Abstract I investigate the effect of financial innovation on portfolio risks when traders have belief disagreements. I decompose traders’ average portfolio risks into two components: the uninsurable variance, defined as portfolio risks that would obtain without belief disagreements, and the speculative variance, defined as portfolio risks that result from speculation. My main result shows that financial innovation always increases the speculative variance through two distinct channels: by generating new bets and by amplifying traders’ existing bets. When disagreements are large, these effects are sufficiently strong that financial innovation increases average portfolio risks, decreases average portfolio comovements, and generates greater speculative trading volume relative to risk-sharing volume. Moreover, a profit-seeking market maker endogenously introduces speculative assets that increase average portfolio risks.

Belief disagreements have been suggested as a major contributing factor to the recent subprime mortgage crisis. This paper theoretically evaluates this hypothesis. I assume that optimists have limited wealth and take on leverage so as to take positions in line with their beliefs. To have a significant effect on asset prices, they need to borrow from traders with pessimistic beliefs using loans collateralized by the asset itself. Since pessimists do not value the collateral as much as optimists do, they are reluctant to lend, which provides an endogenous constraint on optimists' ability to borrow and to influence asset prices. I demonstrate that the tightness of this constraint depends on the nature of belief disagreements. Optimism concerning the probability of downside states has no or little effect on asset prices because these types of optimism are disciplined by this constraint. Instead, optimism concerning the relative probability of upside states could have significant effects on asset prices. This asymmetric disciplining effect is robust to allowing for short selling because pessimists that borrow the asset face a similar endogenous constraint. These results emphasize that what investors disagree about matters for asset prices, to a greater extent than the level of disagreements. When richer contracts are available, relatively complex contracts that resemble some of the recent financial innovations in the mortgage market endogenously emerge to facilitate betting.

I illustrate the effect of financial innovation on portfolio risks by using an example with risk-sharing needs and belief disagreements. I consider two types of innovation: product innovation, formalized as an expansion of new financial assets; and process innovation, formalized as a reduction in transaction costs. When belief disagreements are large, both types of innovation increase portfolio risks. Moreover, endogenous financial innovation is directed towards speculative assets that increase portfolio risks.

Does the provision of leverage to retail traders improve market quality or facilitate socially inefficient speculation that enriches financial intermediaries? We evaluate the effects of 2010 regulations that cap leverage in the U.S. retail foreign exchange market. Using three unique data sets and a difference-in-differences approach, we document that the leverage-constraint reduces trading volume by 23%, alleviates high-leverage traders’ losses by 40%, and reduces brokerages’ operating capital by 25%. Yet, the policy does not affect the relative bid-ask prices charged by the brokerages. These results suggest the policy improves belief-neutral social welfare without reducing market liquidity.

Abstract We theoretically investigate the interaction of endogenous risk intolerance and monetary policy following a large recessionary shock. As asset prices dip, risk-tolerant agents’ wealth share declines. This decline reduces the market’s risk tolerance and triggers a downward loop in asset prices and aggregate demand when the interest rate policy is constrained. In this context, large-scale asset purchases are effective because they transfer unwanted risk to the government’s balance sheet. These effects are sizable when the model is calibrated to match the estimates of aggregate asset demand inelasticity. The COVID-19 shock illustrates the environment we seek to capture.

Abstract We provide a continuous-time “risk-centric” representation of the New Keynesian model, which we use to analyze the interactions between asset prices, financial speculation, and macroeconomic outcomes when output is determined by aggregate demand. In principle, interest rate policy is highly effective in dealing with shocks to asset valuations. However, in practice monetary policy faces a wide range of constraints. If these constraints are severe, a decline in risky asset valuations generates a demand recession. This reduces earnings and generates a negative feedback loop between asset prices and aggregate demand. In the recession phase, average beliefs matter because they not only affect asset valuations but also determine the strength of the amplification mechanism. In the ex ante boom phase, belief disagreements (or heterogeneous asset valuations) matter because they induce investors to speculate. This speculation exacerbates the crash by reducing high-valuation investors’ wealth when the economy transitions to recession, which depresses (wealth-weighted) average beliefs. Macroprudential policy that restricts speculation in the boom can Pareto improve welfare by increasing asset prices and aggregate demand in the recession.

We provide evidence of the stock market consumption wealth effect by using a local labor market analysis. An increase in local stock wealth driven by aggregate stock prices increases local employment and payroll in nontradable industries and in total, with no effect on employment in tradable industries. In a model of geographic heterogeneity in stock wealth, these responses imply an MPC of 3.2 cents per year and that a 20 percent increase in stock valuations, unless countered by monetary policy, increases the aggregate labor bill by at least 1.7 percent and aggregate hours by at least 0.7 percent two years after the shock. (JEL E21, E24, E52, G12, G51, R22, R23 )

We investigate the role of macroprudential policies in mitigating liquidity traps. When constrained households engage in deleveraging, the interest rate needs to fall to induce unconstrained households to pick up the decline in aggregate demand. If the fall in the interest rate is limited by the zero lower bound, aggregate demand is insufficient and the economy enters a liquidity trap. In this environment, households' ex ante leverage and insurance decisions are associated with aggregate demand externalities. Welfare can be improved with macroprudential policies targeted toward reducing leverage. Interest rate policy is inferior to macroprudential policies in dealing with excessive leverage. (JEL D14, E23, E32, E43, E52, E61, E62)

Central banks (the Fed) and markets (the market) often disagree about the path of interest rates. We develop a model where these different views stem from disagreements between the Fed and the market about future aggregate demand. We then study the implications of these disagreements for monetary policy, the term structure of interest rates, and economic activity. In our model, agents learn from the data but not from each other—they are opinionated. In this context, the market perceives monetary policy “mistakes” and the Fed partially accommodates the market’s view to mitigate the impact of perceived “mistakes” on output and inflation. The Fed plans to implement its own view gradually, as it expects the market to receive more information and move closer to the Fed’s belief. Disagreements about future demand translate into disagreements about future interest rates. Disagreements also provide a microfoundation for monetary policy shocks: after a surprise policy announcement, the market (partially) learns the Fed’s belief and the extent of future “mistaken” interest rate changes. We categorize these shocks into three groups: Fed belief shocks, market reaction shocks, and tantrum shocks. Tantrum shocks are the most damaging, as they arise when the Fed fails to forecast the forward rates’ reaction. These shocks motivate gradualism and communication policies that reveal the Fed’s belief, not to persuade the market (which is opinionated) but to prevent the market from misinterpreting the Fed’s belief. Finally, we also find that disagreements affect inflation and create a policy trade-off between output and inflation stabilization akin to “cost-push” shocks.Institutional subscribers to the NBER working paper series, and residents of developing countries may download this paper without additional charge at www.nber.org.

ABSTRACT We analyze optimal monetary policy and its implications for asset prices when aggregate demand has inertia. If there is a negative output gap, the central bank optimally overshoots aggregate asset prices (above their steady‐state levels consistent with current potential output). Overshooting leads to a temporary disconnect between the performance of financial markets and the real economy, but accelerates the recovery. When there is a lower bound constraint on the discount rate, good macroeconomic news is better news for asset prices when the output gap is more negative. Finally, we document that during the COVID‐19 recovery, the policy‐induced overshooting was large.