How much do parents value a safe environment, green spaces and a good education for their children? Such things are priceless - except that, of course, they are not. The best things in life may be free, but buying a house in the vicinity of the best things in life is expensive.

Economic researchers use house prices like a movie jewel-thief uses an aerosol spray. The aerosol isn't important by itself, but it reveals the otherwise invisible laser beams that will trigger the alarm. The house prices aren't necessarily of much direct interest, but indirectly they reveal our willingness to pay for anything from a neighbourhood free of known sex offenders to the more familiar example of a popular school.

In principle this is easy. Compare the market price of two otherwise identical houses, one of which enjoys the amenity in question (a nice view, a quiet street, access to an excellent school) while the other does not. In practice, houses are rarely identical, and all sorts of valuable amenities from good schools to good neighbours to low crime are likely to be jumbled up together.