q-theory in a Time of AI

A staple of mid-1980’s investment modeling, q-theory was an alternative to the Keynes’ marginal efficiency of capital or Jorgenson user cost of capital approaches. A version of q –the ratio of market value to replacement value of a corporation’s capital — was central in Summers’ 1981 BPEA paper modeling corporate investment behavior. And yet, q-theory is now almost completely absent from recent discussion of the level of capital investment (in contrast to the debate nearly two decades ago). Why? First a picture of q, the market price relative to replacement cost of physical capital.

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