Perhaps ironically, one of the most powerful challenges to any Keynesian diagnosis of economic ailments also focuses on inadequate investment spending, but from a wholly different perspective. That challenge is today most closely associated with the economist Robert Higgs.
Higgs' careful look at the data on the Great Depression and World War II convinced him that (1) a U.S. economy producing genuine prosperity wasn't restored until 1946, and (2) investors hunkered down, especially from 1935-40, because New Deal regulations -- along with President Franklin Roosevelt's increasingly vocal hostility to enterprise and successful risk-takers -- created too much uncertainty about how government would treat profits and wealth accumulation.
The "regime uncertainty" -- described by Higgs as "a pervasive uncertainty among investors about the security of their property rights in their capital and its prospective returns" -- unleashed by actual and threatened New Deal interventions made private innovation and entrepreneurial effort simply too unattractive. So private investment spending largely ground to a halt during FDR's reign.Both are essentially arguing that the focus should not be on the demand side, as both Keynesians and monetarists have argued, but on the supply side. To understand the reason that recovery was so slow in the Great Depression, and by extension the reason we see so little recovery now, look not to the theories of macroeconomics, but to some of the literature on economic growth and development that argues that secure property rights and an impartial legal system are keys to economic growth.