11.6. The Stable Marriage Problem 317
the woman he most prefers among those on his list until he must cross her off,
at which point he serenades the next most preferred woman on his list. So from
the man’s perspective, the woman he is serenading can only change for the worse.
Sounds like a good deal for the women.
But it’s not! The fact is that from the beginning, the men are serenading their
first choice woman, and the desirability of the woman being serenaded decreases
only enough to ensure overall stability. The Mating Ritual actually does as well as
possible for all the men and does the worst possible job for the women.
To explain all this we need some definitions. Let’s begin by observing that while
The Mating Ritual produces one stable matching, there may be other stable match-
ings among the same set of men and women. For example, reversing the roles of
men and women will often yield a different stable matching among them.
But some spouses might be out of the question in all possible stable matchings.
For example, given the preferences shown in Figure 11.11, Brad is just not in the
realm of possibility for Jennifer, since if you ever pair them, Brad and Angelina
will form a rogue couple.
Definition 11.6.7.Given a set of preference lists for all men and women, one per-
son is in another person’srealm of possible spousesif there is a stable matching
in which the two people are married. A person’soptimal spouseis their most pre-
ferred person within their realm of possibility. A person’spessimal spouseis their
least preferred person in their realm of possibility.
Everybody has an optimal and a pessimal spouse, since we know there is at least
one stable matching, namely, the one produced by the Mating Ritual. Now here is
the shocking truth about the Mating Ritual:
Theorem 11.6.8.The Mating Ritual marries every man to his optimal spouse.
Proof. By contradiction. Assume for the purpose of contradiction that some man
does not get his optimal spouse. Then there must have been a day when he crossed
off his optimal spouse—otherwise he would still be serenading (and would ulti-
mately marry) her or some even more desirable woman.
By the Well Ordering Principle, there must be afirstday when a man (call him
“Keith”) crosses off his optimal spouse (call her Nicole). According to the rules of
the Ritual, Keith crosses off Nicole because Nicole has a preferred suitor (call him
Tom), so
Nicole prefers Tom to Keith. ()
Since this is the first day an optimal woman gets crossed off, we know that Tom
had not previously crossed off his optimal spouse, and so
Tom ranks Nicole at least as high as his optimal spouse. ()