In social choice theory, **Arrow's impossibility theorem**, the **general possibility theorem** or **Arrow's paradox** is an impossibility theorem stating that when voters have three or more distinct alternatives (options), no ranked voting electoral system can convert the **ranked preferences** of individuals into a community-wide (complete and transitive) ranking while also meeting a specified set of criteria: * unrestricted domain *, * non-dictatorship *, * Pareto efficiency *, and * independence of irrelevant alternatives *. The theorem is often cited in discussions of voting theory as it is further interpreted by the Gibbard–Satterthwaite theorem. The theorem is named after economist and Nobel laureate Kenneth Arrow, who demonstrated the theorem in his doctoral thesis and popularized it in his 1951 book * Social Choice and Individual Values *. The original paper was titled "A Difficulty in the Concept of Social Welfare".^{ [1] }

- Statement
- Independence of irrelevant alternatives (IIA)
- Formal statement of the theorem
- Informal proof
- Part one: There is a "pivotal" voter for B over A
- Part two: The pivotal voter for B over A is a dictator for B over C
- Part three: There exists a dictator
- Interpretations
- Remark: Scalar rankings from a vector of attributes and the IIA property
- Other possibilities
- Approaches investigating functions of preference profiles
- Rated electoral system and other approaches
- See also
- References
- Further reading
- External links

In short, the theorem states that no rank-order electoral system can be designed that always satisfies these three "fairness" criteria:

- If every voter prefers alternative X over alternative Y, then the group prefers X over Y.
- If every voter's preference between X and Y remains unchanged, then the group's preference between X and Y will also remain unchanged (even if voters' preferences between other pairs like X and Z, Y and Z, or Z and W change).
- There is no "dictator": no single voter possesses the power to always determine the group's preference.

Cardinal voting electoral systems are not covered by the theorem, as they convey more information than rank orders.^{ [2] }^{ [3] } However, Gibbard's theorem extends Arrow's theorem for that case. The theorem can also be sidestepped by weakening the notion of independence.^{[ citation needed ]}

The axiomatic approach Arrow adopted can treat all conceivable rules (that are based on preferences) within one unified framework. In that sense, the approach is qualitatively different from the earlier one in voting theory, in which rules were investigated one by one. One can therefore say that the contemporary paradigm of social choice theory started from this theorem.^{ [4] }

The practical consequences of the theorem are debatable: Arrow has said "Most systems are not going to work badly all of the time. All I proved is that all can work badly at times."^{ [5] }

The need to aggregate preferences occurs in many disciplines: in welfare economics, where one attempts to find an economic outcome which would be acceptable and stable; in decision theory, where a person has to make a rational choice based on several criteria; and most naturally in electoral systems, which are mechanisms for extracting a governance-related decision from a multitude of voters' preferences.

The framework for Arrow's theorem assumes that we need to extract a preference order on a given set of options (outcomes). Each individual in the society (or equivalently, each decision criterion) gives a particular order of preferences on the set of outcomes. We are searching for a ranked voting electoral system, called a *social welfare function* (*preference aggregation rule*), which transforms the set of preferences (*profile* of preferences) into a single global societal preference order. Arrow's theorem says that if the decision-making body has at least two members and at least three options to decide among, then it is impossible to design a social welfare function that satisfies all these conditions (assumed to be a reasonable requirement of a fair electoral system) at once:

- Non-dictatorship
- The social welfare function should account for the wishes of multiple voters. It cannot simply mimic the preferences of a single voter.
- Unrestricted domain, or universality
- For any set of individual voter preferences, the social welfare function should yield a unique and complete ranking of societal choices. Thus:
- It must do so in a manner that results in a complete ranking of preferences for society.
- It must deterministically provide the same ranking each time voters' preferences are presented the same way.

- Independence of irrelevant alternatives (IIA)
- The social preference between x and y should depend only on the individual preferences between x and y (
*pairwise independence*). More generally, changes in individuals' rankings of*irrelevant*alternatives (ones outside a certain subset) should have no impact on the societal ranking of the subset. For example, if candidate*x*ranks socially before candidate*y*, then*x*should rank socially before*y*even if a third candidate*z*is removed from participation. (See Remarks below.) - Monotonicity, or positive association of social and individual values
- If any individual modifies his or her preference order by promoting a certain option, then the societal preference order should respond only by promoting that same option or not changing, never by placing it lower than before. An individual should not be able to hurt an option by ranking it
*higher*. - Non-imposition, or citizen sovereignty
- Every possible societal preference order should be achievable by some set of individual preference orders. This means that the social welfare function is surjective: It has an unrestricted target space.

A later (1963)^{ [6] } version of Arrow's theorem replaced the monotonicity and non-imposition criteria with:

- Pareto efficiency, or
**unanimity** - If every individual prefers a certain option to another, then so must the resulting societal preference order. This, again, is a demand that the social welfare function will be minimally sensitive to the preference profile.

This later version is more general, having weaker conditions. The axioms of monotonicity, non-imposition, and IIA together imply Pareto efficiency, whereas Pareto efficiency (itself implying non-imposition) and IIA together do not imply monotonicity.

The IIA condition has three purposes (or effects):^{ [7] }

- Normative
- Irrelevant alternatives should not matter.
- Practical
- Use of minimal information.
- Strategic
- Providing the right incentives for the truthful revelation of individual preferences. Though the strategic property is conceptually different from IIA, it is closely related.

Arrow's death-of-a-candidate example (1963, page 26)^{ [6] } suggests that the agenda (the set of feasible alternatives) shrinks from, say, X = {a, b, c} to S = {a, b} because of the death of candidate c. This example is misleading since it can give the reader an impression that IIA is a condition involving *two* agenda and *one* profile. The fact is that IIA involves just *one* agendum ({x, y} in case of pairwise independence) but *two* profiles. If the condition is applied to this confusing example, it requires this: Suppose an aggregation rule satisfying IIA chooses b from the agenda {a, b} when the profile is given by (cab, cba), that is, individual 1 prefers c to a to b, 2 prefers c to b to a. Then, it must still choose b from {a, b} if the profile were, say: (abc, bac); (acb, bca); (acb, cba); or (abc, cba).

