Evolutionary biology
Herwig Birg has called the inverse relationship between income and fertility a “demo-economic paradox”"." Evolutionary biology dictates that the more successful individuals, or in this case countries, would seek to develop optimum conditions for their life and reproduction. However, in the last half of the 20th century it has become clear that the economic success of developed countries is being counterbalanced by a demographic failure (that is, a below replacement fertility rate) that may prove destructive for their future economies and societies.