a definition isn't necessarily useful for all purposes. Of course, it would be great to have, you know, brain level or sign ups level definition of intelligence. And that's something we're kind of working towards. But that doesn't mean that the actual IQ tests aren't measuring something that we can then use to predict people's success and, you know, in education or in occupations or in everyday life and so on to to some degree.

And they absolutely do measure that. And so when people say, you know, you don't have a definition of intelligence and intelligence is just it's just, you know, defined as what the tests are testing, that's kind of okay to me. That seems absolutely fine. We know that this is a useful thing and we can use it to predict stuff. And that's kind of all we're claiming at this point. And people have come up with with with definitions.

They've trying to give definitions that range from quite vague to, you know, trying to be more theoretical, you know, sound in a kind of a psychological or sometimes neuroscientific theory. But at this point, I think the data just aren't good enough to really make a strong theory of exactly what intelligence is. And I think we would be kind of wasting our time if we if we try to do that. And at some point we'll have the data that we can that we can really make a good stab at that.

But I don't think that's No. So a variant on that objection would be the argument that there are multiple different types of intelligence and IQ just sort of captures one facet of what we colloquially see as intelligence.

So, for example, there's also emotional intelligence. There is creativity, spatial reasoning, et cetera. So this argument goes IQ scores may predict some important things in life, but we shouldn't talk about it like it's the only or even the most important kind of cognitive trait. Yeah.

And, you know, that's that's if you just want to define stuff as intelligence and call stuff intelligence, then that's totally fine. And I think that's absolutely that's absolutely okay. So if you look at someone like Howard Gardner, who is a professor at Harvard, who came up with the idea of multiple intelligences, who has all these these different intelligences, like there's there's analytic intelligence, which is kind of more like the stuff that we talk about when we talk about IQ tests.

But there's also a whole bunch of other stuff like interpersonal and intrapersonal intelligence. And he's added new ones, like I think there's one called a naturalistic intelligence and existential intelligence and all these kind of existential and I'm sorry, I have to pause you and ask what that is.

Well, it's not like it's defined on the basis of data or anything. I mean, Howard Gardner kind of has just decided that these intelligence is our intelligence. So this one, he says that it's the use of collective values and intuition to understand others and the world around them. No, I meant to be like it's about seeing the big picture and that is existential intelligence. And there's also a whole bunch of other ones. You know, there used to be seven.

I think there are nine. And instead of, you know, collecting loads of data and running factor analysis to see where the patterns in the data lie, which is what generally people in the world of intelligence do, Gardner is going to come up with these with these things, which he kind of thinks are intelligence. And then there's other people who who say things like, well, there's also common sense, which is separate from intelligence and rationality, which is separate intelligence and and wisdom, street smarts, as opposed to book smarts, for example.

Yes. Yes, exactly. And and you can totally define these as as intelligence is if you want. And that's and that's fine. But I think a lot of these involve things which you wouldn't necessarily consider intellectual abilities. So a lot of them involve things like personality. So, for instance, something like emotional intelligence. Clearly there's going to be a kind of analytic intellectual aspect to that, which is, you know, your speed of understanding other people's emotions and your ability to represent their mental states.

And some people are going to be better at doing that in others. But also there's an aspect of of of of personality in there. So an aspect of how much you you like talking to other people and and how much you can intuitively empathize other people and so on. So it seems to me that there's a there's a useful separation between the analytic type stuff and the personality type stuff. And those are the kind of the two prongs of individual differences, psychology.

There's there's intelligence on one hand, there's personality on the other. And an emotional intelligence is kind of like a like a perfect mix up of the two of them. You know, it kind of it kind of includes aspects of both. And I think it's it's probably more useful to talk about them separately. And in fact, there are studies showing that once you take into account personality as measured on personalities and intelligence, as measured in IQ tests, emotional intelligence doesn't predict much more of people's, you know, job performance and so on.

So it's a kind of combination of the two of those. And if you want to call lots of other things intelligence, then that's fine, I guess, in my view is more useful to describe or at least things a skills. So, you know, the the sort of naturalistic intelligence that Gardner talks about is like a like an intuitive understanding of the natural world and enjoyment of the natural world. That's something you can work on. And some people clearly have more interest in that than others.

But I would call that a skill or an interest more than intelligence. This is totally immaterial to much of the sciences, is just people playing around with definitions. What I'm really interested in, what most intelligence researchers are interested in is the really rigorous stuff where you've collected lots of data, you've done analysis to work out what goes with what's in intelligence. It really is the case that when you give people pretty much any cognitive task, they do correlate positively together.

