# Hedging gamma, theta or other risks

Speaking on a high level, in the Black-Scholes model the $$f\left(T,S_{T}\right)$$ payoff's value dynamic is given by

$$df\left(t,S_{t}\right)=\left(\frac{\partial f}{\partial t}\left(t,S_{t}\right)+\frac{1}{2}\frac{\partial^{2}f}{\partial x^{2}}\left(t,S_{t}\right)\sigma^{2}S_{t}^{2}\right)dt+\frac{\partial f}{dx}\left(t,S_{t}\right)dS_{t},$$

where $$S$$ is the spot price process of an underlying, $$\sigma$$ is a constant volatility parameter. We try to catch this dynamic with a replicating portfolio of the form

$$dX_{t}=r\left(X_{t}-\Delta_{t}S_{t}\right)dt+\Delta_{t}dS_{t},$$

where $$r$$ is the constant risk free rate, $$\Delta_{t}$$ is the amount of underlying to hold in order to construct the replicating portfolio. To have $$X=f$$ $$\forall t$$, $$dX_{t}=df_{t}$$, we have $$\frac{\partial f}{\partial x}\left(t,S_{t}\right)=\Delta_{t}$$ and get a partial differential equation to equate the $$dt$$s term

$$\frac{\partial f}{\partial t}\left(t,x\right)+\frac{1}{2}\frac{\partial^{2}f}{\partial x^{2}}\left(t,x\right)\sigma^{2}x^{2}=r\left(f(t,x)-\frac{\partial f}{\partial x}\left(t,x\right)\right),$$

which PDE's solution give the value of the $$f\left(T,S_{T}\right)$$ payoff at $$t$$, when the underlying price is $$S_{t}$$.

So in the “derivation” above, it is reasonable to say that via holding $$\Delta_{t}=\frac{\partial f}{\partial x}\left(t,S_{t}\right)$$ at every $$t$$, the risk sourcing from the spot price change is hedged. But how are the $$\frac{\partial f}{\partial t}\left(t,S_{t}\right)$$ theta and $$\frac{\partial^{2}f}{\partial x^{2}}\left(t,S_{t}\right)$$ gamma risk are hedged? Is it fair to say that (theoretically) these “risks” are hedged via the act of asking for $$X_{0}=f\left(0,S_{0}\right)$$ price for the trade in order to construct a replicating portfolio?

How are these risks (gamma, theta, kappa etc) are hedged in practice/ in real life?

• In the BS framework, profits(losses) from gamma are equal to the losses(profits) from theta. In reality, IV > RV most of the time, so you’d profit from just selling options. If you want a literally example; path dependent options can be used to hedge gamma, e.g. barrier options. Or you can setup a gamma-neutral spread such that the multiple options you have give a neutral gamma. I.e. portfolio of short and long an options such that the sum of their gamma’s equal 0. Commented Feb 17 at 12:38

In the Black-Scholes model Gamma and theta do not need to be hedged because the BS PDE says that they balance each other (I'll take $$r = 0$$): $$\frac{\partial f}{\partial t} + \frac12 \sigma^2 S^2\frac{\partial^2 f}{\partial S^2} = 0$$

The need for Gamma hedging comes from model mis-specification. What that means is this: You price an option assuming that the spot price satisfies $$dS = \sigma S dW$$ where $$\sigma$$ is some constant. However, the actual dynamics of the spot price may be $$dS = \sigma_R S dW, \quad \sigma_R \neq \sigma$$ where for simplicity let's assume that $$\sigma_R$$ is also constant, and the subscript $$R$$ stands for realized.

Then the change in the value of a delta-hedged position is (by Ito-calculus) $$df - \frac{\partial f}{\partial S} dS = \frac{\partial f}{\partial t} dt + \frac12 \frac{\partial^2 f}{\partial S^2} (dS)^2$$ Now $$(dS)^2$$ is $$\sigma^2_R S^2 dt$$, since we mis-spcified the model, and by the BS PDE $$\frac{\partial f}{\partial t} = - \frac12 \sigma^2 S^2 \frac{\partial^2 f}{\partial S^2}$$ since we priced $$f$$ assuming that $$dS = \sigma S dW$$.

So the P/L from the delta-hedged position is $$df - \frac{\partial f}{\partial S} dS = \frac12 S^2 \frac{\partial^2 f}{\partial S^2} \left( \sigma_R^2 - \sigma^2 \right) dt$$

So if you want zero p/l you'll need to hedge Gamma by trading another option in such a manner that delta hedging that option gives an offsetting Gamma P/L.

In reality $$\sigma_R$$ is not even constant and you get all kinds of other P/L terms, but the main point you should internalize is that models are almost surely mis-specified and that other options are needed to hedge away the mis-specification, should you wish to do so.

• Kinda make sense, but not fully. Using your reasoning and according to the PDE, they only balance each other perfectly when $r=0$, as your example suggests. Isn't theta the same as (-"dollarized gamma")+(infinitesimal change in the bank account) and in this case indeed the balance is perfect? Commented Feb 17 at 20:35