Actually you'll find there are cultures around the globe where you are expected to share more than 50% if you receive a windfall... The less than 50% thing is very much a western world phenomenon; and a US one in particular.
it's kinda hard to compare between these two kinds of society, people deal with very different things everyday
[0] http://www.researchgate.net/publication/31914331_J._Henrich_...