And you get this general intelligence factor, which explains, you know, about half of the reasons people different across all mental tests and are not general intelligence is something which people don't really like to think about too much. But it's that. But it's but it's there. And this is this is what Gardner's theory is often held up in opposition to. And so I think it's absolutely fine if Gardiner wants to say there are different multiple intelligences. But it's when you say there's no such thing as general intelligence that that you came to.

With that, we come up against the data. So to push back on that a little bit, let's say let's say we just agree that all the cognitive traits, verbal skill, math, skill, creativity would count as a cognitive trait. I think let's say we agree those are all correlated with each other and with IQ scores. Still, isn't it the case that you could design a variation on the IQ test that would give more or less weight to say creativity compared to math skill?

And then the new score call it IQ prime would still be correlated with all these different cognitive skills or traits, but it would be more correlated with some and less with others. So isn't it true that we're implicitly prioritizing some types of cognitive trade or skill over others just in the way the test is designed?

To some degree, but not once you get to a really, really good intelligence test, which includes loads and loads and loads of different cognitive abilities. So there are two papers by if I went to Johnson, who is a professor here in Edinburgh, and she took these amazing datasets with people who had taken dozens and dozens and dozens of cognitive tasks that covered everything from just how quickly they could they could check their letters off a sheet to vocabulary tasks and everything and everything in between, you know, including arithmetic and mental rotation and that.

Absolutely, absolutely. Everything you can think of that would come under the rubric of a of a cognitive task. And she looked at the general intelligence factor that you can extract from these datasets. So you had an intelligence test, one which has maybe 12 tests, and then you had intelligence as to which has another completely separate twelfth test. The general intelligence factors that you extract from those datasets are almost identical. That is when you take just what is common to lots of different kinds of tests, even if they differ quite a lot.

When you take just what is common, that general intelligence factor seems to be something very similar across all different attacks to the general intelligence factors that were extracted from those batteries of very different batteries, of tests correlated together at like point nine, nine or sometimes one. So, you know, they were identical and for all intents and purposes. And so these two papers are called this one called just one G. And then she replicated again and another sample and it's called still just one G.

I really hope she keeps this up. Yeah. Yeah. I've ever just one G. Yeah, yeah, yeah.

It's like when you when you get to the very high level, there really does seem to be just, just one G m at lower levels. If you just give people a couple of tests, you know, if I gave one sample of people just verbal IQ tests in your vocabulary test and like a similarity test, you have to say like what is and what do a dog and a rabbit have in common, you know, that sort of test? And then I gave another sample of people just test it, what you do with speed to, like, press your finger on the button.

When I was on that kind of test, then obviously, you know, we didn't we would expect that those would be similar to general factors that came out of those of those tests, but we wouldn't expect them to be the same. But the point is that if you aggregate across a whole range of mental skills, it really does seem to be something that that's common to them all. And that's what is theorized to be the kind of the general core of of of intelligence.

No, there's lots of different theoretical reasons as to where that G comes from. And there's developmental theories of that. And people have been arguing about this literally since nineteen or four. But at the high level, the general factor does seem to be the same. So. In answer to your question, yes is right, but if you if you give someone a test that's biased towards one particular kind of item and that is one type of cognitive skill, then sure you're going to you're going to have problems.

But on the other hand, the best intelligence measures are kind of the more the merrier perspective for you, just adding in as many kinds of skills as possible.

I think the part I'm still having trouble with is this idea of what's common between the different tests. So, like I could imagine, you know, two people, Devon and Taylor and Devins, like, they're like two standard deviations above the mean on creativity, but just average acts like mathematical reasoning skill. And Taylor is the opposite to standard deviations above the mean at math, an average on creativity. And you give them both a bunch of tests that test both creativity and math skill.

Do their IQ scores end up the same or like is there a way is the question of who gets a higher IQ score sensitive to like how many creativity tests you give one versus the other, or.

Yes, the type of test is important too. So, you know, there are some tests which seem to measure general intelligence better than than others. So, you know, something like that vocabulary seems to have a very high what we call gloating. So it's very strongly correlated with the eventual chief actually get out of that test, plus a whole bunch of others. It seems to be a very good indicator of general intelligence, whereas there are others which are less good.

So something like a simple reaction time. So just pressing a button when the light goes on, it doesn't have a very high gloving because it's it's a very simple task. It's not it doesn't get that complex thinking. That said, gee, is supposedly, you know, is measuring. But but, you know, another way of looking at that question is people have different interests and people have different skills. The General Franks are only accounts for half around about half of the variance when you give in most batteries of tests of cognitive tests.

And there's a whole bunch of variance that's that's left over, which might be to do with people's specific skills on particular kinds of tests. Their interest may have led them to develop a particular skill more strongly than others that maybe let their their math skills, you know, slide a little bit while they've been focusing on verbal stuff. And that might be the result of a kind of initial preference for that stuff or an initial higher skill level in those areas.

