Wednesday, 27 November 2013

Homicide and teen births

Part of The Spirit Level hypothesis is that teen births and homicide are somehow caused by inequality (chapters 9 and 10). However, Wilkinson and Pickett face a problem insofar as inequality has been rising in most countries for many years while rates of teenage births and murder have been falling. They attempt to square this circle on page 142 when talking about their worst performer, the USA.

Homicide rates in America, after rising for decades, peaked in the early 1990s, then fell to their lowest level in the early 2000s. In 2005, they started to rise again. Similarly, after peaking in the early 1990s, teenage pregnancy and birth rates began to fall in America, and the decline was particularly steep for African-Americans. But in 2006, the teenage birth rate also started to rise again, and the biggest reversal was for African-American women.

The standard Gini measurement of inequality (see below) isn't very helpful to Wilkinson and Pickett in this instance since it shows inequality to have been rising pretty much continuously since the early 1990s which is exactly when the homicide and teen birth rates started to fall.

Faced with this obstacle, they resort to an obscure discussion paper which paints a quite different picture of the US trend, with...

...inequality rising through the 1980s to a peak in the early 1990s. The following decade saw an overall decline in inequality, with an upturn since 2000.

This finding is contrary to all other evidence and has been described as "the equivalent of the sun orbiting the earth", but it nevertheless allows Wilkinson and Pickett to triumphantly conclude...

So there is a reasonable match between recent trends in homicides, teenage births and inequality—rising through the early 1990s and declining for a decade or so, with a very recent upturn.

In other words, the 1990s saw falling inequality and therefore falling rates of teen births and homicide, whereas the Noughties saw rising inequality and therefore rising rates of teen births and homicide.

This is patently at odds with the facts. As I mentioned in The Spirit Level Delusion, it is a stretch to say that the homicide rate "started to rise again" in 2005. In fact, there was a tiny blip in 2005-06 when the murder rate went from 5.5 per 100,000 to 5.7 (see below). After that, the downward trend returned. By 2011, it was 4.7 per 100,000.

Moreover—and the reason for this little blog post—I recently had cause to look up the US's teen birth rates which last year "reached historic lows for all age and ethnic groups". Here too we see a little blip in the middle of the Noughties followed by a continued decline.

The Spirit Level was published in 2009 and so the data from the most recent years were naturally not included. Nevertheless, it was, at best, rash of its authors to present a slight upturn in the figures as the start of an inequality-fuelled rising trend. No matter which set of inequality figures one uses—and the Gini figures are vastly more credible—Wilkinson and Pickett's argument does not stand up.

We now know for certain that the small increase in the homicide and teen birth figures in 2005-06 was just a blip. The facts are quite clear. Inequality has been rising in the USA while the homicide and teen birth rates have been falling. There is simply no correlation between these variables. We need to look elsewhere for an explanation.