Reciprocal Score Voting: Difference between revisions
extended analysis
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Each ballot is assigned to one or more factions, based on the top rated candidates in that ballot. Then the ballots of each faction are used to run mini score voting elections, the results of which represent how well each faction rates every other faction.
Let <math>B^{v|\phi}_j</math> be the ballot rating of the <math>v</math>-th voter towards candidate <math>j</math>, where <math>\phi</math> is the set of factions assigned to that voter. This means <math>B^{v|\phi}_j > B^{v|\phi}_k</math> for all <math>j \in \phi, k \notin \phi</math>.
Define <math>F_{i \to j}</math> be the mean rating given by faction <math>i</math> to faction <math>j</math>, and also that <math>F_{i \to \phi} = \frac{1}{|\phi|} \sum_{j \in \phi} F_{i \to j}</math>, that is, the mean of the rating given to all members of that set of factions, and similarly for <math>F_{\phi \to j}</math>. Note that <math>F_{i \to j}</math> may be regarded as a square matrix, and <math>F_{i \to \phi}</math> and <math>F_{\phi \to i}</math> the mean between the columns and rows (respectively) corresponding to the factions in <math>\phi</math>.
With these established, Reciprocal Score Voting proceeds by re-weighting and aggregating the ballot ratings according to the rule:
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that is, factions only give support to factions they received support from, and in proportion to that support up to a maximum. Note the case of <math>F_{\phi \to j} = 0</math> only occurs when all <math>B^{v|\phi}_j = 0</math>, in which case no reweighing is necessary.
Only when factions agree on their mutual ratings and perfectly reciprocate is that their ''reciprocity ratio'' <math>R(j,\phi) = F_{j \to \phi} / F_{\phi \to j} = 1</math>, in which case votes are left unchanged. In every other situation the side which reciprocated less gets penalized, receiving less support from whatever faction they failed to reciprocate with.
== Analysis ==
In the case of any asymmetry in support, the reciprocity ratio is <math>
This system is [[monotonicity criterion|non-monotonic]] and suffers from a very unusual "reverse [[spoiler effect]]", in which a larger faction may lose an election by not supporting smaller supportive factions. Therefore, larger factions are encouraged to promote smaller factions as much as possible in order to win.
The <math>\min(\cdot, 1)</math> condition above is required so that support is never amplified by asymmetry. This is also necessary so that a smaller faction cannot parasite on the support of a larger faction, which will never rate the smaller faction above its own. A smaller faction artificially rating a larger faction too highly will only receive exactly as much support as the larger faction is willing to give to it.
=== Chicken dilemma ==
This system handles the [[Chicken dilemma]] reasonably well. Suppose a three-candidate (ABC) election, with 0-10 ballots as follows:
{| class="wikitable"
|-
! Faction !! % !! A !! B !! C
|-
| A || ''m'' ''r'' || 10 || ''b'' || 0
|-
| B || ''m'' (1 - ''r'') || ''a'' || 10 || 0
|-
| C || (1 - ''m'') || 0 || 0 || 10
|}
Here, ''m'' is the fraction of voters in the A+B faction, and ''r'' the fraction of A+B voters in the A subfaction. ''a'' and ''b'' represent the scores given to A and B by B and A, respectively. The total scores are:
: A = ''m'' (''r'' 10 + (1 - ''r'') ''a'')
: B = ''m'' (''r'' ''b'' + (1 - ''r'') 10)
: C = ''(1 - m)'' 10
If m > 0.5, A+B is a mutual majority. Either A or B can easily win over A provided there is enough mutual support. (For example, if ''m'' = 52%, they may rate one another as low as 7 and one of them still win.) The goal then is to ensure they mutually support one another.
Without loss of generality (due to symmetry), we can assume the faction A is larger than B (''r'' ≥ 0.5). In this simplified scenario the faction ratings are simple to see: they can be read straight from the ballots (the table above).
The reciprocity ratios between A and B are then simply <math>R(A,B) = a/b</math> and <math>R(B,A) = b/a</math>. The ballots are thus adjusted to:
{| class="wikitable"
|-
! Faction !! % !! A !! B !! C
|-
| A || ''m'' ''r'' || 10 || ''b'' min(1, ''a''/''b'') || 0
|-
| B || ''m'' (1 - ''r'') || ''a'' min(1, ''b''/''a'') || 10 || 0
|-
| C || (1 - ''m'') || 0 || 0 || 10
|}
Giving total adjusted scores:
: A = ''m'' (''r'' 10 + (1 - ''r'') ''a'' min(1, ''b''/''a''))
: B = ''m'' (''r'' ''b'' min(1, ''a''/''b'') + (1 - ''r'') 10)
: C = ''(1 - m)'' 10
If faction B defects by giving ''a'' = 0, the ratings become:
: A = ''m'' (''r'' 10 + 0)
: B = ''m'' (0 + (1 - ''r'') 10)
: C = ''(1 - m)'' 10
Such that B ceases to get any support from A. Thus, there is no incentive for B to defect, and they have an incentive to rate A highly. Even more curiously, suppose ''r'' = 3/4 such that A is a much larger subfaction than B. In most voting systems, such as [[Instant Runoff Voting]], A would be safe not supporting B, but this is not the case under Reciprocal Score Voting. If A defects with ''b'' = 0, the exact same results as if B defected occur! Therefore, even if A has an advantage over B, it it still in their best interest to support the minor faction as much as possible. This is akin to a "reverse spoiler effect", in which the larger mainstream party spoils the election by not supporting the smaller third party.
Since the incentive for defecting is eliminated, but the rewards for collaborating are preserved, Reciprocal Score Voting minimizes concerns with Chicken Dilemma scenarios. In any sufficiently large mutual majority scenario, the likelihood of either A or B winning will be very large (although not guaranteed due to the non-ranked nature of the system).
== Variants ==
After the adjusted score ballots are computed, one can additionally implement any variant of score voting. For example, '''Reciprocal [[STAR Voting]]''' would have the top-two rated RSV candidates in an automatic runoff, and the most preferred candidate wins (preferably using the original ballots, not the adjusted ones).
If one chooses to use the adjusted ballots in the automatic runoff it is possible that factional betrayal reverses the preference of a voter, which could be problematic. For example: a voter rates (A=10 B=8 C=5 D=0), and BC are the top two under RSV. If B did not reciprocate with A and this voter's score is adjusted to B=4, this voter's runoff vote would go towards C, not B. While this could also be interpreted as a form of retaliation in the spirit of RSV, it is harder to justify as it does not directly encourage any particular positive behavior (like reciprocation).
== Implementation ==
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