But it kind of a kind of that through a kind of set of feedback loops means that they end up spending most of their time doing verbal stuff instead of math stuff. Other people end up doing more math stuff and more verbal stuff. And so it looks like they have big differences in their common abilities. But actually, it would be possible if you had, you know, theoretically possible if you had gone back and maybe nudged that person more towards math stuff, they would they may well have come up with the same level of ability at the end.

So that's I think a common misconception is when people talk about general intelligence is they think that that's all of the variation, but actually it's just about half of it. And so there are specific skills to that. Those are interesting to think about as well. You know, what is what is a specific skill? What is your specific skill and verbal ability that's not to do with your general intelligence. OK, I'm going to give you one more variant on this.

This class of objection, this is I'd say this is probably the most frequently cited critique of IQ science among people that I know. It's a long essay by someone named Cosmo Elizee in which he argues that IQ basically argues IQ is just a construction. So I have a quote from him here. The correlations among the components and intelligence test and between tests themselves are all positive because that's how we design tests. So making up tests so that they're positively correlated and discovering that they have a dominant factor is just like putting together a list of big square numbers and then discovering that none of them as prime.

It's a necessary side effect of the construction and nothing more. So this is basically a more formal way of saying the old quip that the only thing IQ tests measure is how well you do on IQ tests. What's wrong with that reasoning?

I think anyone can come up with a scientific finding and say, well, that's a finding if we want to. And and that's, you know, that's their opinion. But actually, I think it is interesting that no matter what people come up with, no matter what kind of cognitive skill they try to measure or test, that's correlated with other controversies. People have deliberately tried to come up with new cognitive tests that don't correlate with with IQ. And as I say, you know, there are some that have better or worse.

Gee, that is their correlation with the general factor. But but but in general, that's really, really difficult to do. You can't find controversies that that aren't related in some way to to to gee, it just it just gets into everything. And I think that's I think that's actually really a really interesting finding about human psychology that that to a very large extent, intelligence seems to be general. Our ability seems to be general. Yes.

And so you can you can see well, we decided to test that way. But actually, people have tried their best to design tests that aren't that way. And it still falls into this correlation with Genom. Are they trying?

Yeah, I think that's the crux of disagreement between you and and the Shalvey camp on this issue is that they seem to think that there are tests that we have devised or that we could have devised that are not correlated with the IQ tests.

And, you know, go ahead. Go ahead and collect some data on it. I'd be. I'd love to. I'd love to see. That's interesting.

Noah Smith, the economist who blogs that no opinion and writes for Bloomberg, he proposed a theory a few years ago in response to Shellie's his essay for why it could be possible for there to be multiple different intelligences that are not actually correlated with each other, despite the fact that all the all these cognitive tests are correlated with each other. So his reasoning was suppose that there are these very simple mental skills or like basic mental skills that are substitutable for each other.

So like, quote, in other words, suppose that any simple information processing task could be solved using spatial modeling or solved using symbolic modeling or some combination of the two that would result in a positive correlation between all simple information processing tasks without any dependence between the two mental abilities, end quote. Does that seem plausible to you? Do we have any evidence of whether or not that's true? So that's not an theory that's that's going to be Thompson's theory from the 1930s.

Maybe it was independently discovered.

Yeah, well, not possible, but that is that's known that Secretary Thompson, who was an intelligence researcher and statistician at the at the start of the 20th century, was very, very influential in, for instance, the British education system. And he came up with what he called the Bonds theory of intelligence, which is which was in opposition to Spearman's G. Factor theory. Charles Spearmon was the guy who first noticed the positive correlations among his s and his.

His idea was, well, there's just one underlying general intelligence, some kind of what he called mental energy, that that that that allows the causes, the the the factors to to the skills to correlate positively together. And that is a perfectly viable explanation of the correlations in the tests. And it's just that there are other explanations, too. In that theory that you just described there in the courtroom from Will Smith, is that and is is what Godfrey Thompson came up with.

And they kind of argue with each other for a long time about it so that the brain might have lots of different kind of very basic processes and that they are differently. What he called samples by the by the IQ cognitive tests that are given. And essentially the debate is continued. And we still don't know whether Spearmon or Thompson was correct about that, because it doesn't really seem to be a good way of of of testing that. I guess the first thing I should say is that this doesn't actually matter in terms of the practical consequences of of intelligence.

That's right. So we know that Thompson would have completely agreed and even someone like Kuzmich legally would agree that there is a general factor, that it's that it is that it's sits there and summarizes people's performance across lots of text. Now, the reason for why it happens is a different question, but that general functions there is still predictive of all the stuff that we know IQ tests are predictive of. And so it's still it's still useful as a as a as a measure.

