Arrow’s Impossibility Theorem and Voters’ Transitivity (Post - 4)
“Democracy is the worst form of
government, except all of others that have been tried.”
Winston Churchill!
Preferences plays an important role
in everyone’s daily life. Consumers buy what they prefer. If one prefers chocolate
over vanilla, he or she buys chocolate if both items priced equally. If the
same person prefers vanilla over strawberry, he or she buys vanilla over
strawberry if the prices of vanilla and strawberry priced equally. Furthermore,
this individuals chose chocolate over strawberry if these two priced equally. This
sequences of preferences called a transitive preferences. This simply
illustrates that, if one prefers A to B and B to C, a rational person expected to
choose A over C.
This is not the case in democracy. If
there are three candidates to vote from, it is possible that the candidate who received
the most vote might not be the winner, and it demonstrated by “Arrow’s Impossibility
Theorem.” In social choice theory, Arrow's Impossibility Theorem states that a clear order of preferences can’t be
determined while adhering to mandatory principles of fair voting procedure.
Here is an illustration of typical
election problems. Consider that voters asked to rank their preferences of
candidates A, B, and C.
- 45 votes A > B> C (45 people prefer A over B and B over C.
- 40 votes B > C >A (40 people prefer B over C and C
over A).
- 30 votes C > A > B (30 people prefer C over A and
prefer A over
B).
Candidate A has the most votes, and
he or she would be the winner. However, if B is not running, C would be the
winner, as more people prefer C over A. A would have 45 votes, and C would have
70 votes. Since only one has to represent one particular constituent, election
rules are set for an outcome of one winner. When three candidates run for the
same seat, the top two often go on for run off. The third candidates determined
as a loser automatically. There is a possibility that the loser could be the
winner according to the Arrow’s Impossibility Theorem. The above example
illustrates this finding, when we follow one rule, candidate A wins and using different
rule, candidate C would be the winner.
This outcome violates the
transitivity rule which noted above. When one presented with chocolate,
vanilla, and strawberry, a rational individual would choose always chocolates
if he or she prefers chocolate over vanilla and strawberry doesn’t matter how
one presented it. Voter’s preferences doesn’t follow this pattern and voter’s preferences
are not transitive. Transitivity is the assumption that if one prefers candidate
A to candidate B and candidate B over candidate C, it follows that this voters
must prefer candidate A over candidate C. As noted in the example above, this
transitivity assumption cannot guaranteed under democratic voting system. In simple
term, consumers can get what they prefer at the grocery store; on the other
hand, voters can’t get the candidate they prefer given that there are three or
more candidates running. That is the outcome Kenneth Arrow proved long ago.
A Unifying Impossibility
Theorem
