But it turns out that when you analyze objections to free markets on these grounds, they contain two basic issues: First, goods go to the highest bidder; second, bidders possess different amounts of wealth. Disentangling these two factors is important. When markets produce outcomes that seem unfair, it is usually the second factor — the wealth disparity — that is to blame.

Place bidders on an equal footing and the superior efficiency of the market becomes evident. When two similarly well-off families vie for a large house, for example, the family that places the greater value on the property will outbid the other one.

This insight has been put into practice in many settings. In some universities, for example, students bid on courses using “points,” not dollars. This bidding process can be fair and efficient when universities allocate the points equitably. It would be a very different matter if the richer students invariably received entry to the best classes.

A task force at Feeding America — which included Professor Prendergast, Ms. Morgan and others — adopted this approach. They embraced a bidding system using a virtual currency. Crucially, food banks with the greatest need received the most currency and so could place the highest bids, harnessing the benefits of a free market with fairness in the distribution of the underlying wealth.

The new system started in 2005 and quickly proved successful, sometimes in unexpected ways.

For example, donations seemed to fall into two categories. Food banks sought some items, like diapers, that “sold” at relatively high prices. Some food banks focused on bidding on these items, which had the effect of lowering the prices of staples, like produce. The neediest food banks were able to obtain these staples at bargain prices. This was the kind of positive-sum situation that ideal markets allow: Everyone was a winner.

In another unexpected turn of events, some food banks began to sell their excess stores. The Idaho food bank, with warehouses full of potatoes, sold some of them and bought other commodities with the proceeds. Soon, these transactions accounted for more than 12 million pounds of food redistributed among the food banks each year.

Most noticeably, everything ran more smoothly. Donations in the old system were laboriously offered to individual food banks in succession until a taker could be found. Donations were sometimes not accepted. Imagine how this made donors feel. Why give when what you offer is not wanted?