An aggregate production function provides us a way to organize our thinking about the growth process.
Economists typically assume that the production function exhibits constant returns to scale .
Economists assume that the production function is subject to diminishing marginal productivity in its inputs. This means that if a single input is increased, holding all other inputs constant, output will grow but at a diminishing rate as the input is increased again and again. However, as capital becomes more plentiful, additions to output per worker become smaller. This is because capital is employed in its most productive uses first. As more capital is accumulated, it is used in less productive ways.