TIM HARFORD, "the undercover economist," has assumed Paul Krugman's old mantle as chief explainer of the dismal science to society at large. It is a thankless task which he normally completes with erudition and style. In this Sunday's

Washington Post

, Mr Harford

aimed to explain

Urban architecture matters, too. This is something we feel intuitively but find hard to prove or to quantify. Think of high-rise apartments. Do they make a city safer by packing more people into an area and giving the streets a greater bustle? Or are cities safer if most buildings are low-rise, so that people feel a connection to the street?



Two new-wave economists, Edward Glaeser of Harvard and Bruce Sacerdote of Dartmouth, matched crime figures with data on building height and discovered that the residents of high-rise apartments are much more likely to be victims of crime -- specifically street crime. The effect remains similar after statistically adjusting for poverty, demographics and public housing: It's the height of the building itself that matters.



This is evidence in favor of urbanist Jane Jacobs's persuasive -- but unproven -- insight that "eyes on the street" are what make a neighborhood safe. The hero who sprinted across the street to knock the knife away was sitting on the stoop outside his home, not gazing out his penthouse window.

to readers the insight economics has contributed to the study of crime. It's a nifty read, with one small exception. The problem portion:

File this under probably true but unhelpful. For one thing, it's rather naive to think that housing type might accomplish crime reductions through its effect on behaviour (as opposed to its effect on the composition of the residential population). Mr Harford may believe that replacing the luxury condos with a brownstone will encourage the cocktail mixing professional to instead sip lemonade on his stoop while local youths play stickball in the alley, but I doubt it.

In a more concrete vein, I think it highly likely that building height is instead capturing other neighbourhood variables. It will be more difficult to build tall, for instance, in a highly regulated metropolitan area, and a highly regulated metropolitan area will be more likely to dictate a safer street design, greater police presence, and so on. Height could also be capturing neighbourhood activism. Those troubling community organisations who cause headaches for developers are also the sort to complain excessively to an inattentive police force or put together a community watch. Building height is correlated, in other words, with the strength of social institutions.

Perhaps more importantly, the factoid quoted by Mr Harford does not appear to consider the opportunity cost of building shorter. As Mr Glaeser knows better than anyone, limits on housing supply constitute a tight constraint on urban population growth. As such, any citywide movement to reduce building heights will lead to greater population growth in suburbs and other low-density areas. Those movements come with risks of their own, which cancel out most or all of the urban safety cost due to crime. Low-density areas are subject to obesity and other health risks, the most serious of which is death and injury due to motor vehicle accidents.

That's the danger of focusing on a single urban disamenity. There are tradeoffs associated with building in different ways in different areas. Attention to just one of many public safety factors paints a misleading portrait for readers.

Just another reason why being the layman's economist is such a tough gig. Any honest paragraph of straightforward explanation will usually require a good three pages of caveats and disclaimers.