Welcome to unit six, week six, of this course in human behavioral genetics. Last week we used schizophrenia to illustrate the psychiatric genetic approach. This week I'm going to talk about intelligence as an example of a behavioral genetic approach to quantitative phenotype. Probably there is more behavioral genetic research on intelligence than any other phenotype around. And there's a lot of interest in intelligence. I recognize it's a potentially controversial topic. And I'll try to touch on some of the controversies as we go through this week. But part of the interest in, in, in intelligence is for many I, I recognize not all, but for many psychologists intelligence is a very important variable within the field. It's thought to be a very important contributor, not determiner, but contributor to important outcomes in life: things like how far you go in school how you achieve occupationally, how you're received socially. So because it has been an important variable within psychology it has been a focus of a lot of behavioral genetic research. Just like I began talking about schizophrenia by trying to characterize a little bit about the phenotype before we jumped into the behavioral genetic research on it, I want to say a little bit about what psychologists mean by intelligence before we begin to look at things like twin and adoption studies. And I'm going to do that I'm going to do that in this module by framing my discussion around three questions that are debated within the psychological field that concern intelligence. What is intelligence? Is intelligence unitary or multidimensional? And can we measure it? So let's begin with how to define or how the psychologists define intelligence. About every 20 years the American Psychological Association, APA, puts together a blue ribbon panel and brings them together for maybe a couple days and asks them to produce a consensus definition of intelligence. And this is actually a very difficult process and the last time they did it, I guess, was in 1996. And the outcome of that consensus effort is represented here. There's the publication on it. And this group then defined intelligence as the ability to do things like understand and use complex ideas, to learn from experience, to engage in abstract reasoning. And when you look at that, it, it probably makes sense to you. It looks fairly reasonable as a, as a definition of intelligence. But if you begin to think about it a little bit further, there is, I would argue, one kind of peculiarity about the way they go about defining intelligence. They're not defining a thing here, but rather, they're defining what intelligent individuals do. They engage in abstract reasoning or they learn from experience. It would be analogous to you asking me to define table and I define table not as a flat surface, horizontally-oriented with four legs, as an object, but I define table as something where I place my books or where I, where I go to eat a meal, as to what a table can do, rather than what a table is. One of the controversies around intelligence is that it is difficult to define, and thus these consensus groups, because it isn't defined as a thing, but rather as what scientists call a hypothetical or theoretical construct. All science has hypothetical, theoretical constructs. Intelligence is a hypothetical construct. It's not being defined as a thing, but rather, it's postulated as some attributes people have in order to explain certain observations psychologists make, that people differ in how well they do in school or how well they're able to manipulate material in memory or how well they're able to engage in complex logical reasoning. And the argument is that people, that what accounts for these behaviors is some unobserved hypothetical underlying theoretical construct that we'll call intelligence. Now, for some critics of intelligence the inability to define it as a thing, although admittedly neuroscientists are trying to identify aspects of our brains that might be the physical basis for intelligence at this point. But, nonetheless, the inability to define it as a thing is a source of criticism of intelligence research. But I, I'd like to point out that having hypothetical constructs is not something novel to psychology. It exists in all science. What is gravity? Can you define gravity as a thing? Gravity is a hypothetical or theoretical construct within physics. The, it's posited to exist, in order to account for the motion of celestial bodies. Is there one definition of gravity? No, there are multiple definitions, ranging from Newton's to Einstein's based on relativity theory to quantum mechanics. So gravity, just like intelligence, is not a thing so much as it's a theoretical construct that scientists postulates in order to account for certain observables. Even if you go back to the third week in this course, something that seems as straight forward as a gene, I talked about how geneticists have a hard time even defining gene. Even a gene at this point is a hypothetical or theoretical construct. So psychologists recognize intelligence as a theoretical construct, at this point not a thing. The second debate about intelligence is, is it one thing, or many things? And actually I think this is kind of a false debate. I don't think anybody really believes that intelligence is one thing. I think everyone recognizes multiple things. And it's really a matter of emphasis. Do you emphasize one thing? Or do you emphasize multiple things? Let's see what that point I'm trying to make is. Consider four different ability tests: a verbal ability test, a numerical ability test, a spatial test that might involve rotating objects in space, and a mechanical test. And we might think about how individuals taking this test, how their performance on one test might relate to their performance on another test. If we just speculated about this there may be three different ways we might imagine relationship on one test might relate to relationship on another. They might be positively related. That is, individuals who do well in verbal ability also do well in spatial ability or numerical ability. They're positively