The Expected Move Bracket Isn't Actually Symmetric
The bracket everyone draws
The standard expected move is built from the at-the-money straddle: take the combined call and put premium, multiply by roughly 0.85, and you get a dollar range. Add that number above and below the stock price and you have a symmetric bracket — say, $180 stock, $12 expected move, so an $168–$192 range going into earnings. It's a clean number, and it's the one most tools show. It's also, mechanically, making an assumption that the option chain itself doesn't support.
What the straddle formula assumes
The 0.85-times-straddle shortcut comes from treating the stock's future price as normally distributed around the current spot, with the straddle premium standing in for one standard deviation of that distribution. A symmetric bell curve has, by construction, equal probability mass an equal dollar distance above and below the mean. That's the math producing the equal $12-up, $12-down bracket. The assumption is baked into the formula, not derived from where the market is actually pricing risk.
Skew says the chain disagrees
Look at the same chain's volatility skew and the symmetry breaks down immediately. An out-of-the-money put at, say, 10% below spot almost always carries higher implied volatility than an out-of-the-money call 10% above spot. That's not noise — it's the market pricing crash risk and hedging demand asymmetrically, a pattern that holds on the overwhelming majority of single stocks and indexes. If the put at $168 is priced with meaningfully higher IV than the call at $192, the option chain is explicitly telling you those two prices don't carry equal probability weight, even though they're equidistant in dollars from spot.
Put differently: the ATM straddle prices the width of the move reasonably well, but it says nothing about which side of spot that width should lean toward. Skew is the piece of the chain that answers that question, and the plain expected-move formula ignores it entirely.
Building a bracket that matches the skew
A closer approximation of the market's real implied distribution uses the strikes on either side of spot where delta sits near an equal absolute value — commonly the 16-delta put and 16-delta call, which roughly bracket a one-standard-deviation move under a lognormal assumption. Because IV differs at those two strikes, the dollar distances from spot are no longer equal. On a stock with the typical put-skew shape, the 16-delta put strike usually sits further below spot than the 16-delta call strike sits above it — the downside boundary of the "safety zone" is wider than the upside boundary, reflecting that the market is pricing more room for a large drop than an equally large rally.
This isn't a different data source — it's the same option chain, read one column further. Anywhere you're scanning strikes for expected move or strangle strikes, checking the delta and IV at both wings against each other — rather than assuming they mirror the ATM number — takes only a few seconds longer.
Why this matters going into an event
For a defined-risk structure like an iron condor sold around an earnings date, treating the expected move as symmetric means the short put strike is, in relative probability terms, closer to spot than the short strike on the call side — the position is quietly carrying more downside risk than the same-width upside risk suggests. For a straight long strangle, the asymmetry cuts the other way: the put leg is paying up for a wider, richer-priced tail than the call leg is. Neither error is fatal on its own, but a bracket built from the straddle formula alone will consistently understate how much of the market's priced-in risk sits on the downside. Checking the wing deltas and IVs against each other before setting strikes catches the gap the symmetric number misses.