1. Social Networks 101 P ROF. J ASON H ARTLINE AND P ROF. N ICOLE I MMORLICA

2. Evaluate your peers Experiment: Q. How many points were earned in the first week of class? Write your answer on your index cards and pass it to your TAs.

3. Lecture Seven: Structural balance and wisdom of the crowds

4. Lecture Seven, Part One: Structural balance

5. The enemy of my enemy

6. … is my friend.

7. Signed graphs + - Friends Enemies

8. Three-node signed graphs + ++ + -- - ++ - -- Balanced Unbalanced

9. Balanced or not? + -- -- + - -+ + + -

10. In real life …

11. 1907 UK France RussiaItaly Germany Austria-Hungary WWI

12. And a bit later … Why did the US support Pakistan in the Bangladeshi war of independence?

13. Where is Bangladesh?

15. 1970s US Soviet Union ChinaIndia Pakistan Bangladesh ?

16. Structural balance What do balanced networks look like?

17. Suppose all people know each other.

18. Complete networks Definition. A network is complete if all people know each other.

19. Complete networks Definition. A network is complete if all people know each other.

20. Balanced networks Definition. A complete network is balanced if all triangles are balanced.

21. Structural balance In a complete balanced network, either + + A + B - or all people like each other there are two opposing camps

22. Argument. a Pick any node: A = all friends of a B = all enemies of a Define: 1. Every two nodes in A are friends 2. Every two nodes in B are friends 3. Every node in A is an enemy of every node in B

23. Argument. a A = all friends of a B = all enemies of a there are two opposing camps

24. Argument. a A = all friends of a B = all enemies of a Or everyone is friends if a has no enemies.

25. Optional material Generalizing to arbitrary graphs.

26. Lecture Seven, Part Two: Wisdom of the crowds

27. Collective intelligence The average of our beliefs is a very close approximation to the truth. Closer than any individual belief!

28. Who wants to be a millionaire? Audience lifeline – 91% right Call-an-expert – 65% right Wisdom of the crowds, J. Surowiecki

29. Jan. 28, 1986

30. Challenger explosion Wisdom of the crowds, J. Surowiecki

31. When it works Diversity of opinion Each person should have private information even if it's just an eccentric interpretation of the known facts.

32. When it works Independence People's opinions aren't determined by the opinions of those around them.

33. When it works Decentralization People are able to specialize and draw on local knowledge.

34. When it works Aggregation Some mechanism exists for turning private judgments into a collective decision.

35. Explanation Each person has their own personal noisy signal of the truth (diversity, independence, decentralization). By aggregating, we cancel out the noise, and the truth is revealed.

36. Time for

37. Evaluate your peers Experiment: Q. How many points were earned in the first week of class? A. 540.21 points.

38. Next time Information cascades