[论文解读] Evidence of market manipulation in the financial crisis

Vedant Misra, Marco Lagi+1 more|arXiv (Cornell University)|Dec 14, 2011

Financial Markets and Investment Strategies参考文献 36被引用 8

一句话总结

本文提供了关于2007年11月对花旗集团进行协调性做空袭击的量化证据，期间短期卖空行为激增——表现为借入股票数量急剧上升，股价相应下跌——且与基本面新闻无关，表明存在故意的市场操纵。作者认为，2007年7月废除熔断规则（uptick rule）使此类操纵成为可能，若该规则仍有效，此类操纵本可被阻止。

ABSTRACT

We provide direct evidence of market manipulation at the beginning of the financial crisis in November 2007. The type of manipulation, a "bear raid," would have been prevented by a regulation that was repealed by the Securities and Exchange Commission in July 2007. The regulation, the uptick rule, was designed to prevent manipulation and promote stability and was in force from 1938 as a key part of the government response to the 1929 market crash and its aftermath. On November 1, 2007, Citigroup experienced an unusual increase in trading volume and decrease in price. Our analysis of financial industry data shows that this decline coincided with an anomalous increase in borrowed shares, the selling of which would be a large fraction of the total trading volume. The selling of borrowed shares cannot be explained by news events as there is no corresponding increase in selling by share owners. A similar number of shares were returned on a single day six days later. The magnitude and coincidence of borrowing and returning of shares is evidence of a concerted effort to drive down Citigroup's stock price and achieve a profit, i.e., a bear raid. Interpretations and analyses of financial markets should consider the possibility that the intentional actions of individual actors or coordinated groups can impact market behavior. Markets are not sufficiently transparent to reveal even major market manipulation events. Our results point to the need for regulations that prevent intentional actions that cause markets to deviate from equilibrium and contribute to crashes. Enforcement actions cannot reverse severe damage to the economic system. The current "alternative" uptick rule which is only in effect for stocks dropping by over 10% in a single day is insufficient. Prevention may be achieved through improved availability of market data and the original uptick rule or other transaction limitations.

研究动机与目标

• 调查2008年金融危机之前金融市场上是否发生了市场操纵，特别是做空袭击。
• 确定2007年7月废除熔断规则是否通过消除关键市场稳定机制，使此类操纵成为可能。
• 分析融券余额与交易量数据，识别与正常市场行为不符的模式。
• 评估观察到的卖空活动是否可由内幕交易或新闻事件解释。
• 倡导监管改革，包括恢复熔断规则并提高市场透明度，以防止未来操纵行为。

提出的方法

• 分析金融行业来源提供的每日收盘融券余额数据，以追踪借入股票数量的变化。
• 将融券余额变化与2007年11月1日的日交易量及股价变动进行相关性分析。
• 将卖空活动与股东持股交易量进行比较，以隔离基于借入的卖空行为。
• 检查六天后股价回报模式，以检测卖空头寸是否被协调平仓。
• 使用纽约证券交易所的卖空与交易数据，并通过时间与报告差异的校准，验证卖空量估算的准确性。
• 评估其他可能机制，如‘借股买入’（short to buy）交易，这些交易可能使卖空行为在公开数据中被掩盖。

实验结果

研究问题

• RQ1 2007年11月1日花旗集团是否存在显著且异常的卖空增加，且无法由新闻或基本面因素解释？
• RQ2 融券余额变化的时间与规模是否与股价下跌及随后的股票回补同步，表明存在协调性市场操纵？
• RQ3 观察到的行为是否更可能归因于内幕交易而非做空袭击？
• RQ4 当时缺乏熔断规则在多大程度上使此类操纵成为可能？
• RQ5 当前的市场数据报告实践是否足以检测或防止此类操纵？是否需要额外的监管与透明度措施？

主要发现

• 2007年11月1日，花旗集团股价下跌15%，交易量急剧上升，同时融券余额显著增加。
• 11月1日借入股票的卖出量大到足以占总交易量的相当大比例，表明存在协调行动。
• 六天后，同量级的股票在单日内被归还，表明卖空头寸被同步平仓。
• 未观察到长期股东卖出量的相应增加，排除了基本面新闻作为股价下跌原因的可能性。
• 数据表明可能存在‘借股买入’交易，这些交易可能掩盖了股票来源，使标准市场报告难以察觉。
• 当时缺乏熔断规则使操纵成为可能，作者据此得出结论：应恢复原始的熔断规则，以防止未来发生类似做空袭击。