Why do economies grow? What fixes the long-run rate of growth? These are some of the simplest, but also hardest, questions in economics. Growth of lack of it has huge consequences for a country's citizens. But for various reasons, growth theory has had long fallow patches. Happily, this is changing.In 1956 Robert Solow developed what became the standard neo-classical model of economic growth. Counties grow, on this theory, by accumulating labour and capital.