George Ryan Huston & Thomas Joseph Smith (both of Florida State University, Department of Accounting) have posted Do Taxes Matter: Evidence of Individual and Corporate Tax Incentives on the Choice to Hold Shares Acquired from Exercise of Employee Stock Options on SSRN.  Here is the abstract:

This paper extends prior stock option literature by examining whether individual and corporate tax incentives are associated with the decision to hold shares acquired through option exercises. Aboody, Hughes, Liu, and Su (2008) refute the previously-held assumption that individuals immediately sell shares acquired through option exercise. We extend their work by focusing on the trade-off between tax incentives and the costs of holding, including liquidity constraints, opportunity costs, and future share price decline. We find that insiders hold shares for at least a year in 18.26 percent of exercises. Additionally, insiders are more likely to hold shares obtained through exercise of ISOs (specifically ISOs that are deeper in the money) relative to NQSOs, as ISOs allow for the transfer of pre-exercise gains from ordinary income to capital gain treatment. Finally, we find corporate tax incentives associated with employee stock sales mitigate insiders’ likeliness to hold.