Back in January, David Sucher posted on City Comforts an item about what we can charitably describe as the less rigorous studies of this issue. Since then there has been discussion on Crooked Timber and no doubt other sites.
Rather belatedly I'm linking to an ODPM guide (link is to a summary) on the same topic. I haven't had the time to read it yet, but with luck it avoids some of the more sensationalist claims being put about
