Monotonic core solutions: Beyond Young's theorem

We introduce two new monotonicity properties for core concepts: single-valued solution concepts that always select a core allocation whenever the game is balanced (has a nonempty core). We present one result of impossibility for one of the properties and we pose several open questions for the second property. The open questions arise because the most important core concepts (the nucleolus and the per capita nucleolus) do not satisfy the property even in the class of convex games.J. Arin acknowledges financial support from Project 9/UPV00031.321-15352/2003 of the University of the Basque Country, Projects BEC2003-08182 and SEJ-2006-05455 of the Ministry of Education and Science of Spain and Project GIC07/146-IT-377-07 of the Basque Goverment