Assume two people want to split a pizza in a fair way. In our context, fair means that if both people follow the procedure, each person gets half a pizza, or, if not, only has themselves to blame. The obvious answer seems to be “split it in the middle”, but that leads to all kinds of further questions and complications: Who does the splitting? How is “the middle” determined? Even if we had protocols for that, it would still leave the possibility of one person not being satisfied with how they were carried out—sloppy measurements or sloppy cutting.
The standard way to avoid all such problems is to let one person cut the pizza in two halves, and the other person choose who gets which half. If the cutter thinks they got the smaller piece it’s because they didn’t cut fairly or precisely enough, and if the chooser thinks their piece is smaller, they should have chosen the bigger one.
What if there are three people? Before we try to come up with a procedure, let’s consider people’s motivations first. In the case of two people, whatever one person wins, the other loses. With three people, one person might elect to get a smaller piece for themselves in order to also make one of the other two worse off. Or, more realistically, two people might collude against the third. We’d like a procedure that ensures that even under those circumstances, fairness is guaranteed.
One generalization of the two-people procedure that comes to mind is to let one person—let’s call them “A”—cut, and then let the two others choose, first B, then C. Unfortunately, if A cuts in such a way that there is one large piece and two pieces that are each less than
1/3, and B chooses the large piece, C doesn’t get their fair share (neither does A, but that’s A’s fault). Randomizing the order of cutter and choosers is not a solution, either, because there is still the possibility of somebody getting the short end of the stick.
Here’s a procedure that works: A cuts the pizza, B chooses their slice. C has two options: they can either choose one of the remaining two slices, or they can call shenanigans. In the latter case, B picks a second slice, then both slices are put together and C divides them into two halves. B picks one half, C gets the other.
To see why this procedure is fair, let’s consider each person in turn. A will always get the piece that B and C didn’t pick, so if A cut fairly, they get their fair share. B will either get the piece they pick, or at least half of two pieces they pick, so if they pick the two largest pieces, they will get at least one third of the pizza. C will either get the piece they pick, or, in the case of shenanigans, half of the two pieces that B picked. It’s not obvious that they will get at least a third in the latter case. We assume that C only calls shenanigans if the two pieces that are left after B picked one are both smaller than one third—if one piece is at least a third, C should of course just pick it, thereby getting their fair share. The larger of the two pieces will have size
1/3-x, the smaller one
y>=0. If the two pieces are the same size,
y is zero. The piece that B picked therefore has size
1/3+2x+y. B now picks one of the smaller pieces. If B wants to minimize C’s share, B must pick the smallest piece, i.e.
1/3-x-y, which then gets put together with B’s original piece for C to cut. The two pieces together have size
2/3+x, so if C cuts fairly, they will actually get a size larger than their fair share.
I don’t know whether this procedure can be generalized to a larger number of people.
Update: As many commenters have thankfully pointed out, I have solved a problem that smarter people have already solved for more general cases. For a good overview of a few methods, read this article. My personal favorite is the Last Diminisher algorithm. The method I came up with is a variant of the three-person Lone Divider procedure.
It is important that it is B, not C, who picks the second slice when C calls shenanigans. ↩