In the 60s and 70s, economists including Nicholas Kaldor and James Tobin came up with an alternative investment theory: the q-investment theory, sometimes also referred to as Tobin’s q-investment theory. At its core, Tobin’s q theory of investment relates fluctuations in investment to changes in the stock market. Although the theory gained popularity only in the 70s, first elements of the theory can already be found in works of John Maynard Keynes. In his General theory of employment, interest and money, Keynes mentioned already that investment might be linked to the stock market.
Understanding investment activity in an economy is not trivial. The erratic nature of firm level investment activity is somewhat of a mystery to me and it took me quite some time to get a vague idea of what could be the generating process behind such an erratic behavior. I think understanding capital adjustment costs was the key to understand why it can be rational for firms to invest in a spasmodic way. In this post I would like to shortly summarize part of what I learnt so far and list different types of capital adjustment costs found in the literature.