and mathematical assumptions.
The underlying is a tradable asset.
There is a single, risk-free interest rate.
The underlying can be shorted.
Proceeds from short sales can be invested at the risk-free rate.
All cash flows are taxed at the same rate.
The underlying's returns are continuous and normally distributed with a constant volatility.
Traders have devised various workarounds to address these limiting assumptions (see Appendix One). The most important of these is the concept of the implied volatility surface. If the BSM were an accurate descriptive theory, all options on a given underlying would have one volatility. This is not true. For the BSM equation to reproduce market option prices, options with different strikes have different implied volatilities (the smile) and options with different maturities have different implied volatilities (the term structure). These implied volatilities make up the IV surface. An example is in shown in Figure 1.1.
The IV surface exists partially because the BSM is mathematically misspecified. The underlying does not have returns that are continuous and normally distributed with a constant volatility. However, even a model that perfectly captured the underlying dynamics would need a fudge factor like the implied volatility surface. Some of the reasons for its existence have nothing to do with the underlying. Different options have different supply and demand, and these distort option prices. Because of this, there is often an edge in selling options with high volatilities relative to others on the same underlying (see the section on the implied skewness premium in Chapter Four).
Equation 1.7 gives the average PL of any hedged option position, but there is a wide dispersion of results for this mean, and the spread of this distribution decreases with the number of hedges. Figure 1.2 shows the PL distribution of a short straddle that is never re-hedged, and Figure 1.3 shows the distribution when the straddle is re-hedged every day. The underlying paths were generated from 10,000 realizations of a GBM. The implied and realized volatility were equal so we expect an average PL of zero.
FIGURE 1.1 The implied volatility surface for SPY on September 10, 2019.
FIGURE 1.2 The terminal PL distribution of a single short one-year ATM straddle that is never re-hedged. Stock price is $100, rates are zero, and both realized and implied volatilities are 30%.
FIGURE 1.3 The terminal PL distribution of a single one-year ATM straddle that is hedged daily. Stock price is $100, rates are zero, and both realized and implied volatilities are 30%.
The dependence of the standard deviation of the PL distribution on the number of hedges is shown in Figure 1.4.
FIGURE 1.4 The standard deviation of the terminal PL distribution of a single one-year ATM straddle as a function of the number of hedges. Stock price is $100, rates are zero, and both realized and implied volatilities are 30%.
The reason to hedge less frequently and accept a wider standard deviation of results is that hedging costs money. All hedges incur transaction costs (brokerage, exchange fees, and infrastructure costs). Costs like this are an easily forgotten drain on a portfolio. Individually they are small, but they accumulate. To emphasize this point, Table 1.1 compares the summary statistics of results for the daily hedged short straddle when there is a transaction cost of $.10 per share and when hedges are costless.
The difference between these two cases is roughly equivalent to misestimating volatility by two points.
In practice, aggressive re-hedging is done by market-making firms and some volatility specialists. The vast majority of retail and buy-side users seldom or never hedge. The relevant theory for those hoping to approximate continuous hedging is discussed in Sinclair (2013). In this book we will generally assume that no re-hedging takes place. These results are also applicable to those who hedge infrequently. They can just assume that the original position has been closed and a new one opened. So, a one-year position that is hedged after a month would thereafter have the expected distribution of an 11-month option.
TABLE 1.1 Statistics for the Short One-Year ATM Daily Hedged Straddle With and Without Hedging Costs (stock price is $100, rates are zero, and both realized and implied volatilities are 30%.)
| Statistic | Costless Hedges | $.10/Share Hedges |
| Average | −$6.10 | −$121.54 |
| Median | −$49.85 | −$111.68 |
| Percent profitable | 44% | 30% |
Conclusion
The BSM model gives the replication strategy for the option. The expected return of the underlying is irrelevant to this strategy. The only distributional property of the underlying that is used in the BSM model is the volatility. A hedged position will, on average, make a profit proportional to the difference between the volatility implied by the option market price (by inverting the BSM model) and the subsequent realized volatility. The choice of the option structure and hedging scheme can change the shape of the PL distribution, but not the average value. These choices are far from immaterial, but successful option trading depends foremost on finding situations in which the implied volatility is mispriced.
Summary
Arbitrage-free
