It was first proposed by American sociologist Samuel H. Preston in 1975.  

• It highlights that an increase in per capita income of a country does not cause much of a rise in the life expectancy of its population beyond a point. 
  • When a poor country begins to grow, its per capita income rises and causes increase in life expectancy initially due to nutrition, sanitation and access better healthcare.
  • However, it begins to flatten out after a certain point.