So this is a this is a kind of an orthogonal question to that, which is why do these attacks correlate positively together? And, you know, we're just going to have evidence on that until we have a better brain level understanding of intelligence. You know, and there's papers from mathematical modeling, papers from kind of 2009, 2010, and that's that my peer pressure and Deery worked on. And he was a big fan of Godfrey Thompson. And so he thought, well, we'll try and run some mathematical models that compare Thompson's theory to Spearman's theory.

And the conclusion of those papers is they're both completely viable models of how the G factor exists. So we don't really have evidence to clinch that debate on either side. But I do think it's a mistake to to put that up as a as a response to someone who's saying IQ tests, performance correlates with loads of stuff and is predictive of all the stuff and is correlated with biological variables and so on. And I think a lot of people link to that, Scheelite, see, see and linked to the idea of the bonds theory of intelligence, so on as a kind of a knock down against the practical validity of IQ tests.

And that's completely irrelevant to that practical validity.

One thing that I've noticed in the in the discussion of there being multiple intelligences is that there's like two things that people are conflating. One is the idea of these different intelligences being distinct and the other is them being uncorrelated. And so people will often say, like, look, there are different intelligences and they'll give evidence that there's different cognitive skills, are distinct from each other, don't like all, correlate perfectly. And then they sort of there's like the slippery thing they do where they they act like they've just shown that those intelligences are uncorrelated with each other.

So they'll say things that like imply those things are uncorrelated, like, you know, we can't like you can't learn anything from an IQ score. So, you know, the fact that there are these multiple components debunks IQ or something like that. But in order for that to be fair, you would have to show that they like there's no common factor, right? Yeah, absolutely.

And it's a common problem in papers. So I review a lot of papers where the you know, maybe it's a neuroscience paper where we've got brain scans or it's a genetic paper. And they say, well, we've measured, say, working memory or we've measured mental rotation or we've measured some of their some some specific cognitive ability. But the question is, have you and if you've only given one test, it's very hard to see whether you're actually just measuring some general ability or you're measuring something that is not necessarily the name of that test.

Just because we call a test your mental rotation doesn't mean that it it doesn't also include, you know, some sort of visual perception aspects and some some reasoning aspects and so on. We haven't got tests that can really pinpoint specific cognitive processes, you know, at this point or I suspect ever, because there's going to be lots of different kinds of processes that are involved in performance and not on any individual context. And so it's very mistaken newspapers to say, well, we've measured you know, we found the brain areas that are responsible for mental rotation as well.

OK, but you have to check those brain injuries aren't just involved in all kinds of tasks as well. So so that the fact of the correlation shingled of tasks and confuses even scientists, not just people who are who are arguing against IQ tests or whatever, it confuses people who are interested in intelligence and are trying to measure it at the level of the brain and and so on. And it's a common thing that I see and in reviews. Interesting.

I'm going to shift tracks now to a different kind of objection, which is how do we know that what the test is measuring is cognitive ability as opposed to other things like motivation to do well on a test or familiarity with the kinds of questions that you get asked on an IQ test, like abstract questions?

I think there's multiple different ways of looking at that. So I think, first of all, there is going to be an aspect of motivation and there is an aspect of familiarity with with any test. And, you know, a good test will make sure that the person that they're testing is not looking at the window and completely uninterested in what they're doing. And, you know, we'll try and we'll try to focus people's attention on that. On the test.

Much of the data from IQ tests come from high stakes testing. So you'd expect people's motivation to be fairly high for that. But if you ask people to fill in questionnaires about their level of motivation and also stuff like self-control and things like that and concentration levels and partially out of of the IQ test, when you when you then get eventual score, some of the variation is due to those is likely due to motivation. It makes perfect sense.

And also when you give people the same test again and again and again and again, the the special effects. So people get better when they when they practice the specific kind of, you know, the specific test and also the specific kind of of test. You know, you can raise your IQ by four or five points. I think the most recent net analysis shows if you just do some tutorials on that particular kind of test. So, you know, anyone who does IQ testing is well aware that these are that these are potential objections and will try in, you know, predictive tests to try and account for that by by measuring things like motivation and self-control and and so on.

And and, you know, try to pass that out of the of the analysis. But it is definitely something which is actually similar to my previous answer, where it's very hard to pinpoint one particular cognitive process, whether with a cognitive test. Well, it's actually hard to pinpoint cognitive processes, you know, full stop with a cognitive test because you're also picking up on all this other stuff. So what are the ways around that are just measuring the motivation?

