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With a finite set of at least three alternatives and unrestricted strict preference orders, every onto deterministic strategyproof social choice function is a dictatorship in social choice. Restricting to two alternatives or to a restricted preference domain removes essential hypotheses.

An outcome is Pareto efficient when no feasible alternative weakly improves every agent's utility and strictly improves at least one. For strict preference orders over distinct alternatives, an outcome unanimously ranked below another is inefficient. Onto strategyproof social choice functions on an unrestricted domain are Pareto efficient: rank-raising monotonicity and unanimity rule out selecting a unanimously dominated alternative.

The Gibbard-Satterthwaite theorem concerns a deterministic social choice function on all profiles of strict preferences over a finite set of at least three alternatives. If it is onto and strategyproof, it is a dictatorship in social choice: one fixed voter always obtains its top alternative. Equivalently, every onto nondictatorial rule on this unrestricted domain is manipulable. Dictatorships themselves are strategyproof. We prove the implication for two voters.

First derive the rank-raising monotonicity lemma. If changing one report changes the selected alternative from $a$ to $b$, strategyproofness in the old profile requires $a\succ b$ in the old order, while strategyproofness in the new profile requires $b\succ a$ in the new order. Therefore a change that never lowers the selected $a$ relative to any formerly lower alternative cannot change the outcome.

Onto-ness now implies unanimity. For every $a$, some profile selects it. Raise $a$ to first place in each voter's order; the preceding lemma keeps the outcome at $a$. Moreover the outcome cannot be $b$ when both voters rank some $a$ above $b$: starting from such an outcome, raise $b$ to second place directly below $a$ in each order. These changes never lower $b$ relative to anything, so would preserve $b$ at a profile unanimously topping $a$, a contradiction. Thus the rule respects unanimous pairwise preference.

For each pair $a,b$, promote that pair to the first two places in each report, preserving the voter's order between them. Both dominate every outsider unanimously, so the outcome is $a$ or $b$. The result depends only on the two voters' comparisons of $a,b$, not on the lower alternatives: changing a tail cannot reverse the choice between two alternatives whose relative order did not change, since one direction of that change would be a profitable report. These binary choices define a complete strict social relation satisfying pairwise unanimity and independence from other comparisons.

This relation is transitive. To see why, suppose its choices on some triple form a directed cycle. Put those three alternatives first in both reports, preserving their individual relative orders. Unanimity excludes all outsiders, so the rule chooses one of the three, say $a$. Promoting $a$ with either other member to the first two positions does not lower $a$ and therefore preserves its selection. Thus $a$ must win both its binary comparisons, contradicting the cycle. A complete strict relation with no directed three-cycle is transitive. Also the original social choice is its top: for any original selected $a$, promoting $a$ and any $b$ preserves $a$, so it wins every binary comparison.

It remains to prove two-voter dictatorship for this binary relation. Choose any conflict between $a,b$. Suppose voter 1 prefers $a$ to $b$, voter 2 prefers $b$ to $a$, and the social relation follows voter 1. Say voter 1 wins this ordered comparison. For any third alternative $c$, consider the two profile patternswith other alternatives below the triple. In pattern I, the social relation has $a\succ b$ by independence and $b\succ c$ by unanimity, hence $a\succ c$. This proves that voter 1 wins the conflict $a$ against $c$. In pattern II, it has $c\succ a$ by unanimity and $a\succ b$ by independence, hence $c\succ b$, proving that voter 1 also wins $c$ against $b$.

Thus winning $a$ against $b$ implies winning $a$ against every third $c$ and every such $c$ against $b$. Applying the implication to $a$ against $c$ gives $b$ against $c$, and then to $b$ against $c$ gives $b$ against $a$. The implication therefore supplies both directions of all pairs involving the original alternatives and any new one. For an arbitrary ordered pair $x,y$ distinct from a fixed base alternative $a$, use the already obtained win of $x$ against $a$ and the third alternative $y$ to get the win of $x$ against $y$. Hence voter 1 wins every conflict. Unanimous comparisons also follow its preference, so the social relation is always voter 1's full order, and the social choice is its top.

If the initially chosen conflict follows voter 2, swap the voter roles in the same argument. We have therefore proved every onto two-voter strategyproof rule with at least three alternatives is dictatorial. This two-voter dictatorship from binary choice proof establishes the required special case without assuming the general theorem's conclusion.

Let $A$ be a finite set of alternatives with $|A|\ge 3$, and let each agent report a strict total order on $A$. A profile is the tuple of these orders. The domain is unrestricted: every tuple of orders is allowed. A deterministic social choice function $f$ selects one alternative from each profile. It is onto if every alternative is selected at some profile. It is strategyproof if no agent, holding all other reports fixed, can obtain an outcome strictly preferred according to its true order by reporting another order. A dictatorship in social choice means that one fixed agent's highest-ranked alternative is always selected, regardless of all other reports.

The Gibbard-Satterthwaite theorem says that every onto, deterministic strategyproof social choice function on this unrestricted domain is a dictatorship in social choice. Equivalently, with at least three possible alternatives an onto nondictatorial rule must permit manipulation. Both the number of alternatives and the unrestricted preference domain are essential hypotheses.

For the requested top-bottom decisiveness lemma, suppose $f(R,S)=a$, with $a$ top in $R$ and bottom in $S$. Fix any $R^{\prime }$ with $a$ top. If $f(R^{\prime },S)\ne a$, agent $1$ with true order $R^{\prime }$ could report $R$ and obtain its best outcome $a$, contrary to strategyproofness. Hence $f(R^{\prime },S)=a$. Now fix any $S^{\prime }$. If $f(R^{\prime },S^{\prime })=b\ne a$, agent $2$ with true order $S$ could report $S^{\prime }$ and obtain $b$, which is better than its bottom alternative $a$. ThereforeCall this agent $1$ being decisive for $a$. The same argument with agents interchanged applies to agent $2$.

