Hello, this is Matthew Jackson from Stanford University, and this is our first lecture in social and economic networks. And I'm going to start with just a brief introduction of some of the material we'll be covering. And, of course, the most important question we can start with is, why study networks? And I think from a social scientist's perspective, it's that many economic, political, and social interactions are embedded in social settings, and the structure of those relationships is very important in both determining how people behave and, and determining what outcomes are. So things like trades and good and services, most markets are actually not centralized but, but occur between different parties and bilateral relationships. Sharing of information, sharing a favor, risk, transmission of viruses, opinion. How did you find out about a job? often through somebody you knew, you know. how do you choose who you vote for? How do you make decisions about products? A lot of time you're talking to different individuals, what did they hear? how did you hear about your information? Political alliances can be represented in these networks, trade alliances, there's all kinds of, of different settings where network structure is very important. And critical to this is the fact that the networks actually influence the behavior. So if we look at crime, we look at employment, we look at people's investment in human capital, education. how they vote, whether they smoke all kinds of decisions they make are, are embedded in these settings and are influenced by the social structure. So in, in most important from our perspective, is that networks come in different sizes, and understanding how they're shaped, what they look like is going to very important in understanding what the outcomes are. And so there's, a lot, to understand and to model. So,[COUGH], the primary questions we're gonn be interested in, in this course, will start with just some background on, on structure of social networks. What we know about social networks. we'll must of the course we'll be looking at how networks form, so do the right ones form? If we could actually affect the network formation from different perspectives, would we want to? How do networks influence behavior? So how, how what's the relationship between how dense a network is and what the outcomes are? And so forth. now this is an area, obviously, which has been researched in, in many different disciplines. And so when we look at the literature's, there's sociology, economics, computer science, statistical physics, mathematics, random graph theory, and so what we'll do her is try and synthesize these and bring together a random view point. And pulling models from different perspectives. And trying to understand what we've learned and also what are important areas for future research as we go along. Now, in terms of areas for future research and current research. We'll be looking at, at theoretical foundations, modeling basically of, of network formation, modeling of dynamics. design of networks, how then understanding how networks influence behavior. There will be some emphasis at the, the end of the course on co-evolution, so that, what does that mean? That means that, that who my friends are, influences my behavior, but my behavior also influences who my friends are, so there's a co-determination. It's not as if one is set in stone and affects the other, but both evolve in together. We'll be, be looking at, at a lot of empirical work and experimental work as we go along. Observing networks, seeing what patterns we do see and, and also you know with an emphasis on testing theory and understanding regularities and patterns that are out there in the data. one other thing that we'll see as the course goes along is methodology. So they'll be a whole series of definitions about networks. for instance, you know, understanding who's central in a, in a network could be measured many different ways. And we'll try and say something about what are good and bad ways of, of doing it for different applications. So there might not be a single way to approach a problem, but understanding what are the different methods and, and is there something we can say about the methods themselves. central focus in this course is really going to be on the models and the types of, of techniques we'll be using are one, pulled from random graph theory, pulled from mathematics. The other, we'll be using some strategic and game theoretic techniques, and we'll also be using some hybrid models that involve some, both choice and chance. And looking at some statistical models for fitting and analyzing networks, dealing with data. goals, I'm not going to presume, prior knowledge of network analysis. I'm going to try to introduce you to variety of different approaches, so the idea here is really breadth, more than depth, so its an idea of giving you, some exposure, so you know what's out there. The types of different tools, which tools might be appropriate in different settings. There's a lot more that can be said about each of the subjects we're going to talk about, but this will be more or less an introduction, to give you an idea of exactly what the tools are that might be appropriate for different parts of analysis. It'll also give you some sense of different disciplines' techniques and, what the kinds of questions and perspectives that they take. In terms of, one important aspect when I start the course here is, is really emphasis. Why do we care about modelling things to begin with? And I think this is an important question that, that will shape the structure of, of what kinds of models we work with and, and how they're formed. And, you know, when we look at models, one thing they do for us is give us perspective into why we see certain things. So why do social networks have short average path lengths for instance. Why is it that there's six degrees of seperation in the world. Well, we'll see an answer to that, that will come out of random graph model. So, just understanding the structure of how things arise at random, can help us understand why we might see something like that. So understanding a basic tree structure that underlies social networks, will help us understand path length. models also about to compare the statistics. So if we understand that models changfe as we change different parameters, that can help us make predictions about how the world might change. So, how, how does the component structure change with density. If a, if a network has more and more links, what does that do to the overall component structure of the network. It will help us make predictions out of samples, so if you want us to come in with a new policy for instance you are trying to, to stamp out a flu, epidemic. how effective does the vaccine have to be in order to, to limit, the extent of a, the epidemic. That's a question we can begin to answer with network analysis. things will also the models will allow for statistical estimation. So, if we wana understand, for instance, is their significant clustering which means, you know, are my friends friends with each other. does that happen, because of some social force, or is it happening just at random? we can test models. So we can take models and, and then ask does this appear that this happened at random, or does it appear that it something else was going on. So there'll be statistical tests that we can use once we have models for analyzing that kind of question. in terms of a basic outline of the course, it's going to break into three parts. The first part's going to be background and fundamentals. So, definitions. How do we analyze networks? What are some basic, properties of networks, characteristics. And along with this will be empirical background. The second part of the course, and the central part of the course is going to be network formation models. So we'll look at random graph models, and then we'll also look at strategic formation models when people are actually making choices. the third part of the course is networks and behavior. That's going to then take networks and understand how the shape of networks and the structure of networks, Who do you know, how many people do you know, who do they know and so forth. How does that influejnce what you're dedcisions are, your behavior and so forth. So we'll look at things like diffusion and contagions. We'll look at learning models. And then finally, what's known as games on network or situations where what I do depends on the choice of my friends. So if there's a new app out there, do I want to get it? Well it might depend on how many of my friends get it and that might depend on how many of their friends get it, and so forth. And so how do we analyze that in a network context. So more or less these three main parts are going to be the core structure of the course. And there's a text book which is completely optional, that I've written where a lot of the material is going to be pulled for. In terms of this outline, the numbers on the side here indicate the chapters, so one two, three four five and so forth, these indicate the relative chapters out of the book that, that correspond to the lecture structure of the course. So we'll be moving along through the book with, with a, a couple of exceptions in terms of which chapters are covered in which part. So that's the basic outline. And so let's get started.