What is Preston curve?
The Preston curve refers to a certain empirical relationship that is witnessed between life expectancy and per capita income in a country.
It was first proposed by American sociologist Samuel H. Preston in his 1975 paper “The changing relation between mortality and level of economic development”.
Preston found that people living in richer countries generally had longer life spans when compared with people living in poorer countries.
This is likely because people in wealthier countries have better access to healthcare, are better educated, live in cleaner surroundings, enjoy better nutrition etc.
When a poor country begins to grow, its per capita income rises and causes a significant increase in life expectancy initially as people are able to consume more than just subsistence calories, enjoy better healthcare etc.
The average per capita income of Indians rose from around ₹9,000 per year in 1947 to around ₹55,000 per year in 2011.
During the same period, the average life expectancy of Indians rose from a mere 32 years to over 66 years.
Problems in the curve
However, the positive relationship between per capita income and life expectancy begins to flatten out after a certain point.
In other words, an increase in the per capita income of a country does not cause much of a rise in the life expectancy of its population beyond a point, perhaps because human life span cannot be increased indefinitely
Experts, however, have disagreed over the causal relationship between income levels and human development indicators.
Many economists have used this positive relationship to argue that the way to improve development outcomes in a country is to encourage economic growth.
The rapid economic growth of India and China over the last few decades, which has helped improve life expectancy and other development indicators, has been cited as an example of faster economic growth leading to better development outcomes.
Other experts, meanwhile, have argued that most improvements in life expectancy have come from a shift in the Preston curve rather than due to a movement along the curve.
That is, higher life expectancy has been achieved by countries even at low per capita income levels.
Such improvement in life expectancy at low income levels, according to these experts, could be due to improvements in medical technology, such as the development of life-saving vaccines.
So, in this view of the Preston curve, improvement in life expectancy and other development outcomes is seen as the result of public investment in human development.
Critics of this view however, argue that technological advancement itself is linked to income levels; richer countries tend to possess better technologies.
It should also be noted that poor countries can benefit from technologies that have already been developed by richer countries.
They may thus be able to achieve higher life expectancy even at very low levels of income, thus providing a boost to their development indicators despite their low income levels
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