最適なストックオプション契約と早期行使の可能性について^*

抄録

This paper examines the optimal contracts on stock options and the possibility of early exercise from a different standpoint of conventional option pricing models by using a two-period contracting model.
Recently, the use of stock-option-based compensation has increased in many firms. On stock options, many theoretical or empirical analyses have been examined by option pricing models. However, we call into question these studies with respect to the incentive side for early exercise. Compared with usual call options, since stock options may be exercised early before the option maturity date, the issue of early exercise has been often pointed out. Noting that stock options are usually granted with some restricted terms until exercise, however, it is not clear whether early exercise can be always optimal for managers and shareholders on incentive side.
In this paper, we use the two-period contracting model, which is based on Acharya et al. (2000), considering the two stage of the decision-making structure between a manager and a shareholder. On Acharya et al. (2000), however, it is assumed that stock options are held until the maturity date. We develop the model for analysis of this subject supposing the possibility of early exercise. We examine the effect of early exercise on shareholder value compared with the case of restricted exercise at the maturity date.
The main conclusions in this paper are as follows. In the case of the maximum effort level that managers can choose being low, early exercise may be optimal for managers. However, such early exercise results in reducing the shareholder value. Therefore, it is better for shareholders to lay down the some restricted terms until exercise. In contrast, in the case of being high, stock options are always held until the option maturity date, which is the optimal behavior for shareholders and managers.