Why is the harrod-domar growth model not widely used these days?

Answer 1

The reason why it is not formally taught as a viable growth model is due to its inherent weaknesses. The weaknesses lie in the assumptions of the model. When creating an economic theory, you can make any assumptions you want, regardless of how unrealistic they may be. If the model starts to fall apart when you rest the weakest assumptions, it loses credibility.

One problem with the model is that the price for labor and capital (wage rate and interest rate) are fixed. Along with this assumption, the model assumes that each input is used in equal proportions. In reality we know that these assumptions don't hold.

Another problem with the model is that is assumes investors (savers) are only influenced by changes in output. The greater the output, the more investors will invest capital which in turn increases output. This is known as the accelerator principle and it does not hold up in empirical studies. Investors are influenced by the amount of risk they must take given the expected rate of return they will receive on their investment.

A model that rests the assumptions of the H-D model is the Solow Model (aka Solow-Swan Model). It uses some of H-D framework but then expands on it to allow for flexibility in the use of both capital and labor as flexible inputs to output. A great source for a more detailed but easy to understand explanation is Wikipedia. Check out the related link. After reading this, review the commentary on the Solow Model. Hope this help.