social choice: the impossible dream michelle blessing february 23, 2010 michelle blessing february...

Social Choice: The Impossible Dream

Social Choice: The Impossible Dream

Michelle BlessingFebruary 23, 2010Michelle Blessing

February 23, 2010

Outline...• What is social choice theory?

• How do we define a “good” voting system?

• Voting between two candidates

• Voting among three or more candidates

• Arrow’s Impossibility Theorem

• DiscussionQuickTime™ and a

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• Social choice theory: how can we measure individual interests and preferences and combine them into one collective decision?– Finding an outcome that reflects “the will of the

people”

Assumption: The number of voters is odd!

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Preference List BallotA rank order of candidates: often pictured as a vertical list with the most

preferred candidate on top and the least preferred on the bottom

1 Green

2 Orange

3 Yellow

4 Red

5 Purple





Which is your favorite skittle?

Class Election Rank the following Olympic sports in the

order that you enjoy watching them:

• L ~ Luge• I ~ Ice Skating• D ~Downhill Skiing QuickTime™ and a

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Choosing Between Two Candidates

Majority rule: Each voter indicates a preference for one of the two candidates and the candidate with the most votes wins.





Advantages of Majority Rule

1. All voters are treated equally.

2. Both candidates are treated equally.

3. It is monotone: If a new election were held and a single voter changed her ballot from the loser of the previous election to the winner, but everyone else voted exactly as before, the outcome of the new election would be the same.

Can we find a better voting system?

May’s Theorem

Among all possible two-candidate voting systems that never result in a tie, majority rule is the only one that treats all voters equally, treats both candidates equally, and is monotone.

• Proven by Kenneth May in 1952• Mark Fey extended the theorem to an infinite number of voters in 2004

What about elections with three or more candidates?

Several different possibilities for voting systems exist:

1. Condorcet’s Method2. Plurality Voting3. The Borda Count4. Sequential pair-wise voting5. The Hare System







1. Condorcet’s Method A candidate is a winner if he or she would defeat every other

candidate in a one-on-one contest using majority rule.

First A B C

Second B A A

Third C C B• A defeats B (2 to 1)

• A defeats C (2 to 1) Therefore, A wins!

• B defeats C (2 to 1)

Condorcet’s Voting Paradox

With three or more candidates, there are elections in which

Condorcet’s method yields no winners!

First A B C

Second B C A

Third C A B• A defeats B (2 to 1)

• C defeats A (2 to 1) No winner!

• B defeats C (2 to 1)

2. Plurality Voting• Only first place winners are considered• The candidate with the most votes wins• Fails to satisfy the Condorcet Winner Criterion, e.g.

2000 US presidential election• Manipulability

Condorcet’s Winner Criterion

For every possible sequence of preference ballots, either there is no Condorcet winner, or the voting system produces exactly the same winner for this election as does Condorcet’s Method.



3. The Borda Count• Assigns points to each voter’s rankings and then sums these points to arrive at a group’s

final ranking.

• Each first place vote is worth n-1 points, each second place vote is worth n-2 points, and so on down.

Method: count the number of occurences of other candidate names that are below this candidates name.

Applications: senior class rank, sports hall of fame, track meets, etc.

First A A A B B

Second B B B C C

Third C C C A A

Problem with the Borda Count...

• Does not satisfy the property known as “independent of irrelevant alternatives”.

Independence of Irrelevant Alternatives

It is impossible for a candidate B to move from non-winner status to winner status unless at least one voter reverses the order in which he or she had B and the winning candidate ranked.

Failure of the IIAFirst A A A B B

Second B B B C C

Third C C C A A

First A A A C C

Second B B B B B

Third C C C A A

Borda scores: A = 6, B = 7, C = 2 B is the winner!

Borda scores: A = 6, B = 5, C = 4 A is the winner

But no one has changed his or her mind about whether B is preferred to A!

4. Sequential Pairwise voting• Start with a (non-ordered) list of the candidates.

• Pit the first candidate against the second in a one-on-one contest

• The winner then moves on to the third candidate in the list, one-on-one.

• Continue this process through the entire list until only one remains at the end.

• Example: choosing a favorite color:

Problem with Sequential Pairwise Voting...

• It fails to satisfy the “Pareto Condition.”

Pareto Condition

If everyone prefers one candidate A to another candidate B, then B should not be the winner!

5. The Hare System

• Arrive at a winner by repeatedly deleting candidates that are “least preferred”, in the sense of being at the top of the fewest ballots.

• If a single candidate remains after all others have been eliminated, it alone is the winner (otherwise, it is a tie).

“[The Hare System] is among the greatest improvements yet made in the theory and practice of government.” ~ John Stuart Mill



Applying the Hare System

Rank 5 4 3 1

First A C B A

Second B B C B

Third C A A C

Rank 5 4 3 1

First A C C A

Second C A A C

First place votes: A = 6, B = 3, C = 4

Therefore, delete B!

First place votes: A = 6, C = 7 Therefore, C wins!

Problem with the Hare System

• It fails to satisfy the property of monotonicity.

Monotonicity

If a candidate is a winner, and a new election is held in which the only ballot change made is for some voter to move the former winning candidate higher on his or her ballot, then the original winner should remain the winner!

A summary of voting systems for three or more candidates

Voting System Problem

Condorcet’s Method Not always a winner

Plurality Voting Fails to satisfy the Condorcet Winner Criterion, manipulability

Borda Count Fails to satisfy the Independence of Irrelevant Alternatives (IIA) property

Sequential Pairwise Voting Fails to satisfy the Pareto Condition

Hare System Fails to satisfy monoticity

Can we do better? • Is it possible to find a voting system for three or more

candidates as “ideal” as majority rule for two candidates?

Arrow’s Impossibility Theorem

With three or more candidates and any number of voters, there does not exist - and there will never exist - a voting system that always produces a winner, satisfies the Pareto condition and independence of irrelevant alternatives, and is not a dictatorship.



Another possibility? Approval Voting• We have seen that any search for an

idealvoting system of the kind we have discussed is doomed to failure.

• One alternative possibility is approval voting: Instead of sing a preference list ballot, each voter is allowed to give one vote to as many of the candidates as he or she finds acceptable. – No limit is set on the number of candidates for whom

an individual can vote. – The winner under approval voting is the candidate

who receives the larges number of approval votes.



Approval voting is used to elect new members to the National Academy of Science and the Baseball Hall of Fame.

Discussion

• Other applications?

• Which method do you think is most easily manipulated?

• Which might be a good method for electing the US president, as an alternative to the Electoral College, if any?

Homework: #10, 29

social choice: the impossible dream michelle blessing february 23, 2010 michelle blessing february...

Documents

voting systems

hare system slide

condorcets voting paradox

candidates majority

condorcets method

good voting system

better voting system

defeats b