nobody really:

The results of the ultimatum game do vary a great deal between cultures. Here's an example study with interesting results:

https://www.middlebury.edu/NR/rdonlyres/94E229FB-760A-4F67-ACE2-7AA76B258603/0/Wilson_paper.pdf

(or http://tinyurl.com/c46egx if that's too long to handle)

From the abstract: "proposers in the "ultimatum game" almost invariable made offers that split a day's wage at 50/50... we find that responders invoke an exceedingly strong norm of a 50/50 split or nothing at all."

Take a look at tables 2 and 3a/3b--both groups that Bahry and Wilson studied had about 50% rejection rates of offers that would have given the whole pot away. And 8 out of 24 people who were offered more than half the pot actually rejected the offer!

Now, that response can be interpreted as a desire to signal that altruism is more important to the responder than money. But there aren't significant genetic differences between different cultures, where the outcomes of the ultimatum game vary widely. So how can evolutionary psychology explain why these results vary?

I'm skeptical of evolutionary psychology as a scientific endeavor, because the basic theory can explain anything in retrospect--but I've never seen a researcher make a prediction based on EP and then verify it via testing. Of course, that doesn't mean it hasn't happened--I've read very little on the field.