Here is a complete proof of the two-agent theorem. First, onto plus strategyproofness gives unanimity. If $a$ occurs at some profile, change agent $1$ to any order with $a$ top: a different outcome would allow that agent to obtain $a$ by returning to its old report. Then change agent $2$ to any order with $a$ top in the same way. Thus every profile with both agents ranking $a$ top selects $a$.

We also need Pareto efficiency. A useful rank-raising monotonicity lemma follows from strategyproofness: if the current outcome is $b$, and one agent changes its order without placing above $b$ any alternative previously below $b$, the outcome remains $b$. Otherwise an outcome $c\ne b$ would satisfy $b\succ c$ in the old order, since a profitable deviation is forbidden there, but $c{\succ }^{\prime }b$ in the new order, since reporting the old order is forbidden there. Those comparisons contradict the allowed change. If both agents prefer $a$ to a selected $b$, change their orders one at a time to put $a$ first and $b$ second. These changes only raise $b$ relative to other alternatives, so the outcome remains $b$, contradicting unanimity for $a$. Therefore an alternative unanimously dominated by another cannot be selected.

Choose distinct $a,b$. Give agent $1$ order $b\succ a\succ \text{the rest}$ and agent $2$ order $a\succ b\succ \text{the rest}$. Pareto efficiency forces the outcome to be $a$ or $b$. If it is $b$, move $b$ to the bottom of agent $2$'s order while keeping $a$ top. A switch to $a$ would be a profitable deviation for its old true order. Every other alternative is unanimously dominated by $a$. Hence the outcome stays $b$, and the top-bottom decisiveness lemma makes agent $1$ decisive for $b$. If the outcome is $a$, the symmetric argument makes agent $2$ decisive for $a$. Consequently some agent is decisive for some alternative. Rename that agent $1$ and that alternative $a$.

Both agents cannot be decisive for this same $a$. To see this, choose $b,c\ne a$ distinct, with every remaining alternative below these three, and considerAt $(R,S^{\prime })$, agent $1$ can obtain $a$ by placing it top, so strategyproofness restricts the outcome to $a$ or $b$. Both agents prefer $b$ to $a$, so Pareto efficiency forces $b$. At $(R^{\prime },S^{\prime })$, agent $1$ still ranks $b$ top and can obtain it by reporting $R$, so the outcome must again be $b$. On the other hand, at $(R^{\prime },S)$, decisiveness of agent $2$ for $a$ restricts the outcome to $a$ or $c$. Both agents prefer $c$ to $a$, so it must be $c$. Since $c$ is top in $S^{\prime }$, agent $2$ can obtain it at $(R^{\prime },S^{\prime })$ by reporting $S$, forcing outcome $c$ there. This contradiction proves that agent $2$ is not decisive for $a$.

Now fix $b\ne a$ and any agent-$1$ order $R$ with $b$ first and $a$ second. At every $(R,S)$, agent $1$ can force $a$ by placing it top, so the outcome must lie in $\{a,b\}$. If $a$ occurs for even one report $S$, changing agent $2$ to an order with $a$ first and $b$ second preserves $a$, because it can return to $S$ to obtain its top choice. Lower $a$ to the bottom of agent $1$'s order, keeping $b$ top. A switch to $b$ would be a profitable deviation under its old order, and any other outcome is unanimously dominated by $b$. Thus $a$ remains selected at a profile where agent $2$ ranks it top and agent $1$ ranks it bottom. The top-bottom decisiveness lemma would make agent $2$ decisive for $a$, a contradiction.

Therefore every $(R,S)$ with agent $1$ ranking $b$ first and $a$ second selects $b$. Choose $S$ with $b$ bottom and apply the top-bottom decisiveness lemma once more: agent $1$ is decisive for $b$ in every order placing $b$ top. This holds for each $b\ne a$, and it already holds for $a$. Agent $1$ is a dictator in social choice, proving the two-agent theorem.

With three agents and only two alternatives, the two-alternative majority rule is onto and strategyproof and has no dictator in social choice. An agent whose vote is not pivotal cannot change the outcome. A pivotal agent obtains its preferred alternative by voting truthfully and its less preferred alternative by reversing its vote. No one is a dictator in social choice, since the other two agents can both oppose its preference. There is no tie with three agents. This does not contradict the theorem because its hypothesis $|A|\ge 3$ fails.

If a strategyproof social choice function selects $a$, raising $a$ in one agent's order without moving any formerly lower alternative above it preserves the outcome. A different outcome would make one of the changes between the old and new reports a profitable deviation. Arbitrary changes among alternatives wholly above or wholly below $a$ are allowed.

Social choice theory studies how individual preferences determine a collective decision. A social choice function selects one alternative from a preference profile; strategyproofness asks whether agents can benefit by misrepresenting their preferences.

A mechanism or social choice function is strategyproof if truthful reporting is a weakly dominant strategy: for every true type and every collection of other reports, no misreport yields strictly higher utility or a strictly preferred outcome. This is stronger than merely requiring truthful reports to form a Nash equilibrium.

For two agents and a strategyproof social choice function, if an alternative is selected when one agent ranks it first and the other ranks it last, it is selected whenever the first agent ranks it first, whatever the other agent reports. Changing the first agent's order preserves its attainable top choice; changing the second agent's order cannot offer it an improvement over a selected bottom alternative.