Volatility skew
ボラティリティ・スキュー。権利行使価格ごとにインプライド・ボラティリティが異なる現象で、通常は暴落への警戒を反映してプットのIVが高くなる。
Volatility skew describes how implied volatility differs across strikes for options on the same underlying with the same expiration. In theory, every strike would carry the same implied vol, but in real markets it doesn't work that way. For most equities and indexes, out-of-the-money puts trade at a higher implied vol than out-of-the-money calls, because investors pay up for downside protection. That downward-sloping shape is the classic equity skew.
A trader reads the skew to spot which options are relatively expensive or cheap in vol terms, not just in dollars. Say a stock sits at 100, the 90 put shows 35% IV and the 110 call shows 22%. Selling that pricey put (as a cash-secured put or the short leg of a put spread) means you're being paid for the market's fear. If you buy the cheaper call, you're getting upside vol at a discount.
The common mistake is treating a high IV number as automatically "overpriced." Skew is often steep for a reason: earnings, a pending event, or genuine crash risk. Selling the fat put can hand you a large loss if the stock actually drops. Read the skew for relative value, but always ask why the shape looks the way it does before leaning into it.
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