correlated. People in general do, if they're positively correlated, maybe they're people generally do well across all tests. They're good at these types of things. Alternatively, we can certainly easily conjure a scenario where they're negatively associated. For example, verbal ability might be negatively associated with numerical ability. If I'm spending all my time studying math maybe my vocabulary suffers. And so high math scores go with low vocabulary scores, a negative association. And we could also conjure where they're unrelated, the four abilities. In fact, one of these is invariably always true, if you actually did this study. Invariably, performance on multiple ability tests are positively correlated. And because they're always positively correlated, for some psychologists, the existence of these positive correlations imply the existence of something common to them. That's why people who do well on one, tend to do well on the other, because they have some sort of general ability that allows them to do well across the board. That general ability they call general intelligence or I'm going to call it, I prefer the term general cognitive ability. It's the common core that allows, that underlies this positive correlation, that allows people, if they're doing well on one to do well on the other test as well. For other psychologists, they're going to emphasize the fact that, well, you, you tend to do well on the other if you do well on one, but it's not a perfect correlation, so these really are distinct abilities. So they'll emphasis the distinct abilities rather than the common ability. In this module, I'm going to emphasize the general cognitive ability, not the distinct abilities. Again, that's primarily a pragmatic decision. Most of the research, behavioral genetic research, is on the general cognitive ability. There is also plenty of research on distinct abilities. They don't, that research isn't as far along as this research. So I'm going to use this one to illustrate the approach. By saying that, I'm not saying for sure that, and this is one criticism of intelligence, that you could summarize an individual's talent up in a single number. You certainly cannot do that. But a single number does tell you something important about an individual. It doesn't tell you everything important about him or her, but it does tell you something important about that individual. The last thing the last debate is, can we measure general cognitive ability? I think actually this is one of the great achievements in psychology, is that the area of psychology devoted to this, psychometrics has been very successful in developing a way, an approach to measure general cognitive ability. And this dates back more than 100 years to this man, a French man, Alfred Binet, who in 1905 the French, the Paris School District approached Binet and asked him to develop a test that would allow the school district to identify children who would struggle In the schools under the normal curriculum. And they wanted to do that to in, to identify students who might need other types of intervention other than the normal academic curriculum. So the school district wanted something that would predict academic success. So Binet, along with his colleague Simone it's not important for us to get into the details of this, but what they did to successfully develop this test of academic to, that would predict academic success, is to sample multiple ability or intellectual domains: verbal debates, mathematical domains, reasoning domains, memory domains. And then they summarized the performance as a single, kind of average score. Again, not because Binet and Simone thought that you could summarize an individuals talents by a single number. That certainly there were distinct abilities here, but rather that this was an effective way to summarize their performance in order to try to predict how they were going to do in school in general. Binet developed what we call an intelligence test. The term IQ, or intelligence quotient, was actually not introduced by Binet. It was something that was introduced by German psychologists, about ten years after Binet but these tests are now typically scored on an IQ scale. So an IQ is a measure of general cognitive ability, actually a fairly good measure of cognitive ability. And if you looked at the distribution of IQ it would look kind of like a bell curve or a normal distribution. It's actually not identical to a normal distribution. IQ isn't exactly normally distributed. It would look like a normal distribution, but there's an important deviation from the normal distribution here that actually will be important, later in this in this unit when we talk about intellectual disability. But it kind of looks like a bell curve and because it's a bell curve IQ's are actually scaled to have a mean of 100 in the population. So the average IQ is 100, but roughly two-thirds of the population will fall between an IQ of 85 and 115. That's the average IQ range. If we go out into the tails a little bit less than 2.5% are in what's called the gifted range, IQs above 130. And here's actually the deviation from normality. If this was a perfectly symmetric normal curve, then as many people would be falling above an IQ of 130 as would fall below an IQ of 70. But actually, there are a few more people down in this lower tail of the IQ distribution. This is associated with intellectual disability, IQs less than 70. So intelligence is a hypothetical construct. Psychologists certainly recognize that there are multiple aspects of intelligence, but certainly one very important aspect is what's called general cognitive ability. General cognitive ability can be measured pretty well by an IQ score that has this property that its approximately normally distributed in the population with a mean of 100. Does that mean the IQ score is, summarizes an individual's talents? No. Is it the most important thing about an individual? No. Is it an important aspect of an individual? Yes. So next time we'll begin with this introduction of, to intelligence out of the way, we'll begin talking about twin and adoption studies of general cognitive ability.
