Manuel and Natasha Face the Same Budget and Choose Differently

Both have $40, yogurt costs $1 and a film rental $4, so both face the same line: forty yogurts, or ten films, or anything between. Their indifference curves differ, and that is the whole of the difference between them. Manuel settles at three films and 28 yogurts; Natasha at seven films and twelve yogurts.

Raise both incomes and both move out to a new line, but not in the same direction. Manuel spends most of the extra on films, moving to seven films and 32 yogurts. Natasha spends most of hers on yogurt, moving to eight films and 28. Same budget, same change in budget, different answers: the model puts preferences and constraints in separate boxes, and only the constraint is shared. The slider sets the new income.