Austerity versus Growth? Which economic model is sustainable? If it weren't for those pesky bond vigilantes, it may be only politics. Let's not get too excited that either path will work. Let's look at the implications for investors with a focus on the U.S. dollar.

To have budgets sustainable is as simple as matching revenue and expenses. Almost. Politicians have figured out that running a deficit may still yield a stable debt-to-GDP ratio, assuming there is economic growth. When U.S. politicians brag their budget forecasts bring down the absolute level of the deficit, it's the debt-to-GDP ratio at best that may improve, assuming their rosy projections of economic growth prove correct. In the Eurozone, governments have - theoretically - committed themselves to running no more than 3% deficits, suggesting that such deficits lead to sustainable debt-to-GDP ratios. Not quite. Our research shows that, in order to achieve a long-term debt-to-GDP ratio in the low 50% range, deficits of no more than half of GDP growth may be run. If GDP growth is 3% a year, the deficit ought to be no higher than 1.5%. If 3% deficits are allowed, but GDP growth is only half of that - a more realistic assumption for many countries in today's environment, the debt-to-GDP ratio will stabilize at above 200%. For a visualization of where the debt-to-GDP ratio will peter out given different growth and deficit scenarios, consider this table:

Why do we care about debt-to-GDP ratio? Allowing a high debt-to-GDP ratio is akin to playing with fire. Just look at the housing bust for an illustration as to what's so dangerous about piling up too much debt: when interest rates are low, life feels great, the standard of living has gone up. But when interest rates move up, those with a higher debt load have to make disproportionate cuts to their standard of living to service their debt. According to the Congressional Budget Office (CBO), the U.S. paid an average of 2.2% of GDP in interest from 1972 until 2011. In its "Extended Alternative Fiscal Scenario" that assumes current policies, as opposed to current laws, remain in place, net interest payments will soar to 9.5% of GDP in 2037. In 2011, $454.4 billion in interest was paid in a $15 trillion economy; that is, 3% of GDP was spent in servicing interest payments. If the U.S. were to spend 9.5% of GDP to service its debt rather than 3%, it requires a cut of 6.5% of GDP in other services. It dwarfs the fiscal cliff (the reference to Bush era tax cuts running out coupled with spending cuts, absent of Congressional action) that might put a 3% to 5% dent on GDP.

The pessimistic CBO outlook assumes what we judge to be an unrealistically low 2.7% average interest rate on debt payments. Should the market demand more compensation - generally referred to as bond vigilantes holding policy makers accountable - financing the deficits may no longer be feasible. Just ask Greece; or Spain.

With 10 year Treasuries yielding 1.6% in the United States, some argue that higher deficits are warranted. As the discussion above shows, the logic behind the view is fundamentally flawed, if not reckless. In the U.S., we differentiate between discretionary and mandatory spending. Mandatory refers to contractual obligations, the entitlements, most notably regarding Social Security and Medicare. Put simply, it may not be realistic in the long run to finance thirty years of retirement with forty years of work. As interest payments take a bigger chunk of GDP, discretionary spending is squeezed out. Be that an infrastructure project at the federal level; teachers at the state level; or firefighters or animal shelters at the local level.

In real life, there are some added complexities: we all fight for our benefits, take them for granted once granted and will fight vigorously to retain them. Also keep in mind that emergencies, such as natural disasters or wars, are typically not reflected in budget forecasts, as they might be considered "one off" items; even if that were correct, emergency spending adds to the deficit.

As debate rages whether austerity or growth is the solution to the plight of the developed world, keep in mind that there is a history spanning centuries, if not millennia, of governments spending too much money. In the old days, neighboring countries were "taxed" to fill domestic coffers: taxation, as in conquering other countries and taking treasures and slaves. Debt is a modern form of slavery, except that it is voluntary servitude. If history is any guide, don't get your hopes up too high that these issues will be resolved. But investors may be able to navigate the waters, mitigate some of the risks or even profit from opportunities that arise in this environment.

The growth camp suggests that, with enough growth, a high debt load can be carried. That may be the case if such growth is not financed through debt. In recent years, trillions have been spent to achieve billions in growth - not exactly a recipe for sustainable budgeting.

The austerity camp suggests that, with enough austerity, books can be balanced. Except that, as services are cut, the economy is at risk of entering a downward spiral, increasing, rather than decreasing deficits.

And while these two ideologies are being discussed, central banks struggle to either keep the banking system afloat (as in the Eurozone) or are helping to finance the government deficit (as in the U.S.). Indeed, the key difference between the U.S. and European model is that U.S. Treasuries appear to be backed by a) the taxing power of Congress and b) a lender of last resort, the Federal Reserve (Fed) with its printing press; whereas European sovereign debt is backed only by the taxing power of the national governments. The European Central Bank (ECB) has made it clear that its role is to support the banking system, not the sovereigns. In contrast, the Federal Reserve (Fed), while not admitting to outright financing of government deficits, has shown a willingness to do so in practice. As such, European sovereign debt is trading more like municipal bonds trade in the U.S., with weak "municipalities" (think Spain) paying a huge premium.

To be fair, the growth camp wants more than deficit spending; and the austerity camp wants more than cost cutting. Indeed, in a recent analysis Saving the Euro, we argued policy makers should instead focus on competitiveness, common sense and communication. Ultimately, both camps believe their philosophies should attract more investment as confidence is increased. In practice, the proverbial can is kicked down the road. By the time the can reaches the end of the road, it may be very beaten up. Indeed, an increasing number of investors are spreading their investments across multiple "cans" - we refer to these "cans" as diversified baskets of currencies, including gold.

What's different between the U.S. and Europe is that the U.S. has a significant current account deficit. In our experience, countries with current account deficits tend to favor growth oriented strategies as they need to attract money from abroad to finance their deficit and, as such, to support the currency. While a bond market in shambles is a major drag on growth in the Eurozone, the Euro has been able to hold up reasonably well given the fact that the Eurozone's current account is roughly in balance. A misbehaving bond market might have dire consequences for the U.S. dollar, even if one disregards the odds that the Federal Reserve may make U.S. Treasuries even less attractive by financing government spending (when a central bank buys a country's own debt by printing money, such securities are intentionally over priced, potentially weakening the currency).

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