1.  Trust

2. Defining Trust  In Class Exercise: What are things you would do with someone you trust, but would not do with someone you don’t trust?  Loan money  Ask for a movie / restaurant / hotel recommendation  Tell a secret  Ask for a recommendation letter or for a reference  Ask for advice  Ask the person to take care of my pets / house while I am away  Lend my car  Go on a trip

3. Why trust?  Loan money - We expect the person will pay us back  Ask for a recommendation - The person's recommendation will match our taste and the movie or restaurant or hotel will be good.  Tell a secret - The person will keep a secret, not tell others, and not judge us for it.  Ask for a recommendation or reference - The recommendation will be positive and help us get the position we are applying to.

4. General Definition  Trust is putting oneself in a vulnerable position based on the belief that another person will act with our best interest in mind.

5. Categories of Trust  Trust with material possessions  Belief about reliability  Trust with secrets  Trust regarding physical safety  Johnson-George and Swap (1982)

6. How Trust Develops  Calculation-based trust – A rational decision about whether to trust someone, where the costs and benefits of trusting are factored in.  Personal-based trust – A person's propensity to trust, developed over the course of their life.  Cognition-based trust - The instant rapport and trust that can develop between people who share similar backgrounds, beliefs, and values. It often is based in first impressions.  Institution-based trust - How trust may form in the presence of guarantees and protections offered by an institution.

7. Trust Asymmetry  Trust is not always equal between people  Extreme example: parents and children  A child must have almost complete trust in his parent while the parent should have very little trust in a child (on substantive matters)  More common asymmetries are smaller  E.g. Bosses and employees  Employees tend to trust superiors more

8. Context and Time  Trust changes over time  Trust will vary among contexts  I may trust Bob to recommend a restaurant, but not to dog sit  Trust can also transfer between contexts  I trust Bob from knowing him at work, and so I will trust him to water my plants if I ask (even though I have no basis for trusting his plant responsibility)

9. Measuring Trust  Some people are more trusting than others. This is called a propensity to trust  This can be measured with a simple thought experiment called the Investment Game

10. The Investment Game  You are given $10. You can keep it all or invest some (or all) of it with an unknown person.  Whatever you choose to invest is tripled and given to the unknown person.  E.g. If you invest $5, they get $15. If you invest $10, they get $30  That person can keep it all, or return any amount of it to you as a return on your investment.  How much do you invest?

11. Trustworthiness  We can compute trustworthiness with the same game by asking how much money you would return if you receive the investment.  A great survey by Evans and Revelle (2008) also measures trustworthiness. Take it and see where you fall on the scales

12. Trust in Others  The investment game can be repeated for specific other people instead of strangers  Other surveys to measure trust. Examples:  Overall Trust  If we decided to meet somewhere for lunch, I would be certain ______ would be there.  I could expect _____ to tell me the truth.  Emotional Trust  I could talk freely to ____ and know he/she would want to listen.  _____ would never intentionally misrepresent my point of view to others.  Reliability  If my alarm clock was broken and I asked ______ to call me at a certain time, I could count on receiving the call.  If I were injured or hurt, I could depend on _____ to do what was best for me.

13. Trust in Social Media  Apply these same estimates to people we know online  Ask people explicitly to rate trust in others  E.g. Epinions  Issue: most people online are strangers

14. Trust Inference  Infer trust between two unknown people using network structure  If A-B have trust, and B-C have trust, how much should A trust C? ABC t AB t BC t AC

15. Trust Inference Algorithms  Use network structure to infer trust  Example approach  Find neighbors who are trusted.  Ask them how much to trust the stranger.  Average their responses weighted by how much we trust each neighbor.  Neighbors repeat this if they do not know the stranger.

16. Inferring over many paths Source Sink B A E D C I H F G 8 7 3 9 5 8 6 10 4 9 2 1 9 8 5 3

17. Applications of Trust  Once trust is computed, how can we use it?  Filtering information  e.g. show reviews only from the most trusted people  Sorting Information  Show Facebook posts from my most trusted friends first, and least trusted friends last  Aggregating Information  Give more weight to restaurant ratings from trustworthy people and less weight to lower-trust people when computing an average rating.

18. Brainstorming  Think of examples of places you interact with user- generated content online.  If you could compute how much to trust the person responsible for each post, how would you use that to help you?

19. Conclusions  Trust has an element of risk and belief in another person.  There are many ways to estimate trust users have in their friends.  In social media, we want to know how much to trust strangers and there are computational methods to do that.  Trust can be used to improve the way people interact with information in social media.