

SPX is pricing 12 vol at the money and 20 at the 10 delta put
As of Monday the September 16 SPX expiration trades about 12.2 implied vol at the money, the market's price for future movement. The same expiration prices its 10 delta put at 19.7 and its 10 delta call at 10.4. That is one index on one date, with the downside strike costing almost twice the vol of the upside one.
Delta works as a rough probability tag, so a 10 delta option has roughly a one in ten chance of finishing in the money. Those two strikes are mirror image long shots. The put sits at 7,200, about 7% below Monday's 7,755. The call sits at 8,100, only 4.5% above.
The distances are lopsided for the same reason the vols are. Downside moves are priced as bigger and faster, so you have to walk 7% away to find a one in ten put but only 4.5% to find the matching call. The put still costs about $17.70 against $8.90 for the call.
The cheapest vol on the whole September line is not at the money. It sits around the 10 delta calls at about 10.4, almost 2 points under ATM, partly because funds that sell calls against stock for income keep that side supplied all year.
The gap also grows with time. The 25 delta put trades 1.7 vol points over the 25 delta call for Tuesday's expiration, 3.5 points for September, 4.3 for October, and about 7 points a year out.
The part I find interesting is that this is the skew, the extra charge on downside strikes, near its flattest of the year. The slope of the put side sits in the bottom decile of the past twelve months, and 30 day vol has been higher on about 90% of days in that window. Even at its sleepiest, the index never prices the two sides symmetrically.
For those who buy index puts for protection, do you pick the strike by its dollar price or by what it costs in vol terms? I ask because the far wing is the cheapest line on the board in dollars and the most expensive one in vol.