Partially, as I say, making sure that the testing environment is not full of distractions and so on, but also testing people on multiple different tests and taking this this general factor phone? No, that's just what that has to do with that. All of the tests and removes the any error that might be to do with one test, which people are like, oh, I don't care about this, or I don't like this test. And another way is testing the multiple times as well in multiple different scenarios.

And this is, you know, increases the reliability of the of the measures in all psychology research. We have measurement issues and we know that humans are complicated and messy. And when you try and collect data on them, there's lots of different reasons for the the the way that the performance might be on any test. And so making sure that you're that you're increasing the reliability of your test by using, you know, well-known standardized tests rather than just ones you made up.

Suddenly, lots of psychologists use tests that they just made up in there and they're experiments and we don't know the reliability of and by taking latent variables and taking variables across time, you can you can sum up to some extent get around these problems. But, yeah, these are these are totally and completely accept those objections. They don't mean that there isn't a core of cognitive ability that you are actually measuring. And, you know, when people say things like, well, if you get better IQ tests, then then that means that the IQ test is not measuring intelligence.

Sure. But some people are going to be able to practice the tests more efficiently. And that might be something that we might want to call intelligence.

Yeah, I think a useful thing to do with all of these objections to conceptual objections to IQ tests is to try to apply those same objections to other things besides intelligence, like physical fitness, for example, has a bunch of different components that are correlated but distinct, you know, like running the speed or the number of, you know, upper body strength or things like that. That doesn't necessarily mean that it's meaningless to talk about physical fitness as a general thing.

And then also, in this case with practice, you can get better at, you know, the long jump or are, you know, your motivation to or like energy levels might vary from day to day. That doesn't mean that there's like a core underlying thing that it's meaningful to call physical fitness that that we could measure across multiple days, etc..

Absolutely, and, you know, with any human day, as I say, there's always going to be noise and some of that noise is going to be due to other other factors. But you testing large numbers of people on large samples of come tests. This is really the best way out of that problem, I think. And, you know, intelligence testing has really been pretty impressive in recent years in increasing the sample sizes of the tests and increasing the quality of the research.

Losing a huge amount still to be desired, as there is in all psychology, I think. But yet people are well aware of these of these objections. If you look through some some discussion sections of papers, you'll see limitations sections where people are like, well, you know, the people might not have not motivated because it wasn't a high stakes test. And so we need to take that into account and so on. But it doesn't mean that there isn't a core there of of actual cognitive ability.

And so we were just talking about changes like things that could have caused variation in an individual's IQ score test to test or over time. But then there's also variation over time in societies average IQ score. So the the Flynn effect, which I'm sure many listeners have heard of, is that IQ scores seem to have gone up. I think it's like 30 points over the many the multiple generations since we've been measuring it. Feel free to correct that number. But do you think that that's undermines do you think that that undermines not the idea that IQ is a thing we're measuring that's like meaningful, but does it undermine the IQ is an innate like, I guess a genetic thing?

Well, it's often used as a as a as an argument against IQ. Being related to genetics is something we haven't talked about. But there is a lot of evidence that, you know, variation in IQ scores is explained to some quite substantial extent by variation in genetics. We can talk about that. Yeah, if you like that. But yeah, the Flynn effect is often held up as a as a well, here's the environment influencing intelligence. And so therefore it can't be anything real or anything biological and and yes, with three IQ points per decade across the 20th century, there's some evidence that the Flynn effect is kind of is kind of leveling off in some rich industrialized Western countries, not so much in the developing world where it seems to be continuing apace, which is fascinating to see.

But this doesn't really say anything about the the actual ability of IQ tests to tell us something about people's abilities or people's success in their life, because it's a it's a mean level effect. There's still variation and the variation in IQ, if you imagine the sort of the normal curve, the bell curve of intelligence and the Flynn effect is simply that curve shifting long to write as the as the generations go on. There are many explanations that have been put forward for this better nutrition, more effective schooling, better health care and so on.

There's lots of different reasons why the Flynn effect might be might be occurring and both of the kind of societal and the kind of biological or nutritional level. But that doesn't say anything about the actual size of the the variation around that means so that meat is increasing, but there's still variation in that variation. Still tells you, you know, the people who are at the high end of that normal curve of intelligence are on average going to be the ones that do better at school but do better.

The jobs that live longer yields evidence of IQ being linked to longer lifespan. And and so it's fairly irrelevant where the for the millions around that. And we always knew from from twin studies and so on that there's a large environmental portion to the variation in IQ, environmental being, just stuff that isn't genes in this case. But there still is the case that if you measure IQ at any point along that Flyn cycle, you know, if you have if you have twin studies of IQ from the 1940s and twin studies like, you know, you get fairly similar results, which is that, you know, the monozygotic twins are much more strongly correlated with each other than the dizygotic twins are.

