Matching without prices

you are in this shape if

• Participants game the application order instead of stating true preferences
• Unravelling: offers creep earlier and earlier to beat rivals
• The unmatched pile up while acceptable matches exist

the moves

Run deferred acceptance

Collect full preference lists, tentatively match, release and re-match until stable — no one ends up with someone they'd both trade away.

Make honesty the best strategy

Choose a mechanism where stating true preferences is optimal, and say so — strategy-proofness is a feature users can feel.

Chain the incompatible

When direct swaps are blocked, build chains through the pool so each gives to a stranger and receives from another.

where it was solved

1. 1952NRMPMedical labour marketsThe US residency Match: a clearing house running what was later proven to be the deferred-acceptance algorithm, redesigned by Alvin Roth in 1998.Killed decades of exploding-offer chaos; still allocates every US resident today.
2. 2003NYC Department of EducationSchool choiceReplaced the strategic high-school application scramble with a deferred-acceptance match designed by the same economists.Students left unassigned by the process fell from roughly thirty thousand to three thousand in the first year.
3. 2004New England Program for Kidney ExchangeOrgan transplantationEconomists applied matching theory to incompatible donor-patient pairs, later extended into long non-simultaneous chains.Thousands of transplants that could not otherwise happen; the 2012 economics Nobel cited the work.
4. 2005Feeding AmericaFood bankingIn 2005 Feeding America replaced its queue-and-allocate system with an internal market. Every food bank gets a budget of a synthetic currency called 'shares' and bids against other banks for truckloads of food as they become available, revealing what it actually wants instead of accepting what a planner assigned. Crucially, bids can go negative: a food bank can be paid in shares to take a truckload of, say, unwanted pickles, so undesirable donations get absorbed by whichever bank values the shares more than it dislikes the pickles, instead of rotting because no one had standing to refuse them.The market now allocates roughly 300 million pounds of food a year across the network. Food banks self-sort by real local need rather than a planner's guess, and the negative-price mechanism specifically solved the chronic problem of unwanted donations sitting unclaimed until they spoiled.

what breaks in transit

• A mechanism that isn't strategy-proof teaches participants to lie, then optimises the lies.
• The money taboo is the constraint, not an inefficiency to engineer around — 'add prices' proposals kill these systems politically.
• Stability guarantees hold on paper; interviews, couples and side deals leak strategic behaviour back in at the edges.

adjacent plates

• 09Stranger trust