The concept of distance decay is often used by geographers when trying to describe and explain spatial patterns.
Distance decay is the “attenuation of a pattern or process with distance” (Dictionary of Human Geography, Blackwell 1986). It is one of the fundamental concepts behind many geographic models (e.g. Christaller, von Thünen) and the basis of W. Tobler’s first law of geography: “everything is related to everything else, but near things are more related than distant things.” (Tobler 1970)
A recent study published in Addiction found that the principle of distance decay applies to the purity of drugs sold in the USA. Most drugs entering the USA do so via the Mexico-USA border. It presumably costs more, and entails higher risks, to transport drugs further north than just over the border.
The health risks associated with illicit, non-prescription drugs are related to drug purity. Addiction rates also vary with purity. Surprisingly, the geographic pattern of drug purity within a single country has rarely been investigated in detail before.
The researchers analyzed the drugs seized in about 240,000 drugs incidents for the period 1990-2004. They also looked at the correlation between drug purity and the time, and size, of each drug seizure.
The researchers found that the purity of methamphetamine, cocaine and heroin generally decreased with distance from the border. Beyond 1450 km (900 miles), however, heroin purity increased again towards the northeast. The pattern for heroin, and a similar pattern for methamphetamine after 2000, suggest that perhaps some of these drugs were imported via the eastern seaboard of the USA (heroin) or from southeastern Canada (methamphetamine). The findings also suggest that methamphetamine from Canada may be on a larger scale than previously thought.
For all three drugs, the purity of small amounts seized close to the border showed more variation than was true for seizures further north.
Given the conclusions of this study, researchers may be better able to predict the areas which are likely to have higher rates of drug dependence and overdoses.
And some people still think geography is boring?
Sources: Cunningham J.K., Maxwell J.C., Campollo O., Cunningham K.I., Liu L.M., Lin H.L.: “Proximity to the US-Mexico border: a key to explaining geographic variation in US methamphetamine, cocaine and heroin purity”. Addiction. 2 August 2010.
Tobler, W. 1970 “1970: a computer movie”. Economic Geography 46: 234-240.