In an article in the August 2009 issue of Nature , Myrskylä, Kohler and Francesco Billari argue that the previously negative relationship between "development", as measured by the Human Development Index (HDI), and birth rates has become J-shaped. The HDI is a composite of life expectancy, income, and level of education. Development promotes fertility decline at HDI levels below but further advances in HDI cause a small rebound in birth rate.  In many countries with very high levels of development, fertility rates are now approaching two children per woman — although there are exceptions, notably Germany, Italy and Japan. 
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