And from that, you can derive the fact that IQ is is is is heritable and that mean difference, that the full effect doesn't doesn't really see that in any way. You know, again, and just like you did there with the physical fitness thing, think about other traits that have increased in height, has increased across the 20th century. People got taller, taller, probably for similar reasons. You know, the nutritional reasons, for instance, that might that might have stopped kids being stunted and so on and generally increased height across across even healthy people.

And it doesn't say anything about whether height is heritable to to to make the observation that that that height has increased across the 20th century. And so I think it suggests that.

I mean, maybe the missing piece of the piece a lot of people might be missing is that a trade can be heritable, but its expression in a particular individual can still be influenced by the environment. So you could like you could think of like the potential for for height as the heritable thing. And then, you know, whether you reach your full potential can be influenced by, you know, whether you get adequate nutrition as a child. Yeah, it's the most common misconception when talking about heritability that people think that heritability is the same thing as, you know, and still immutable, never going to be able to change it.

And I think that's where a huge amount of the anxiety comes from. When people talk about heritability, not just of intelligence, but, you know, the other areas that are controversial about heritability, psychiatric disorders and so on. And they've been extremely controversial when people have talked about their heritability. Inheritability of schizophrenia, for instance, is something which has been, you know, debated hugely, although the evidence is very strong, that it's that it's highly heritable.

And that doesn't mean that you can't change it. So thinking back to hating your hate is strongly heritable, up to 80 percent heritable. You know, 80 percent of the differences in hate between people are due to differences in their genes. And and yet you can you can cause someone to be stunted if you if you malnourish them as a as a kid. And that's a completely environmental effect on their height, which we know has had trouble with. The other classic example that's given.

It's slightly different because it involves like a technology is that is is is short-sightedness right. So and we know that myopia is it's really strongly heritable, 80 to 90 percent heritable, and yet it can be cured instantly by one environmental input, which is wearing a pair of glasses. And it's immediately go on effectively. It's no longer a part of your life if you know, no matter what your genetic propensity for milk is. We can we can we can we can sort of know that doesn't mean that we have these abilities to some to suddenly boost someone's IQ.

We don't have those yet. That's that's not that's not in our technological abilities right now. But it's not theoretically, you know, the possibility of those technologies is not at all spoken against by the fact that IQ is it's a book called Intelligence and How to Get It by Richard Nesbitt's, in which I haven't read it, I'll admit.

But the summary is he's arguing that we don't have good evidence that intelligence is is significantly or mostly hereditary. Can you do you understand why you disagree?

Well, yeah, there are there are objections to twin studies, and so up until very recently, the evidence that we had that IQ was was was heritable was was twin and family studies. So as I said, you know, the monozygotic twins were the correlations. Like you said, the monozygotic twins were compared to the correlations for the dizygotic twins, that is identical twins and fraternal twins. And the identical twins always had much higher correlations. And from that, you can work out that a portion of the variation in IQ is due to genes and there are objections to that twin methodology.

There are problems, for instance, with well, maybe it's the case that that identical twins are actually treated differently from non identical twins, maybe people, you know, dressed their identical twins. Similarly, I see no objection. And and so maybe that has some aspect of an effect on how they see themselves and that might affect their IQ. I've never quite got the logic of that. I don't know why dressing people similarly makes their IQ more similar.

But and but that's an objection that's often made to twin studies. And people have written, you know, what length of the weaknesses of twin studies misclassifying cyclicity, you know, where you think you've got diagnostic twins and they're actually monozygotic. There are a whole range of of of debates and objections and back and forth in this sphere. But I think the consensus in the world of genetics is that whereas there are some reasons that the the heritability of of traits, not just IQ, but, you know, you can use one study to examine any traits, whether it's a disease like schizophrenia or a trait like height or trait like IQ.

There's some reason to believe that the heritability might be slightly inflated in twin studies. So it might be a bit higher than it can actually be associated with genes. But the constellation of evidence that we've got, not just from twin studies, but know these days from DNA as well, from direct testing of DNA. So the twin studies were really on the right track all along. So nowadays we have evidence where we can take people who are completely unrelated or as unrelated as any, you know, randomly selected humans are and and give them a DNA array that that you would text the variation to maybe, you know, four or 600000 points of DNA and give them an IQ test and essentially say, you know, are the people who are more similar in the DNA, more similar in their intelligence, and that gives a positive heritability number as well.

So you get a good chunk of variation is associated with with the genetic differences. And nowadays we're actually finding the specific genes as well that are related to variation in IQ. And those can be put together to produce a heritability number as well. And it comes well. So you have twin studies, family studies, study of twins adopted away into into different environments and so on. And the kind of whole, as I see the constellation of evidence hangs together really well, that there really are no genetic influences on variation in intelligence.

