本研究重新檢視長期以來關於增資（Seasoned Equity Offering, SEO）後營運績效不佳之謎，並透過區分融資可得性的因果效果與發行時機的內生性，釐清其背後機制。儘管理論上，股權融資應有助於企業進行價值提升的投資，然而實證文獻卻一致發現企業於增資後其獲利能力持續下滑。為解決發行決策的內生性問題，本文採用兩種互補的識別策略：其一為以2008年美國證券交易委員會（SEC）放寬貨架註冊制度為準自然實驗所建構之雙重差分法（Difference-in-Differences, DID），其二為使用價格比率工具變數之兩階段最小平方法（Two-Stage Least Squares, 2SLS）。

本文以1999年至2024年間美國工業公司為樣本，實證結果提供以下幾點重要發現。首先，DID分析顯示，外生性融資可得性的提升本身並不會導致企業績效惡化，顯示資本供給的增加並非績效下滑的根本原因。其次，研究發現觀察到的績效不佳主要來自於在有利評價條件下進行增資的企業，亦即當股價高於其基本面價值時發行股權。此一負面效果在發行後最長可持續三年，且於代理成本較高的企業中更為顯著。

上述結果與基於代理理論的過度投資解釋一致，顯示市場擇時所帶來的過剩流動性，可能使經理人進行次優的資本配置，進而導致持續性的效率損失。整體而言，本研究透過實證證據調和公司金融文獻中的分歧觀點，指出SEO後績效不佳主要反映的是管理者誘因的內生性與評價驅動的發行時機，而非融資限制的實質效果。本文結果亦凸顯公司治理在抑制股權融資後資本錯置風險上的關鍵角色。

This study revisits the long-standing seasoned equity offering (SEO) operating underperformance puzzle by disentangling the causal effects of financing access from endogenous issuance timing. While standard theory suggests equity financing should facilitate value-enhancing investment, empirical evidence consistently documents a persistent decline in post-issue profitability. To address the inherent endogeneity of issuance decisions, we employ two complementary identification strategies: a difference-in-differences (DID) design centered on the 2008 SEC shelf-registration deregulation and a two-stage least squares (2SLS) framework utilizing a Price ratio based instrument.
Using a comprehensive sample of U.S. industrial firms from 1999 to 2024, our results provide several key insights. First, the DID analysis reveals that exogenous improvements in financing access do not, by themselves, lead to a deterioration in firm performance, suggesting that capital availability is not inherently harmful. Second, we find that the observed underperformance is primarily driven by firms that issue equity under favorable valuation conditions—specifically when stock prices exceed fundamental benchmarks. This negative effect persists for up to three years following issuance and is more pronounced in firms with higher agency costs.
Consistent with an agency-based interpretation of overinvestment, these findings suggest that excess liquidity from market timing allows managers to undertake sub-optimal capital allocation, leading to sustained inefficiencies. Overall, this study reconciles conflicting predictions in the corporate finance literature by demonstrating that post-SEO underperformance reflects endogenous managerial incentives and valuation-driven timing rather than the real effects of financing constraints. Our evidence highlights the critical importance of corporate governance in mitigating the potential for capital misallocation following equity issuance.

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