Now I am confused

In today's Wall Street Journal, economist Ed Prescott (subscription required) writes about five macroeconomic myths. This passage left me scratching my head:

Myth No. 5: Government debt is a burden on our grandchildren. There's no better way to get people worked up about something than to call on their sympathies for their beloved grandkids. The last thing that I want to do is to burden my own grandchildren with the sins of profligacy. But we should stop feeling guilty -- at least about government debt -- because we are in better shape than conventional wisdom suggests.

Theory and practice tell us that the optimal amount of public debt that maximizes the welfare of new generations of entrants into the workforce is two times gross national income, or GDP. This assumes 1% population growth, 2% productivity growth, 4% real after-tax return on investments, and that people work to age 63 and live to age 85. Currently, privately held public debt is about 0.3 times GDP, and if we include our Social Security obligations, it is 1.6 times GDP. In either case, we could argue that we have too little debt.

What's going on here? There are not enough productive assets -- tangible and intangible assets alike -- to meet the investment needs of our forthcoming retirees. The problem is that the rate of return on investment -- creating more productive assets -- decreases as the stock of these assets increases. An excessive stock of these productive assets leads to inefficiencies.

Total savings by everyone is equal to the sum of productive assets and government debt, and if there is an imbalance in this equation it does not mean we have too little or too many productive assets. The fix comes from getting the proper amount of government debt. When people did not enjoy long retirements and population growth was rapid, the optimal amount of government debt was zero. However, the world has changed, and we in fact require some government debt if we care about our grandchildren and their grandchildren.

If we should worry about our grandchildren, we shouldn't about the amount of debt we are leaving them. We may even have to increase that debt a bit to ensure that we are adequately prepared for our own retirements.

Usually, even when I disagree with a fellow economist, I can understand his point of view. But I am puzzled about this passage. Ed must have some model in mind, but I am not sure what it is.

To some degree, it sounds like Ed is worried that the U.S. economy might accumulate more capital than what Ned Phelps called the "Golden Rule" level. As I discuss in my intermediate macroeconomics textbook, Phelps used a Solow growth model to show that an economy can potentially save beyond the point that maximizes steady-state consumption. Peter Diamond subsequently showed that such excessive saving can arise even when overlapping generations of consumers make individually optimal saving decisions. This situation is sometimes called "dynamic inefficiency." When an economy is dynamically inefficient, government debt can have the virtue of depressing the capital stock toward its optimal level.

Andy Abel, Larry Summers, Richard Zeckhauser, and I once wrote a paper on this topic. We concluded that while dynamic inefficiency is possible in theory, it is not a problem in practice. Is Ed now coming to the opposite conclusion, or does he have something entirely different in mind? I am not sure.

Update: Click here.