And as I've previously said, that doesn't say anything about whether we can change intelligence or whether it's immutable or whatever. It's just that's just irrelevant to that. But it does mean that when we're looking for the reasons that people are are similar right now or different from each other right now, some of that has to be to do with their their genetic makeup. And we're making progress, as I say, in identifying the specific points in the DNA, where there are papers coming out every few weeks now with large studies.

And we're talking hundreds of thousands of people that that are that are illustrating the the points on the DNA that are, you know, where there's variation. It's related to differences in test scores and also the other traits that are related like like education. So it's really easy to measure education. You just ask people how many years of schooling they had and then you can you can get you know, the next study is going to come out, has over a million people in it with the DNA test and information education.

And it's finding tons of of of points in the DNA that are related to better educational outcomes. And a lot of those overlap with with IQ as well. So this is a really exciting and fast moving field now. And to make arguments that, you know, well, actually, there's no genetic basis to IQ is really out of date. You know, that was an argument that was had in that. And I have to say one by the by the IQ is heritable, say, in the 1970s.

How do you have any is there any way for us to tell now, like what percentage of IQ or related things like like educational attainment will eventually be explainable, like we'll be able to find the genes for.

Yeah, well, we can take we can take the twin study estimate. As the kind of highest point, sort of like the the ceiling on how much variation in IQ is explained by genetics has to come from something like twin studies because twin studies measure all possible genetic influences that, you know, they're just they're right. They're fully formed people. We're not measuring. We're not trying to take a DNA test and actually measure all the different aspects of their DNA.

We're just saying, you know, it's a matter of relatedness and how genetically related you are to someone or not. So actually, that's the upper limit. That's the upper boundary. And so see, for IQ on average across studies, that's about 50 percent. That means that we won't get higher than that in in looking at the DNA studies in terms of heritability and the original DNA, DNA studies at the moment, if you just look at kind of common variations of variation that most people, you know, but maybe you see a quarter of people have have one lesson in the DNA where everyone else has the opposite letter.

If you look at that kind of variation and you can get up to 20, maybe 30 percent of the variation in IQ is explainable by that kind of variation. If you look at rarer genetic variants, you're getting much closer to that, to that ceiling, to that twin ceiling. So and, you know, there has been progress made on this. And we're getting towards the point where we can and we can certainly measure using DNA most to all of the of the of the genetic difference.

It's now finding specifically what those variants are is the next question. And that's, you know, we know even less of that. And then finding out the actual biological processes, the actual, you know, the functions of those genes and how they relate to other genes, try to relate to environment and so on, is the kind of massive landscape of work that genetics has to do in the next few years. But I think it's been a pretty exciting thing over the last couple of years, even since my book came out, where I just made the prediction that we would in the next few years find some, you know, replicated variants that are associated with with with IQ scores.

And that prediction was correct. But there's been so much more.

Besides, I have a question about your Twitter account. Again, there was an exchange on Twitter in which someone said, here's a tip for young people interviewing for jobs. Just take an IQ test and put that on your resume. And if they like, you know, still care about your degree or other things, then ignore them. You're not worth your time. And setting aside the strategic wisdom of that advice or lack thereof, you replied that you would not be interested in someone's IQ score if they applied to a job for you.

Why is that if IQ is so predictive of all these important things? Yeah, well, I mean, so I mean, the first thing is that, yeah, the kind of group level we have really strong evidence from big epidemiological studies that IQ is predictive of all sorts of good stuff and, you know, from from education to occupation. And as I said, you know, people with hierarchies tend to live longer for various reasons and all that.

So, you know, having a higher IQ is it's going to be better in that respect. What made me when I saw this person KVI, when they're applying for a position here and I was asked to comment on some of the CVI aspects when they include their IQ, there's lots of things that come to mind. So first of all, it's just kind of socially inept to to see the top of your CV.

My IQ is I can't remember what the number was. It was relatively high. I can't remember exactly what number was. And so so that's a signal not I mean, setting aside the fact that there's no there was no validation of that. There's just a number on this deal. Yeah. If maybe there was some transcripts attached to it from some independent body that verified this person's IQ and maybe take a bit more seriously. But I think it goes back to this question of is it all that matters?

Right. We know that people who are the individuals who can have a super high IQ can do really badly in lots of areas. And people who are who are maybe lower can can can be totally competent, all sorts of stuff to the discussion on where did meet me. Can you reconsider that? And I thought, you know, and there are some good arguments. So clearly people who have not had all the educational opportunities that they might have might have wanted and they maybe didn't have all the right contacts and we didn't have the right location, you know, in the world to to get the education that they they wanted or that would have been appropriate for them.