In different words, Arrow defines IIA as saying that the social preferences between alternatives x and y depend only on the individual preferences between x and y (not on those involving other candidates).

Let A be a set of **outcomes**, *N* a number of **voters** or **decision criteria**. We shall denote the set of all full linear orderings of A by L(A).

A (strict) **social welfare function** (**preference aggregation rule**) is a function

which aggregates voters' preferences into a single preference order on A.^{ [8] }

An *N*-tuple (*R*_{1}, …, *R*_{N}) ∈ L(A)^{N} of voters' preferences is called a *preference profile*. In its strongest and simplest form, Arrow's impossibility theorem states that whenever the set A of possible alternatives has more than 2 elements, then the following three conditions become incompatible:

- Unanimity, or weak Pareto efficiency
- If alternative,
**a**, is ranked strictly higher than**b**for all orderings*R*_{1}, …,*R*_{N}, then**a**is ranked strictly higher than**b**by F(*R*_{1},*R*_{2}, …,*R*_{N}). (Unanimity implies non-imposition.) - Non-dictatorship
- There is no individual,
*i*whose strict preferences always prevail. That is, there is no*i*∈ {1, …,*N*} such that for all (*R*_{1}, …,*R*_{N}) ∈ L(A)^{N},**a**ranked strictly higher than**b**by*R*implies_{i}**a**ranked strictly higher than**b**by F(*R*_{1},*R*_{2}, …,*R*_{N}), for all**a**and**b**. - Independence of irrelevant alternatives
- For two preference profiles (
*R*_{1}, …,*R*_{N}) and (*S*_{1}, …,*S*_{N}) such that for all individuals*i*, alternatives**a**and**b**have the same order in*R*as in_{i}*S*, alternatives_{i}**a**and**b**have the same order in F(*R*_{1}, …,*R*_{N}) as in F(*S*_{1}, …,*S*_{N}).

Based on two proofs appearing in * Economic Theory *.^{ [9] }^{ [10] } For simplicity we have presented all rankings as if ties are impossible. A complete proof taking possible ties into account is not essentially different from the one given here, except that one ought to say "not above" instead of "below" or "not below" instead of "above" in some cases. Full details are given in the original articles.

We will prove that any social choice system respecting unrestricted domain, unanimity, and independence of irrelevant alternatives (IIA) is a dictatorship. The key idea is to identify a *pivotal voter* whose ballot swings the societal outcome. We then prove that this voter is a partial dictator (in a specific technical sense, described below). Finally we conclude by showing that all of the partial dictators are the same person, hence this voter is a dictator.

Say there are three choices for society, call them **A**, **B**, and **C**. Suppose first that everyone prefers option **B** the least: everyone prefers **A** to **B**, and everyone prefers **C** to **B**. By unanimity, society must also prefer both **A** and **C** to **B**. Call this situation *profile 0*.

On the other hand, if everyone preferred **B** to everything else, then society would have to prefer **B** to everything else by unanimity. Now arrange all the voters in some arbitrary but fixed order, and for each *i* let *profile i* be the same as *profile 0*, but move **B** to the top of the ballots for voters 1 through *i*. So *profile 1* has **B** at the top of the ballot for voter 1, but not for any of the others. *Profile 2* has **B** at the top for voters 1 and 2, but no others, and so on.

Since **B** eventually moves to the top of the societal preference, there must be some profile, number *k*, for which **B** moves **above A** in the societal rank. We call the voter whose ballot change causes this to happen the **pivotal voter for B over A**. Note that the pivotal voter for **B** over **A** is not, a priori, the same as the pivotal voter for **A** over **B**. In part three of the proof we will show that these do turn out to be the same.

Also note that by IIA the same argument applies if *profile 0* is any profile in which **A** is ranked above **B** by every voter, and the pivotal voter for **B** over **A** will still be voter *k*. We will use this observation below.

In this part of the argument we refer to voter *k*, the pivotal voter for **B** over **A**, as *pivotal voter* for simplicity. We will show that pivotal voter dictates society's decision for **B** over **C**. That is, we show that no matter how the rest of society votes, if Pivotal Voter ranks **B** over **C**, then that is the societal outcome. Note again that the dictator for **B** over **C** is not a priori the same as that for **C** over **B**. In part three of the proof we will see that these turn out to be the same too.

In the following, we call voters 1 through *k − 1*, *segment one*, and voters *k + 1* through *N*, *segment two*. To begin, suppose that the ballots are as follows:

- Every voter in segment one ranks
**B**above**C**and**C**above**A**. - Pivotal voter ranks
**A**above**B**and**B**above**C**. - Every voter in segment two ranks
**A**above**B**and**B**above**C**.

Then by the argument in part one (and the last observation in that part), the societal outcome must rank **A** above **B**. This is because, except for a repositioning of **C**, this profile is the same as *profile k − 1* from part one. Furthermore, by unanimity the societal outcome must rank **B** above **C**. Therefore, we know the outcome in this case completely.

Now suppose that pivotal voter moves **B** above **A**, but keeps **C** in the same position and imagine that any number (or all!) of the other voters change their ballots to move **B** below **C**, without changing the position of **A**. Then aside from a repositioning of **C** this is the same as *profile k* from part one and hence the societal outcome ranks **B** above **A**. Furthermore, by IIA the societal outcome must rank **A** above **C**, as in the previous case. In particular, the societal outcome ranks **B** above **C**, even though Pivotal Voter may have been the *only* voter to rank **B** above **C**. By IIA, this conclusion holds independently of how **A** is positioned on the ballots, so pivotal voter is a dictator for **B** over **C**.

