本研究探討美國股票市場中隱含波動率之波動所呈現的負向定價溢酬，是否反映由投資人過度樂觀與套利限制共同造成的真實錯誤定價，以及此一效果是否隨時間出現結構性衰退。既有文獻指出，個股自身波動率不確定性較高者，其未來報酬往往較低；然而，其背後的行為機制及其隨時間演變的情形，仍缺乏充分探討。

本研究使用 OptionMetrics 於 1996 年至 2023 年間的日頻選擇權資料，建構一項相對有符號的波動率之波動指標，用以衡量個股上漲與下跌價格變動所引發之隱含波動率變化的相對強度。實證結果顯示，整體錯誤定價主要來自波動率之波動高度集中於股價上漲期間的股票。此一錯誤定價在樣本早期，即自網路泡沫破裂至 2008 年全球金融危機期間，具有高度顯著性；但在近期樣本中已明顯減弱。與此同時，多項常用的套利限制代理變數，包括特質波動率、股票非流動性、買賣價差與公司規模，均呈現系統性且統計上顯著的改善趨勢。

條件式雙重排序分析與橫斷面交互作用迴歸結果進一步證實，以上漲行情為主導的波動率不確定性，其負向報酬溢酬在特質風險較高、流動性較差、買賣價差較大及市值較小的股票中最為明顯；且上述四項套利限制代理變數在早期樣本期間皆會顯著放大錯誤定價。此外，該異常現象在投資人情緒高漲時更為強烈，支持過度樂觀投資人所造成的向上價格壓力，是初始高估的重要來源。

整體而言，本研究顯示，此一不對稱波動率異常源自樂觀投資人與套利限制之間的交互作用；隨著現代金融市場效率提升，該效果已出現結構性弱化。

This study examines whether the negative pricing premium associated with the volatility of implied volatility in the U.S. stock market reflects a genuine mispricing driven by investor over-optimism and limits to arbitrage, and whether this effect has structurally decayed over time. Prior studies document that stocks with high uncertainty about their own volatility earn lower future returns, yet the underlying behavioral mechanism and its evolution over time remain underexplored. Using daily options data from OptionMetrics spanning 1996 to 2023, we construct a relative signed measure of the volatility of volatility that captures the relative intensity of upside versus downside implied-volatility movements for each stock. We find that the aggregate mispricing originates predominantly from stocks whose volatility-of-volatility is disproportionately generated by upside price movements. This mispricing was highly significant during the early sample period, spanning the Dot-Com bust through the 2008 Global Financial Crisis, but has substantially weakened in the recent period, concurrent with a systematic and statistically significant decline in several widely used proxies for limits to arbitrage, including idiosyncratic volatility, stock illiquidity, bid-ask spreads, and firm size. Conditional double-sort analyses and cross-sectional interaction regressions jointly confirm that the negative premium associated with upside-dominated volatility uncertainty is most pronounced among stocks with the greatest idiosyncratic risk, illiquidity, and bid-ask spreads, and the smallest market capitalization, with all four proxies for limits to arbitrage significantly amplifying the mispricing in the early period. Furthermore, the anomaly is significantly stronger during periods of elevated investor sentiment, corroborating that upward price pressure from over-optimistic investors is the source of the initial overvaluation. Our findings suggest that this asymmetric volatility anomaly was a product of optimistic investors interacting with limits to arbitrage---an effect that has structurally weakened as modern market efficiency has improved.

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