Population Concentration

I haven't been able to post anything on here for awhile. I've had several little projects I've been working on, but they require a greater time commitment than I have. Once the full Census data comes out in the next month or two, I'm interested in looking at demographic trends between urban-rural areas.

Basically, something I noticed in trends related to population decline is that often depopulation is localized, while other areas continue to see population growth. For example, although Germany's population is declining over all, almost all of this decline is taking place in eastern Germany, most of western Germany is still experiencing an increase in population.

Although it's not always the case, urban areas in the developed world are continuing to an increase in population, while rural areas are losing population. A notable exception is Detroit. When looking at this trend before, I noticed an interesting pattern in Kansas. Nearly every county that had a population above 50,000 in 2000 increased in population during the past decade, but nearly every county that had a population below 50,000 decreased in population. This meant that the population of Kansas, and many other states, was becoming more concentrated in a few urban areas. I plan to do a comparison of population concentration in each state in 2000 and 2010, but that data is not available yet. So today I did a quick test run on the US as a whole, to see how population concentration has changed with each Census.

To measure concentration I used a measure called the Herfindahl–Hirschman Index (HHI). The highest possible score on this index is 10,000. In this case, a score of 10,000 would imply that everyone in the US lived in one state. The lowest score is 0. This would imply an infinite number of states of equal population. A more realistic benchmark here is 200, this is the score I would receive if all 50 states had the same population.

Year - HHI
1790 905
1800 815
1810 735
1820 687
1830 665
1840 616
1850 563
1860 502
1870 480
1880 432
1890 394
1900 399
1910 393
1920 393
1930 404
1940 400
1950 402
1960 414
1970 421
1980 411
1990 435
2000 436
2010 440

Unsurprisingly, the earliest dates have the highest scores as there were fewer states. In 1790, around 20% of Americans lived in Virginia. As the nation expands, the score declines until it reaches its nadir in 1910-20. Since then there has been a gradual rise in the concentration of the US population, but it still remains low. This frankly isn't very surprising. Most states have large urban centers that help draw in population, and thus there's not a tread of people leaving one state in favor of another (except Michigan), so population concentration has remained fairly constant. Most of the change in population concentration is taking place within each state, as mentioned in the Kansas example.

Just to give an idea of how insignificant this increase in concentration is, since 1950, the 9 nine largest states have made up more than 50% of the US population. Despite the raising score, their share of the population is actually declining, albeit very slowing, from 52% in 1970 to 51% in 2010.