In this part of the argument we refer back to the original ordering of voters, and compare the positions of the different pivotal voters (identified by applying parts one and two to the other pairs of candidates). First, the pivotal voter for **B** over **C** must appear earlier (or at the same position) in the line than the dictator for **B** over **C**: As we consider the argument of part one applied to **B** and **C**, successively moving **B** to the top of voters' ballots, the pivot point where society ranks **B** above **C** must come at or before we reach the dictator for **B** over **C**. Likewise, reversing the roles of **B** and **C**, the pivotal voter for **C** over **B** must be at or later in line than the dictator for **B** over **C**. In short, if *k*_{X/Y} denotes the position of the pivotal voter for **X** over **Y** (for any two candidates **X** and **Y**), then we have shown

*k*_{B/C}≤ k_{B/A}≤*k*_{C/B}.

Now repeating the entire argument above with **B** and **C** switched, we also have

*k*_{C/B}≤*k*_{B/C}.

Therefore, we have

*k*_{B/C}= k_{B/A}=*k*_{C/B}

and the same argument for other pairs shows that all the pivotal voters (and hence all the dictators) occur at the same position in the list of voters. This voter is the dictator for the whole election.

Although Arrow's theorem is a mathematical result, it is often expressed in a non-mathematical way with a statement such as *no voting method is fair*, *every ranked voting method is flawed*, or *the only voting method that isn't flawed is a dictatorship*.^{ [11] } These statements are simplifications of Arrow's result which are not universally considered to be true. What Arrow's theorem does state is that a deterministic preferential voting mechanism—that is, one where a preference order is the only information in a vote, and any possible set of votes gives a unique result—cannot comply with all of the conditions given above simultaneously.

Various theorists have suggested weakening the IIA criterion as a way out of the paradox. Proponents of ranked voting methods contend that the IIA is an unreasonably strong criterion. It is the one breached in most useful electoral systems. Advocates of this position point out that failure of the standard IIA criterion is trivially implied by the possibility of cyclic preferences. If voters cast ballots as follows:

- 1 vote for A > B > C
- 1 vote for B > C > A
- 1 vote for C > A > B

then the pairwise majority preference of the group is that A wins over B, B wins over C, and C wins over A: these yield rock-paper-scissors preferences for any pairwise comparison. In this circumstance, *any* aggregation rule that satisfies the very basic majoritarian requirement that a candidate who receives a majority of votes must win the election, will fail the IIA criterion, if social preference is required to be transitive (or acyclic). To see this, suppose that such a rule satisfies IIA. Since majority preferences are respected, the society prefers A to B (two votes for A > B and one for B > A), B to C, and C to A. Thus a cycle is generated, which contradicts the assumption that social preference is transitive.

So, what Arrow's theorem really shows is that any majority-wins electoral system is a non-trivial game, and that game theory should be used to predict the outcome of most voting mechanisms.^{ [12] } This could be seen as a discouraging result, because a game need not have efficient equilibria; e.g., a ballot could result in an alternative nobody really wanted in the first place, yet everybody voted for.

The IIA property might not be satisfied in human decision-making of realistic complexity because the *scalar* preference ranking is effectively derived from the weighting—not usually explicit—of a *vector* of attributes (one book dealing with the Arrow theorem invites the reader to consider the related problem of creating a scalar measure for the track and field decathlon event—e.g. how does one make scoring 600 points in the discus event "commensurable" with scoring 600 points in the 1500 m race) and this scalar ranking can depend sensitively on the weighting of different attributes, with the tacit weighting itself affected by the context and contrast created by apparently "irrelevant" choices. Edward MacNeal discusses this sensitivity problem with respect to the ranking of "most livable city" in the chapter "Surveys" of his book *MathSemantics: making numbers talk sense* (1994).

In an attempt to escape from the negative conclusion of Arrow's theorem, social choice theorists have investigated various possibilities ("ways out"). These investigations can be divided into the following two:

- those investigating functions whose domain, like that of Arrow's social welfare functions, consists of profiles of preferences;
- those investigating other kinds of rules.

This section includes approaches that deal with

- aggregation rules (functions that map each preference profile into a social preference), and
- other functions, such as functions that map each preference profile into an alternative.

Since these two approaches often overlap, we discuss them at the same time. What is characteristic of these approaches is that they investigate various possibilities by eliminating or weakening or replacing one or more conditions (criteria) that Arrow imposed.

Several theorists (e.g., Kirman and Sondermann^{ [13] }) point out that when one drops the assumption that there are only finitely many individuals, one can find aggregation rules that satisfy all of Arrow's other conditions.

However, such aggregation rules are practically of limited interest, since they are based on ultrafilters, highly non-constructive mathematical objects. In particular, Kirman and Sondermann argue that there is an "invisible dictator" behind such a rule.^{ [13] } Mihara^{ [14] }^{ [15] } shows that such a rule violates algorithmic computability.^{ [16] } These results can be seen to establish the robustness of Arrow's theorem.^{ [17] }

When there are only two alternatives to choose from, May's theorem shows that only simple majority rule satisfies a certain set of criteria (e.g., equal treatment of individuals and of alternatives; increased support for a winning alternative should not make it into a losing one). On the other hand, when there are at least three alternatives, Arrow's theorem points out the difficulty of collective decision making. Why is there such a sharp difference between the case of less than three alternatives and that of at least three alternatives?

