A specter is haunting Europe: small-town underpopulation.

It might seem strange to be worrying about underpopulation in a world headed toward 9 billion humans by 2050. But during that same time period, some 45 countries are likely to see their populations decrease, some by rather large margins. A chart at VoxEu provides the gory details. Bulgaria is projected to lose 35 percent of its population by 2050; Ukraine, 33 percent; Belarus, 28 percent. Also on the list, Japan, German, Portugal, Italy and Greece.

Within individual nations, write German researchers Benny Geys, Friedrich Heinemann and Alexander Kalb, there are also marked rural-urban disparities. In Germany, even as relatively rural regions experience significant population drops, larger urban areas are growing. And there's a reason for this: It's harder for smaller towns to adjust, creating a downward spiral. The number of people in a smaller city may go down, but the costs of providing services are not so flexible, or "elastic," as the economists like to say.

For municipalities up to a size of 6000 inhabitants, the costs for providing public goods fall underproportionally with population size ... As such, a significant strain on local public budgets is to be expected for these entities with population decline. In contrast, larger municipalities are less confronted with the fixed cost problems and should, therefore, be more able to cut costs in proportion with falling population...If the above-mentioned results prove to be robust, demographic change in Europe could bring a further push for the growth of cities and agglomerations (at the detriment of rural areas) ... Already today the agglomerations are the growth leaders and migration magnets all over Western and Eastern Europe (European Commission, 2007). When municipalities in the rural areas find themselves in the population shrinkage-cost trap in the future, this tendency will accelerate. In this sense, cities and agglomerations are likely to be the "winners" of demographic change.

The odd thing about this conclusion is that cities are also "winning" in nations where the population is growing. Cities are winning everywhere; 2007 was the first year in which more than half the humans on the planet lived in metropolitan areas. Agglomerations rule.