中文摘要：

股票投资是中国上市公司的一种重要的投资活动，但对这种投资活动的动机缺少研究。基于2007年至2013年中国沪、深股市上市公司定期报告披露的证券投资信息，采用多元线性回归和Logit回归等统计分析方法，从上市公司股票投资的概率、投资规模、盈亏状况、持股周期4个方面检验上市公司进行股票投资的跟风投资、现金管理和平滑利润3种动机。研究结果表明，股市回报率与上市公司进行股票投资的概率负相关，否定了跟风投资动机；现金流入较多、利润波动较大的上市公司股票投资概率更高，投资规模也更大，支持现金管理和平滑利润动机；只有约7％的上市公司能够通过股票投资实现总利润扭亏为盈，说明上市公司虽然通过股票投资平滑了利润，但没有操纵利润；上市公司所持有的股票中50％～80％的股票持股周期短于1个季度，持股周期越短的股票投资业绩越好，反映了上市公司股票投资期限较短，符合现金管理动机。研究结果支持现金管理和平滑利润动机，否定跟风投资动机。研究结论对投资者理解上市公司的股票投资活动以及监管机构制定相关政策具有重要借鉴意义。

英文摘要：

Security investment is one of the important investments conducted by the Chinese listed firms. However, the studies on the motives of security investments are scant. Based on the security investment information disclosed in financial reports of the listed firms on Shanghai and Shenzhen Stock Exchanges in China from 2007 through 2013, the motives of the listed firm＇s stock investments are studied through the investigations of the probability, size, performance and persistence of the stock investments of the listed firms. Three possible motives for the listed firms to invest on stocks are herding investments, cash management, and earnings smoothing. The research methods include multivariate linear regressions and Logit regressions. Empirical results show that a listed firm＇s decision to whether invest on stocks. That is, once a listed firm starts investing, the probability that it stops investing is around 71.43% ; whereas a non-investing firm has a probability of 77.10% to remain non-in- vesting. The probability that a listed firm invests on stocks is negatively associated with stock market returns. This evidence re- fuse herding investment motive. The listed firms with more cash inflows and more volatile earnings are more likely to invest on stocks and the investment sizes tend to be larger, supporting the motives of cash management and earnings smoothing. In most cases, the listed firms make profits on their stock investments. But only 7% of the listed firms can turn negative total earnings into positive ones simply by including the results of stock investments in total earnings. Therefore, the results suggest that it is difficult for the listed firms to manipulate their accounting earnings figures through strategically choosing the timing of stock investments. Among the stocks held by the listed firms, around 50% to 80% of them are held for less than one quarter. Multivariate regressions on the listed firms＇ stock investment performances illustrate that the listed firms gain more on the shorter term stock