*Nakamura's theorem* (about the core of simple games) gives an answer more generally. It establishes that if the number of alternatives is less than a certain integer called the ** Nakamura number **, then the rule in question will identify "best" alternatives without any problem; if the number of alternatives is greater or equal to the Nakamura number, then the rule will not always work, since for some profile a voting paradox (a cycle such as alternative A socially preferred to alternative B, B to C, and C to A) will arise. Since the Nakamura number of majority rule is 3 (except the case of four individuals), one can conclude from Nakamura's theorem that majority rule can deal with up to two alternatives rationally. Some super-majority rules (such as those requiring 2/3 of the votes) can have a Nakamura number greater than 3, but such rules violate other conditions given by Arrow.^{ [18] }

A common way "around" Arrow's paradox is limiting the alternative set to two alternatives. Thus, whenever more than two alternatives should be put to the test, it seems very tempting to use a mechanism that pairs them and votes by pairs. As tempting as this mechanism seems at first glance, it is generally far from satisfying even Pareto efficiency, not to mention IIA. The specific order by which the pairs are decided strongly influences the outcome. This is not necessarily a bad feature of the mechanism. Many sports use the tournament mechanism—essentially a pairing mechanism—to choose a winner. This gives considerable opportunity for weaker teams to win, thus adding interest and tension throughout the tournament. This means that the person controlling the order by which the choices are paired (the agenda maker) has great control over the outcome. In any case, when viewing the entire voting process as one game, Arrow's theorem still applies.

Another approach is relaxing the universality condition, which means restricting the domain ^{[ disambiguation needed ]} of aggregation rules. The best-known result along this line assumes "single peaked" preferences.

Duncan Black has shown that if there is only one dimension on which every individual has a "single-peaked" preference, then all of Arrow's conditions are met by majority rule. Suppose that there is some predetermined linear ordering of the alternative set. An individual's preference is *single-peaked* with respect to this ordering if he has some special place that he likes best along that line, and his dislike for an alternative grows larger as the alternative goes further away from that spot (i.e., the graph of his utility function has a single peak if alternatives are placed according to the linear ordering on the horizontal axis). For example, if voters were voting on where to set the volume for music, it would be reasonable to assume that each voter had their own ideal volume preference and that as the volume got progressively too loud or too quiet they would be increasingly dissatisfied. If the domain is restricted to profiles in which every individual has a single peaked preference with respect to the linear ordering, then *simple*^{ [19] } aggregation rules, which include majority rule, have an *acyclic* (defined below) social preference, hence "best" alternative.^{ [20] } In particular, when there are odd number of individuals, then the social preference becomes transitive, and the socially "best" alternative is equal to the median of all the peaks of the individuals (Black's median voter theorem ^{ [21] }). Under single-peaked preferences, the majority rule is in some respects the most natural voting mechanism.

One can define the notion of "single-peaked" preferences on higher-dimensional sets of alternatives. However, one can identify the "median" of the peaks only in exceptional cases. Instead, we typically have the destructive situation suggested by McKelvey's Chaos Theorem:^{ [22] } for any *x* and *y*, one can find a sequence of alternatives such that *x* is beaten by *x _{1}* by a majority,

By relaxing the transitivity of social preferences, we can find aggregation rules that satisfy Arrow's other conditions. If we impose *neutrality* (equal treatment of alternatives) on such rules, however, there exists an individual who has a "veto". So the possibility provided by this approach is also very limited.

First, suppose that a social preference is *quasi-transitive* (instead of transitive); this means that the strict preference ("better than") is transitive: if and , then . Then, there do exist non-dictatorial aggregation rules satisfying Arrow's conditions, but such rules are *oligarchic*.^{ [23] } This means that there exists a coalition L such that L is *decisive* (if every member in L prefers x to y, then the society prefers x to y), and each member in L *has a veto* (if she prefers x to y, then the society cannot prefer y to x).

Second, suppose that a social preference is *acyclic* (instead of transitive): there do not exist alternatives that form a *cycle* (). Then, provided that there are at least as many alternatives as individuals, an aggregation rule satisfying Arrow's other conditions is *collegial*.^{ [24] } This means that there are individuals who belong to the intersection ("collegium") of all decisive coalitions. If there is someone who has a veto, then he belongs to the collegium. If the rule is assumed to be neutral, then it does have someone who has a veto.

Finally, Brown's theorem left open the case of acyclic social preferences where the number of alternatives is less than the number of individuals. One can give a definite answer for that case using the *Nakamura number*. See limiting the number of alternatives.

There are numerous examples of aggregation rules satisfying Arrow's conditions except IIA. The Borda rule is one of them. These rules, however, are susceptible to *strategic manipulation* by individuals.^{ [25] }

See also Interpretations of the theorem above.

Wilson (1972)^{ [26] } shows that if an aggregation rule is non-imposed and non-null, then there is either a dictator or an inverse dictator, provided that Arrow's conditions other than Pareto are also satisfied. Here, an *inverse dictator* is an individual *i* such that whenever *i* prefers *x* to *y*, then the society prefers *y* to *x*.

Amartya Sen offered both relaxation of transitivity and removal of the Pareto principle.^{ [27] } He demonstrated another interesting impossibility result, known as the "impossibility of the Paretian Liberal" (see liberal paradox for details). Sen went on to argue that this demonstrates the futility of demanding Pareto optimality in relation to voting mechanisms.

In social decision making, to rank all alternatives is not usually a goal. It often suffices to find some alternative. The approach focusing on choosing an alternative investigates either *social choice functions* (functions that map each preference profile into an alternative) or *social choice rules* (functions that map each preference profile into a subset of alternatives).

As for social choice functions, the Gibbard–Satterthwaite theorem is well-known, which states that if a social choice function whose range contains at least three alternatives is strategy-proof, then it is dictatorial.

As for social choice rules, we should assume there is a social preference behind them. That is, we should regard a rule as choosing the maximal elements ("best" alternatives) of some social preference. The set of maximal elements of a social preference is called the *core*. Conditions for existence of an alternative in the core have been investigated in two approaches. The first approach assumes that preferences are at least *acyclic* (which is necessary and sufficient for the preferences to have a maximal element on any *finite* subset). For this reason, it is closely related to relaxing transitivity. The second approach drops the assumption of acyclic preferences. Kumabe and Mihara^{ [28] } adopt this approach. They make a more direct assumption that individual preferences have maximal elements, and examine conditions for the social preference to have a maximal element. See Nakamura number for details of these two approaches.

