Equal shares, the definition everyone reaches for first

Equal division is the default because it needs no information about anybody.

Equal shares has one enormous practical advantage over its rivals: it requires nothing to be known. To split a bill evenly you do not need to know what anyone ordered, what anyone earns, who booked the table or who has been quietly covering the others for months. You need only a count of heads. This is why equal division is the setting a group falls back to when it is tired, when the information is contested, or when raising the subject at all would be awkward. It is the fairness of last resort, and it works because it is cheap.

Its cost is that it ignores everything. A group of six sharing a platter equally has treated the person who ate nothing and the person who ate half exactly the same, and both of them know it. Over a single dinner nobody minds. Over twenty dinners the person who eats nothing starts to resent the arrangement, and the person who eats half starts to feel watched. Equal shares does not remove the differences between people. It declines to look at them, which is a different thing, and the differences keep accumulating quietly on the other side of the decision.

There is a second, subtler feature worth naming. Equal division is very hard to argue against without appearing to ask for special treatment. If you say the even split is unfair to you, you are proposing that your circumstances be examined, and that feels like a request for a favour even when it is not. This asymmetry gives equal shares an unearned advantage in group discussions. It wins not only because it is simple but because objecting to it is socially expensive, and that is worth knowing when you find a group settling on it very quickly.