[drrotmos]

The statutes of the Nobel Foundation, § 4 reads (in part and translated): "If a work that is being rewarded has been produced by two or three persons, the prize shall be awarded to them jointly. In no case may a prize amount be divided between more than three persons."

Source: http://www.nobelprize.org/nobel_organizations/nobelfoundatio...

[aestra]

It's a controversial restriction. Scientists rarely work alone anymore.

[drrotmos]

Agreed, but it's in the statutes, so the board has to abide by them (or risk liability). It's also non-trivial to change the statutes of a foundation ("stiftelse") in Sweden.