Arrow originally rejected cardinal utility as a meaningful tool for expressing social welfare,^{ [29] } and so focused his theorem on preference rankings, but later stated that a cardinal score system with three or four classes "is probably the best".^{ [2] }

Arrow's framework assumes that individual and social preferences are "orderings" (i.e., satisfy completeness and transitivity) on the set of alternatives. This means that if the preferences are represented by a utility function, its value is an *ordinal* utility in the sense that it is meaningful so far as the greater value indicates the better alternative. For instance, having ordinal utilities of 4, 3, 2, 1 for alternatives a, b, c, d, respectively, is the same as having 1000, 100.01, 100, 0, which in turn is the same as having 99, 98, 1, .997. They all represent the ordering in which a is preferred to b to c to d. The assumption of *ordinal* preferences, which precludes *interpersonal comparisons* of utility, is an integral part of Arrow's theorem.

For various reasons, an approach based on cardinal utility, where the utility has a meaning beyond just giving a ranking of alternatives, is not common in contemporary economics. However, once one adopts that approach, one can take intensities of preferences into consideration, or one can compare (i) gains and losses of utility or (ii) levels of utility, across different individuals. In particular, Harsanyi (1955)^{ [30] } gives a justification of utilitarianism (which evaluates alternatives in terms of the sum of individual utilities), originating from Jeremy Bentham. Hammond (1976)^{ [31] } gives a justification of the maximin principle (which evaluates alternatives in terms of the utility of the worst-off individual), originating from John Rawls.

Not all voting methods use, as input, only an ordering of all candidates.^{ [32] } Methods which don't, often called "rated" or "cardinal" (as opposed to "ranked", "ordinal", or "preferential") electoral system, can be viewed as using information that only cardinal utility can convey. In that case, it is not surprising if some of them satisfy all of Arrow's conditions that are reformulated.^{ [33] } Range voting is such a method.^{ [5] }^{ [34] } Whether such a claim is correct depends on how each condition is reformulated.^{ [35] } Other rated electoral system which pass certain generalizations of Arrow's criteria include approval voting and majority judgment. Note that Arrow's theorem does not apply to single-winner methods such as these, but Gibbard's theorem still does: no non-defective electoral system is fully strategy-free, so the informal dictum that "no electoral system is perfect" still has a mathematical basis.^{ [36] }

Finally, though not an approach investigating some kind of rules, there is a criticism by James M. Buchanan, Charles Plott, and others. It argues that it is silly to think that there might be *social* preferences that are analogous to *individual* preferences.^{ [37] } Arrow (1963, Chapter 8)^{ [38] } answers this sort of criticism seen in the early period, which come at least partly from misunderstanding.

A multi-pronged refutation of Arrow's theorem was published by philosophy Professor Howard DeLong in 1991.^{ [39] }^{ [40] } He challenges the theorem on the basis that Arrow wrongly assumes Preference is transitive property and that Collective Preference is the same as summing up individual preferences. He also claims that Arrow's model fails to model democracy as it exists in the real world as the model ignores the possibility of consensual temporary dictatorships (i.e.: Greek tyrants in times of war) and the effect of allowing lotteries to decide tie breakers and to avoid the problem of the tyranny of the majority, the example used being a group of campers at a summer camp, 8 of whom prefer cake, 7 who prefer ice cream, but funds are limited to one choice or the other on a weekly basis. Under the collective preference of majority rule each week the group would select cake. By drawing lots the choices would more accurately reflect the preferences of the collective than the majority.^{ [41] }

**Approval voting** is a single-winner electoral system where each voter may select ("approve") any number of candidates. The winner is the most-approved candidate.

The **Condorcet paradox** in social choice theory is a situation noted by the Marquis de Condorcet in the late 18th century, in which collective preferences can be cyclic, even if the preferences of individual voters are not cyclic. This is paradoxical, because it means that majority wishes can be in conflict with each other: Majorities prefer, for example, candidate A over B, B over C, and yet C over A. When this occurs, it is because the conflicting majorities are each made up of different groups of individuals.

Within economics, the concept of **utility** is used to model worth or value. Its usage has evolved significantly over time. The term was introduced initially as a measure of pleasure or satisfaction within the theory of utilitarianism by moral philosophers such as Jeremy Bentham and John Stuart Mill. The term has been adapted and reapplied within neoclassical economics, which dominates modern economic theory, as a **utility function** that represents a consumer's preference ordering over a choice set. Utility has thus become a more abstract concept that is not necessarily solely based on the satisfaction or pleasure received.

In welfare economics, a **social welfare function** is a function that ranks social states as less desirable, more desirable, or indifferent for every possible pair of social states. Inputs of the function include any variables considered to affect the economic welfare of a society. In using welfare measures of persons in the society as inputs, the social welfare function is individualistic in form. One use of a social welfare function is to represent prospective patterns of collective choice as to alternative social states. The social welfare function provides the government with a simple guideline for achieving the optimal distribution of income.

The **independence of irrelevant alternatives** (**IIA**), also known as **binary independence** or the **independence axiom**, is an axiom of decision theory and various social sciences. The term is used with different meanings in different contexts; although they all attempt to provide an account of rational individual behavior or aggregation of individual preferences, the exact formulations differ from context to context.

In social choice theory, the **Gibbard–Satterthwaite theorem** is a result published independently by philosopher Allan Gibbard in 1973 and economist Mark Satterthwaite in 1975. It deals with deterministic ordinal electoral systems that choose a single winner. It states that for every voting rule, one of the following three things must hold:

- The rule is dictatorial, i.e. there exists a distinguished voter who can choose the winner; or
- The rule limits the possible outcomes to two alternatives only; or
- The rule is susceptible to tactical voting: in certain conditions some voter's sincere ballot may not defend their opinion best.

In social choice theory, **May's theorem** states that simple majority voting is the only anonymous, neutral, and positively responsive social choice function between two alternatives. Further, this procedure is resolute when there are an odd number of voters and ties (indecision) are not allowed. Kenneth May first published this theorem in 1952.

