There is a whole subcategory of behavioral economics that can be summed up as "people treat potential losses and potential gains very differently". But what you

There is a whole subcategory of behavioral economics that can be summed up as "people treat potential losses and potential gains very differently". But what you're saying isn't the example that is being presented. You have A and B being equal partners with a loan from A to B of $1000. That's different from the kind of ultimatum style (with 100% certainty) game that is proposed. I would say a better example is that A owns 66% of a startup and B owns 33%. They decide to sell the tech for X dollars. However, they are offered a 2X buyout if both of them agree to work for 3 years at the acquiring company. There's no "partial" offer though, it's both partners or neither. How is the 2X split up? The contrived example fails because of the 33% and 66% wouldn't be assigned randomly.