Maybe, you know, taking an IQ test is a kind of way into showing that they have lots of potential and that, in fact, the argument that people like Godfrey Thompson and the other kind of early IQ folks gave at the starting point essentially for changing the UK's education system from one where essentially, if your parents knew people or if your parents were rich, they got you into a good school. And and if not, you were into a really, really broken down old building with the poor teachers and harsh discipline and all that stuff.

And and IQ testing allowed people who were from poor backgrounds to to get into the grammar school so that the high quality schools know that system is no longer in place in the UK, mainly because the fact that the test the skills people used were sent to who who didn't get you, didn't pass the IQ test, essentially were so poor and it caused a huge amount of resentment and so on. But but yeah. So so that kind of idea, that's the IQ test can be a way of of being more socially kind of equitable is a very strong arm.

And that actually made me reconsider. You know, why why did I react like that to someone putting their IQ in reading? I think I think it just comes back to the social ineptness. I think it's it it stands out in a bad way that someone is is is kind of showing off about their IQ and sort of Donald Trump when he often tweets about his his high IQ and so on. And I think it's more a matter of for the individual, you know, having an IQ test in your DNA, if you were a scientist like me is it's fascinating.

And it's great to see, on average how people do. But bragging as an individual about you're really high IQ is is is inept.

Maybe we could solve this problem by allowing the allowing people to include their IQ score on the resume and then also include their score on a test of social skills so that they can avoid sending the negative thrill of bragging about their scores.

Yeah, yeah.

I think, you know, I think in a world where everyone had their IQ. Yeah, yeah. It would be ten in a minute and you know, and it was verified in some way and it was fine. But it's the fact that this person's student by doing that and and you might I think I think fundamentally the thing that you're actually interested in is how people have used their IQ. So, for instance, if you're applying for a job in science, then what you want to see is, you know, lots of really cool papers and lots of good courses on and and lots of lots of grants that they've applied for and got with loads of cool ideas.

And you read the papers and they're full of beautiful writing and interesting and, you know, ideas and so on. That's what you want, rather than someone just saying my IQ 145.

Tony Stewart, before I let you go, I wanted to ask you to recommend a book or blog. Or article or something like that that you have substantial disagreements with, but you still respect, like I think is worth engaging with for various reasons. Do you have anything like that you could recommend to our listeners?

Well, I mean, the thing that immediately comes to mind is the best measure of man, which is that Stephen Jay Gould book from 1991 up there in I think 95 ish that everyone's going to think with this guy is, you know, an IQ test proponent. Why? Yes, that is what I'm about. But, you know, I had I had calls to read again recently, and it's beautiful. It's it's beautifully written as opposed even Google Books, where it's well, actually not the big one, but the structure of evolutionary theory is is pretty dumb.

But other than that, they're all pretty exciting. And they, you know, includes loads of really important critiques of of intelligence. And a lot of them are not accurate. And, you know, you go and look at the actual the actual papers and the historical record and so on, and you find that they're not the case. It's been criticized for and he's been criticized for not. Fully, accurately representing the data in the case of the skull sizes that he talks about, the skull size comparisons that were made by Samuel Morton at the end of the 19th century, his criticisms there.

But there's a whole lot of stuff in that in that book that that is really important to know. And it has this kind of rubric of and I think he calls it learning, learning by debunking, which is, you know, you can you can learn a lot about a subject by looking at how is it is it has gone wrong or has been misconceived, you know, in the same way that you I think I think Gould actually used this example in the same way that you can learn a huge amount of evolution from watching a debate with creationists.

You know, critics are saying all these all these things and then biologists come in and and see why they're wrong. I find that really fascinating and an engaging way to to learn new stuff. And ironically, and you can learn a lot about IQ tests from from from learning what Gould himself got wrong. And and so I think there's a kind of meta debunking going on there that you can you can read his book, which is which is about debunking and learn how how the evidence is actually fairly strong in many cases for IQ testing.

But there are you know, there are also some good arguments in there. And as I say, it's an entertaining and fascinating read anyway. So I think anyone who is is truly interested in this subject should read that. But in the knowledge that, you know, as any science book that was written in 1995 or originally in 1981 has to be a time of great news, a huge amount of good evidence that it didn't exist back then. But also even then, there were there were mistakes and critiques of it.

But I think to fully appreciate the debate around this topic, that's one that you've got to read.

Great. Well, thank you for that. And thank you so much for for being on the show. Will, I'll post a link to your excellent book. That's like my favorite introduction to the field of IQ, science and intelligence, all that matters. And yeah.

Thanks so much for being on the show, Stuart. My pleasure. Thanks. This concludes another episode of Rationally Speaking. Join us next time for more explorations on the borderlands between reason and nonsense.