In economics, game theory, and decision theory, the **expected utility hypothesis**—concerning people's preferences with regard to choices that have uncertain outcomes (gambles)—states that the subjective value associated with an individual's gamble is the statistical expectation of that individual's valuations of the outcomes of that gamble, where these valuations may differ from the dollar value of those outcomes. The introduction of St. Petersburg Paradox by Daniel Bernoulli in 1738 is considered the beginnings of the hypothesis. This hypothesis has proven useful to explain some popular choices that seem to contradict the expected value criterion, such as occur in the contexts of gambling and insurance.

The **median voter theorem** states that "a majority rule voting system will select the outcome most preferred by the median voter". It is associated with public choice economics and statistical political science.

The **liberal paradox**, also **Sen paradox** or **Sen's paradox**, is a logical paradox proposed by Amartya Sen which purports to show that no social system can simultaneously

- be committed to a minimal sense of freedom,
- always result in a type of economic efficiency known as Pareto efficiency, and
- be capable of functioning in any society whatsoever.

**Social choice theory** or **social choice** is a theoretical framework for analysis of combining individual opinions, preferences, interests, or welfares to reach a *collective decision* or *social welfare* in some sense. A non-theoretical example of a collective decision is enacting a law or set of laws under a constitution. Social choice theory dates from Condorcet's formulation of the voting paradox. Kenneth Arrow's *Social Choice and Individual Values* (1951) and Arrow's impossibility theorem in it are generally acknowledged as the basis of the modern social choice theory. In addition to Arrow's theorem and the voting paradox, the Gibbard–Satterthwaite theorem, the Condorcet jury theorem, the median voter theorem, and May's theorem are among the more well known results from social choice theory.

**Revealed preference theory**, pioneered by economist Paul Samuelson, is a method of analyzing choices made by individuals, mostly used for comparing the influence of policies on consumer behavior. Revealed preference models assume that the preferences of consumers can be revealed by their purchasing habits.

Kenneth Arrow's monograph * Social Choice and Individual Values* and a theorem within it created modern social choice theory, a rigorous melding of social ethics and voting theory with an economic flavor. Somewhat formally, the "social choice" in the title refers to Arrow's representation of how

The mathematical notion of **quasitransitivity** is a weakened version of transitivity that is used in social choice theory and microeconomics. Informally, a relation is quasitransitive if it is symmetric for some values and transitive elsewhere. The concept was introduced by Sen (1969) to study the consequences of Arrow's theorem.

In social choice theory, **unrestricted domain**, or **universality**, is a property of social welfare functions in which all preferences of all voters are allowed. Intuitively, unrestricted domain is a common requirement for social choice functions, and is a condition for Arrow's impossibility theorem.

In economics and other social sciences, **preference** is the order that a person gives to alternatives based on their relative utility, a process which results in an optimal "choice". Instead of the prices of goods, personal income, or availability of goods, the character of the preferences is determined purely by a person's tastes. However, persons are still expected to act in their best interest.

In cooperative game theory and social choice theory, the **Nakamura number** measures the degree of rationality of preference aggregation rules, such as voting rules. It is an indicator of the extent to which an aggregation rule can yield well-defined choices.

**Intensity of preference**, also known as **intensity preference**, is a term popularized by the work of the economist Kenneth Arrow, who was a co-recipient of the 1972 Nobel Memorial Prize in Economics. This term is used in reference to models for aggregating ordinal rankings.

**Maximal lotteries** refers to a probabilistic voting system first considered by the French mathematician and social scientist Germain Kreweras in 1965. The method uses preferential ballots and returns so-called maximal lotteries, i.e., probability distributions over the alternatives that are weakly preferred to any other probability distribution. Maximal lotteries satisfy the Condorcet criterion, the Smith criterion, reversal symmetry, polynomial runtime, and probabilistic versions of reinforcement, participation, and independence of clones.

Electoral systems are the rules for conducting elections. Comparisons between different systems can focus on different aspects: on suffrage or rules for voter eligibility; on candidate eligibility and the rules governing political parties; on the way elections are scheduled, sequenced, and combined; or on the rules for determining the winner within a given election.

- ↑ Arrow, Kenneth J. (1950). "A Difficulty in the Concept of Social Welfare" (PDF).
*Journal of Political Economy*.**58**(4): 328–346. doi:10.1086/256963. JSTOR 1828886. Archived from the original (PDF) on 2011-07-20. - 1 2 "Interview with Dr. Kenneth Arrow".
*The Center for Election Science*. October 6, 2012.*CES:*you mention that your theorem applies to preferential systems or ranking systems. ... But ... Approval Voting, falls within a class called cardinal systems. ...*Dr. Arrow:*And as I said, that in effect implies more information. ... I'm a little inclined to think that score systems where you categorize in maybe three or four classes ... is probably the best. - ↑ Sen, Amartya (1999). "The Possibility of Social Choice".
*American Economic Review*.**89**(3): 349–378. doi:10.1257/aer.89.3.349. JSTOR 117024.Does Arrow's impossibility ... go away with the use of interpersonal comparisons in social welfare judgments? ... yes. The additional informational availability allows sufficient discrimination to escape impossibilities of this type. ... even weaker forms of comparability would still permit making consistent social welfare judgments, satisfying all of Arrow's requirements

