As investors position themselves for the final quarter of the year, markets have yet to price in the incertitude ahead. There are three reasons for this; all are related to the inherent difficulties of accurately determining risk premiums in this context of unusual economic, financial and political uncertainty.

1. The largest uncertainty is political.

In terms of systemic risk, Europe is the source of the greatest political uncertainty in coming months: The government of Prime Minister Theresa May in the U.K. will engage in tense negotiations with other European Union members over Brexit, even as the EU itself is beset by growing internal divisions on immigration. In addition, Italy will hold a referendum in December that could decide the fate of Prime Minister Matteo Renzi’s government. Meanwhile, Greece’s debt problems continue to fester, and the popularity of Chancellor Angela Merkel, the region’s most influential leader, is being challenged by the migrant crisis.

This unpredictability is particularly hard to price because it raises questions about long-standing tenets of regional economic interaction, including the institutions and rules governing cross-border movements of goods, labor and financial services.

The atypical U.S. presidential election also exposes America’s economy to unusual political uncertainty. Although both candidates have signaled they wish to step back from the enthusiastic embrace of international trade that has strengthened the U.S.’s economic role in the world, Donald Trump has gone a lot further by advocating punishing import tariffs on China and Mexico. Should he win in November, and make good on the promise of trade penalties, there would be significant risk of retaliatory actions from trading partners that could raise the threat of a contraction in economic activity.

Until now, markets have essentially disregarded these uncertainties. Judging from history, political rhetoric, especially during heated election campaigns, does not always translate into action. Also, given the binary nature of the possible outcomes of some of these events (such as the Italian referendum), the appropriate risk premiums are even harder to determine. Finally, for some time, investors have been able to rely on exceptional liquidity injections to contain and reverse the occasional bout of politically induced volatility.

2. Central banks have been the most effective repressors of volatility.

Through the use of an expanding set of unconventional measures, these institutions have been effective in boosting financial asset prices and repressing market volatility -- directly, through large-scale asset purchases, and indirectly, by inducing investors to assume greater investment risks and companies to buy back stock. Both influences have contributed to a “buy-on-the-dips” mindset that has acted as an additional internal stabilizer.

Nonetheless, there is a now a growing feeling that the effectiveness of such central bank measures may be waning. For one, there is evidence of mounting concerns among some officials about the risk of engendering future financial instability (judging from the three dissenters at the recent Federal Open Market Committee meeting, this worry may be fueling an active discussion within the Fed).The questioning of policy is a lot more intense in Japan, where a growing number of indicators suggest that the central bank has suffered a significant blow to its policy effectiveness, and may risk becoming counterproductive.

Over the course of this quarter, investors will get more evidence about whether the center of gravity within the Fed will end up tipping in favor of a rate hike, and whether further stimulus on the part of the Bank of Japan is indeed counterproductive. But the most important indicator will come from the European Central Bank, which will face a new set of challenges as it tries to shield markets from the region’s more complicated politics and its persistently sluggish economy. Questions could arise about how long it can effectively perform this role, particularly as it faces growing political pressures.