Small museum paintings have been popular objects of theft. They are easy to hide and sell to art collectors. But lately, massive well-known outdoor bronze sculptures weighing thousands of pounds have been stolen frequently.

The thieves probably can't tell the difference between a Henry Moore and a John Waddell, but they surely know that the heavier the sculptures, the more scrap metal they contain. It may be a tragedy that a sculpture worth $18 million is melted down as scrap copper sold for only a few thousand dollars, but a few thousand dollars are nothing to sneer at.

Before the price of copper went up 10 times from 30 cents to $3 a pound, the same sculpture would have been worth only a few hundred dollars.

The modern scrap metal thefts are only recent reminders of similar plunders by the Spanish conquistadors who melted down huge quantities of Aztec royal jewelries for their gold content.

Material thefts are not confined to metals. Plastic crates have been stolen for recycling when the price of petroleum-based resin went up with increasing oil prices. Prices of the resin jumped more than 40% after Hurricane Katrina, rising to 87.50 cents per pound in November 2006. Today, prices are hovering in the 70 cents-per-pound range, nearly double the price just three years ago.

Material theft is a graphic example of how insecure property rights could lead to gross misallocation of resources. First, insecure property discourages the creation of added value such as public art. Where they are still on display, the tightened security inevitably mars the viewing experience. Second, the low marginal cost of stealing means that thieves could afford to sell the stolen objects for very low marginal revenue. If the thieves had to pay market prices for the stolen objects, they would never sell them as scraps. Third, the illegality of buying stolen properties also means that buyers are unlikely to offer higher than the market value even if the stolen objects are sold in their original forms. Fourth, private ownership of stolen public art inevitably reduces the social benefit of public access.