- ↑ Suzumura, Kōtarō (2002). "Introduction". In Arrow, Kenneth J.; Sen, Amartya K.; Suzumura, Kōtarō (eds.).
*Handbook of social choice and welfare*.**1**. Amsterdam, Netherlands: Elsevier. p. 10. ISBN 978-0-444-82914-6. - 1 2 McKenna, Phil (12 April 2008). "Vote of no confidence".
*New Scientist*.**198**(2651): 30–33. doi:10.1016/S0262-4079(08)60914-8. - 1 2 Arrow, Kenneth Joseph Arrow (1963).
*Social Choice and Individual Values*(PDF). Yale University Press. ISBN 978-0300013641. - ↑ Mas-Colell, Andreu; Whinston, Michael Dennis; Green, Jerry R. (1995).
*Microeconomic Theory*. Oxford University Press. p. 794. ISBN 978-0-19-507340-9. - ↑ Note that by definition, a
*social welfare function*as defined here satisfies the Unrestricted domain condition. Restricting the range to the social preferences that are never indifferent between distinct outcomes is probably a very restrictive assumption, but the goal here is to give a simple statement of the theorem. Even if the restriction is relaxed, the impossibility result will persist. - ↑ Geanakoplos, John (2005). "Three Brief Proofs of Arrow's Impossibility Theorem" (PDF).
*Economic Theory*.**26**(1): 211–215. CiteSeerX 10.1.1.193.6817 . doi:10.1007/s00199-004-0556-7. JSTOR 25055941. - ↑ Yu, Ning Neil (2012). "A one-shot proof of Arrow's theorem".
*Economic Theory*.**50**(2): 523–525. doi:10.1007/s00199-012-0693-3. JSTOR 41486021. - ↑ Cockrell, Jeff (2016-03-08). "What economists think about voting".
*Capital Ideas*. Chicago Booth. Archived from the original on 2016-03-26. Retrieved 2016-09-05.Is there such a thing as a perfect voting system? The respondents were unanimous in their insistence that there is not.

- ↑ This does not mean various normative criteria will be satisfied if we use equilibrium concepts in game theory. Indeed, the mapping from profiles to equilibrium outcomes defines a social choice rule, whose performance can be investigated by social choice theory. See Austen-Smith & Banks (1999) Section 7.2.
- 1 2 Kirman, A.; Sondermann, D. (1972). "Arrow's theorem, many agents, and invisible dictators".
*Journal of Economic Theory*.**5**(2): 267–277. doi:10.1016/0022-0531(72)90106-8. - ↑ Mihara, H. R. (1997). "Arrow's Theorem and Turing computability" (PDF).
*Economic Theory*.**10**(2): 257–276. CiteSeerX 10.1.1.200.520 . doi:10.1007/s001990050157. JSTOR 25055038. Archived from the original (PDF) on 2011-08-12. Reprinted in Velupillai, K. V.; Zambelli, S.; Kinsella, S., eds. (2011).*Computable Economics*. International Library of Critical Writings in Economics. Edward Elgar. ISBN 978-1-84376-239-3. - ↑ Mihara, H. R. (1999). "Arrow's theorem, countably many agents, and more visible invisible dictators".
*Journal of Mathematical Economics*.**32**(3): 267–277. CiteSeerX 10.1.1.199.1970 . doi:10.1016/S0304-4068(98)00061-5. - ↑ Mihara's definition of a
*computable*aggregation rule is based on computability of a simple game (see Rice's theorem). - ↑ See Chapter 6 of Taylor, Alan D. (2005).
*Social choice and the mathematics of manipulation*. New York: Cambridge University Press. ISBN 978-0-521-00883-9 for a concise discussion of social choice for infinite societies. - ↑ Austen-Smith & Banks (1999 , Chapter 3) gives a detailed discussion of the approach trying to limit the number of alternatives.
- ↑ Austen-Smith, David; Banks, Jeffrey S. (1999).
*Positive political theory I: Collective preference*. Ann Arbor: University of Michigan Press. ISBN 978-0-472-08721-1 . Retrieved 2016-02-16. - ↑ Indeed, many different social welfare functions can meet Arrow's conditions under such restrictions of the domain. It has been proved, however, that under any such restriction, if there exists any social welfare function that adheres to Arrow's criteria, then the majority rule will adhere to Arrow's criteria. See Campbell, D. E.; Kelly, J. S. (2000). "A simple characterization of majority rule".
*Economic Theory*.**15**(3): 689–700. doi:10.1007/s001990050318. JSTOR 25055296. - ↑ Black, Duncan (1968).
*The theory of committees and elections*. Cambridge, Eng.: University Press. ISBN 978-0-89838-189-4. - ↑ McKelvey, Richard D. (1976). "Intransitivities in multidimensional voting models and some implications for agenda control".
*Journal of Economic Theory*.**12**(3): 472–482. doi:10.1016/0022-0531(76)90040-5. - ↑ Gibbard, Allan F. (2014) [1969]. "Intransitive social indifference and the Arrow dilemma".
*Review of Economic Design*.**18**(1): 3–10. doi:10.1007/s10058-014-0158-1. - ↑ Brown, D. J. (1975). "Aggregation of Preferences".
*Quarterly Journal of Economics*.**89**(3): 456–469. doi:10.2307/1885263. JSTOR 1885263. - ↑ Blair, Douglas; Muller, Eitan (1983). "Essential aggregation procedures on restricted domains of preferences".
*Journal of Economic Theory*.**30**(1): 34–53. doi:10.1016/0022-0531(83)90092-3. - ↑ Wilson, Robert (1972). "Social Choice Theory without the Pareto principle".
*Journal of Economic Theory*.**5**(3): 478–486. doi:10.1016/0022-0531(72)90051-8. - ↑ Sen, Amartya (September 1979a). "Personal Utilities and Public Judgements: Or What's Wrong With Welfare Economics?".
*The Economic Journal*.**89**(355): 537–558. doi:10.2307/2231867. JSTOR 2231867. - ↑ Kumabe, M.; Mihara, H. R. (2011). "Preference aggregation theory without acyclicity: the core without majority dissatisfaction" (PDF).
*Games and Economic Behavior*.**72**: 187–201. arXiv: 1107.0431 . Bibcode:2011arXiv1107.0431K. doi:10.1016/j.geb.2010.06.008. - ↑ "Modern economic theory has insisted on the ordinal concept of utility; that is, only orderings can be observed, and therefore no measurement of utility independent of these orderings has any significance. In the field of consumer's demand theory the ordinalist position turned out to create no problems; cardinal utility had no explanatory power above and beyond ordinal. Leibniz' Principle of the identity of indiscernibles demanded then the excision of cardinal utility from our thought patterns." Arrow (1967), as quoted on p. 33 by Racnchetti, Fabio (2002), "Choice without utility? Some reflections on the loose foundations of standard consumer theory", in Bianchi, Marina (ed.),
*The Active Consumer: Novelty and Surprise in Consumer Choice*, Routledge Frontiers of Political Economy,**20**, Routledge, pp. 21–45 - ↑ Harsanyi, John C. (1955). "Cardinal Welfare, Individualistic Ethics, and Interpersonal Comparisons of Utility".
*Journal of Political Economy*.**63**(4): 309–321. doi:10.1086/257678. JSTOR 1827128. - ↑ Hammond, Peter J. (1976). "Equity, Arrow's Conditions, and Rawls' Difference Principle".
*Econometrica*.**44**(4): 793–804. doi:10.2307/1913445. JSTOR 1913445. - ↑ It is sometimes asserted that such methods may trivially fail the
*universality*criterion. However, it is more appropriate to consider that such methods fail Arrow's definition of an aggregation rule (or that of a function whose domain consists of preference profiles), if preference orderings cannot uniquely translate into a ballot. - ↑ However, a modified version of Arrow's theorem may still apply to such methods (e.g., Brams; Fishburn (2002). "Chapter 4". In Arrow, Kenneth J.; Sen, Amartya K.; Suzumura, Kōtarō (eds.).
*Handbook of social choice and welfare*.**1**. Amsterdam, Netherlands: Elsevier. Theorem 4.2 framework. ISBN 978-0-444-82914-6. - ↑ Smith, Warren D.; et al. "How can range voting accomplish the impossible?" . Retrieved 2016-02-16.
- ↑ No voting method that nontrivially uses cardinal utility satisfies Arrow's IIA (in which preference profiles are replaced by lists of ballots or lists of utilities). For this reason, a weakened notion of IIA is proposed (e.g., Sen (1979, p. 129)). The notion requires that the social ranking of two alternatives depend only on the
*levels of utility*attained by individuals at the two alternatives. (More formally, a*social welfare functional*is a function that maps each list of utility functions into a social preference. satisfies*IIA (for social welfare functionals)*if for all lists and for all alternatives , if and for all , then .) Many cardinal voting methods (including range voting) satisfy the weakened version of IIA. - ↑ Poundstone, William (2009-02-17).
*Gaming the Vote: Why Elections Aren't Fair (and What We Can Do About It)*. Macmillan. ISBN 9780809048922. - ↑ Feldman, Allan M.; Serrano, Roberto (2006).
*Welfare Economics and Social Choice Theory*. Springer Science & Business Media. ISBN 9780387293684.It is quite silly in the first place to think that there might be social preferences that are analogous to individual preferences. It is nonsense to talk about social preferences since society itself is nothing more than a collection of individuals, each with his own interests. ... this first reaction to Arrow's Theorem is logically attractive, but it can lead to varieties of nihilism that are unappealing to some people, including us.

