So let's turn now to a second way in which people differ that's been the focus of a lot of work by psychologists, and this is intelligence. Now it's hard to get explicit about what we mean when we talk about intelligence. There was a poll of 1,000 experts were i asked them what intelligence meant to them. And just about all of them mentioned abstract, reasoning, problem solving, and ability to acquire knowledge. And then other things kept coming up memory, mental speed, good with language, good with math, knowledge, creativity and so on. And intuitively, we have intuitions that some people are not very smart and some people are very smart, and intelligence is how we capture it. Now, there's been enormous energy on developing tests for intelligence. And the classic tests developed by Spearman, involved breaking down intelligence into two parts. And there's a distinction between what you can call g and what you would call s. So imagine that there's a series of separate tests, separate components of an intelligence test. Imagine there's six of them, and so you'll do them and you'll get six separate scores. But Spearman points out that you could break this up, you could talk about people's skill at each individual test. And that's teir S, but you could also do the math and realize that there's a common factor, spanning all of these tasks, and you can call that g. And so here's an analogy, imagine if instead of a mental task, it was a physical task, it was an athletic task. So you have to do all sorts of things like running, and gymnastics, and boxing, and so on, weightlifting, and you get your different score on say, six different athletic tests. So you'll get a lot of variance. Some people will be really good at one, and some people will be bad at another, and there'll definitely be differences. But the interesting part is they'll also have sort of a common factor. Somebody who is in really good shape and is a trained athlete will probably do pretty well in all of them. Somebody who is obese or extremely elderly or just out of shape will do badly in all of them. So you could sort of talk about being good at weightlifting and good at running, but you could also have a general factor, like good at athletics. And similarly, for psychology tests, we can talk about being good at math and good at verbal. But also just good at intelligence, in general. Now when you look at how tests are coded, it's actually interesting. We'll get to this later when we talk a bit about differences. But the average of tests are always calibrated to be a 100. So once they're scored, the numbers are fiddled with, so the average is always a 100. And they have what statisticians call a normal distribution. About two-thirds of the people have an intelligence between 85 and a 115. And when it come to something of an inteligence of 145, only a fraction, 0.13% have that. So you get a number with these tests, you get a g and the question is, what does this mean? What does this g mean? And it used to be said, that it doesn't tell you anything. Your score on an IQ test just tells you how good you are at doing IQ tests, but this isn't really true. Your score on IQ tests has a pretty powerful predict of power on school achievements, the sort of jobs you get, how well do you do at life actually. IQ tests have a degree of validity. They seem to test the sort of thing you want to test. Now to some extent though, the success of IQ test is a self-fulfilling prophecy. Because if as a society, you take them as important, they then become important. If you need a high IQ to get into a good college, and you do because the sort of tests like the SATs and GREs that we do for education are basically IQ tests. Then it turns out that having an IQ will be connected to being in a good college and, to the extent that being in a good college is relevant to your future income. [SOUND] IQ gets related to income. To use an analogy, suppose people were tested on the color of your hair and if you have red hair, you immediately get into Harvard, and Yale, and Stanford. Well, it'll turn out that having red hair is going to be strongly related to success in life. Not because of some intrinsic power of red hair, but because of the social value given to this. And if that sounds like a fanciful example, think about skin color. Where skin color is powerfully related to how well you do in life. At least in part, because people take skin color very seriously when making decisions, both consciously and unconsciously. It seems not very debatable that IQ captures something of meaningful relevance. So there's actually research that finds that it correlates, your score on a test correlates with your mental speed and the span of your working memory. Short term memory that we discussed before. People with high IQs as measure by IQ tests are able to think in some way faster and better. And it does make a difference in everyday life.