Preston law

The Preston curve shift

The Preston curve is similar in shape to the logarithmic curve, with income along the x-axis and life expectancy along the y-axis. According to this formal relationship, the variation in life expectancy across countries per US$100 increase in per-capita national income is characterised by a steep increase in life expectancy among countries with low levels of income and a much slower increase in life expectancy among countries with high levels of national income.

A single Preston curve reflects a cross-sectional relationship in one period. Comparing these relationships between different time periods (1900s, 1930s, and 1960s) and taking into account differences in purchasing power, Preston found that the curves shifted upwards with time—ie, to higher life expectancies—with minor changes in shape. This pattern of change between periods suggested that the life expectancy increase over time was mostly attributable to the growing ability of the same amount of income to essentially buy more lifetime. Preston estimated that this accounted for 75–90% of the increase in life expectancy over time. This change was thought to operate through general improvements in knowledge about prevention and treatment diffusing across countries irrespective of levels of national income. The modern version of the Preston curve uses the per-capita GDP (PPP) as the measure of national wealth. More recent analyses have shown that the Preston relationship between life expectancy and per-capita GDP (PPP) has continued over the 1970s–2000s and still fits well with empirical data.

The classic vertical shift of the Preston curve over time is visible even amid declines in national income. Notably, decreases in mortality rates in European countries were sustained between 2003 and 2012 despite the great economic recession of 2008 with the concomitant stagnation or decline in national wealth. In particular, Spain and Italy experienced persistently high longevity despite abrupt deteriorations in the labour market, which led to dramatic drops in the fraction of lifetime spent in employment. More broadly, since the 1980s, life expectancy in many developing countries has increased substantially with little growth in national income. Furthermore, a recent Global Burden of Disease analysis has shown that mortality corresponding to the same level of their Socio-Demographic Index tends to decline with time.

Altogether, although these findings suggest that national income is an important resource for improvement of health, higher national wealth does not automatically result in improved health or increased survival.

However, a study of European countries has found that over the past three decades, increases in life expectancy have been more closely associated with national wealth than in earlier periods. The authors suggest that this is because of the growing contribution of cutting-edge medical technologies to mortality decline whose adoption requires substantial increased investment in equipment and expertise.

GDP (PPP)=gross domestic product by purchasing power parity.