- ↑ Arrow, Kenneth Joseph (1963). "Chapter VIII Notes on the Theory of Social Choice, Section III. What Is the Problem of Social Choice?".
*Social Choice and Individual Values*. Yale University Press. pp. 103–109. ISBN 978-0300013641.these criticisms are based on misunderstandings of my position

- ↑ DeLong, Howard (1991).
*A Refutation of Arrow's Theorem*. University Press of America. ISBN 978-0819182500. - ↑ Colignatus, Thomas (21 July 2008). "Review of Howard DeLong (1991), 'A refutation of Arrow's theorem', with a reaction, also on its relevance in 2008 for the European Union" (PDF). Retrieved September 26, 2016.
- ↑ DeLong (1991), Ch. 5.

- Campbell, D. E. (2002). "Impossibility theorems in the Arrovian framework". In Arrow, Kenneth J.; Sen, Amartya K.; Suzumura, Kōtarō (eds.).
*Handbook of social choice and welfare*.**1**. Amsterdam, Netherlands: Elsevier. pp. 35–94. ISBN 978-0-444-82914-6. Surveys many of approaches discussed in #Approaches investigating functions of preference profiles. - Dardanoni, Valentino (2001). "A pedagogical proof of Arrow's Impossibility Theorem" (PDF).
*Social Choice and Welfare*.**18**(1): 107–112. doi:10.1007/s003550000062. JSTOR 41106398. preprint. - Hansen, Paul (2002). "Another Graphical Proof of Arrow's Impossibility Theorem".
*The Journal of Economic Education*.**33**(3): 217–235. doi:10.1080/00220480209595188. - Hunt, Earl (2007).
*The Mathematics of Behavior*. Cambridge University Press. ISBN 9780521850124.. The chapter "Defining Rationality: Personal and Group Decision Making" has a detailed discussion of the Arrow Theorem, with proof. - Lewis, Harold W. (1997).
*Why flip a coin? : The art and science of good decisions*. John Wiley. ISBN 0-471-29645-7. Gives explicit examples of preference rankings and apparently anomalous results under different electoral system. States but does not prove Arrow's theorem. - Sen, Amartya Kumar (1979).
*Collective choice and social welfare*. Amsterdam: North-Holland. ISBN 978-0-444-85127-7. - Skala, Heinz J. (2012). "What Does Arrow's Impossibility Theorem Tell Us?". In Eberlein, G.; Berghel, H. A. (eds.).
*Theory and Decision : Essays in Honor of Werner Leinfellner*. Springer. pp. 273–286. ISBN 978-94-009-3895-3. - Tang, Pingzhong; Lin, Fangzhen (2009). "Computer-aided Proofs of Arrow's and Other Impossibility Theorems".
*Artificial Intelligence*.**173**(11): 1041–1053. doi: 10.1016/j.artint.2009.02.005 .

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