We have stock option and stock incentive plans for employees and non-employee Directors that provide for grants of restricted stock awards (“RSAs”), restricted stock units (“RSUs”), performance stock units (“PSUs”), and options to purchase shares of our common stock at exercise prices generally equal to the fair value of such stock at the dates of grant. We also maintain a shareholder-approved Employee Stock Purchase Plan (the “ESPP”) for the benefit of our employees. The ESPP allows eligible employees to purchase common stock on a regular basis at the lower of 85% of the market price at the beginning or end of each offering period.

During the three months ended March 31, 2021 the Compensation Committee of our Board of Directors (the “Committee”) authorized awards from approved stock incentive plans of RSUs to certain employees, and RSAs and PSUs to certain Company officers, which, assuming that performance under the PSUs were to be achieved at target levels, together totaled 381,000 shares and had an aggregate grant date market value of $9.5 million.

During the three months ended March 31, 2020 the Committee authorized awards from approved stock incentive plans of RSUs to certain employees, and RSAs and PSUs to certain Company officers, which, assuming that performance under the PSUs were to be achieved at target levels, together totaled 255,000 shares and had an aggregate grant date market value of $6.6 million. The PSUs granted in 2020 represent the right to receive from 60% to 150% of the target number of shares of common stock. In February 2021 the Committee used structured discretion to determine that the 2020 PSUs were earned and should be paid out at 100% of target resulting in a modification of the award which resulted in $960,000 compensation expense during the three months ended March 31, 2021 related to these performance awards.

The Committee authorized, from approved stock incentive plans, grants of stock options to purchase a total of 226,000 and 212,000 shares to certain Company officers during the three months ended March 31, 2021 and 2020, respectively. The exercise prices of the options were equal to the closing stock prices on their respective grant dates.

Employees purchased common stock totaling 36,000 and 30,000 shares in the three months ended March 31, 2021 and 2020, respectively, through the ESPP.

The following weighted-average assumptions were used to determine the fair value of options and shares purchased under the ESPP:


	Three Months Ended
	March 31, 2021
	Stock Options	ESPP
Expected life	5.0 Years	0.5 Years
Expected stock price volatility	0.40	0.46
Risk-free interest rate	0.57%	0.09%

The following table summarizes total stock compensation expenses prior to the capitalization of amounts into deferred preservation and inventory costs (in thousands):


	Three Months Ended
	March 31,
	2021		2020
RSA, RSU, and PSU expense	$	2,050	$	2,156
Stock option and ESPP expense		587		533
Total stock compensation expense	$	2,637	$	2,689

Included in the total stock compensation expense, as applicable in each period, were expenses related to RSAs, RSUs, PSUs, and stock options issued in each respective year, as well as those issued in prior periods that continue to vest during the period, and compensation related to the ESPP. These amounts were recorded as stock compensation expense and were subject to our normal allocation of expenses to inventory costs and deferred preservation costs. We capitalized $157,000 in the three months ended March 31, 2021, and $125,000 in the three months ended March 31, 2020, of the stock compensation expense into our inventory costs and deferred preservation costs.

As of March 31, 2021 we had total unrecognized compensation costs of $14.8 million related to RSAs, RSUs, and PSUs and $3.2 million related to unvested stock options. As of March 31, 2021 this expense is expected to be recognized over a weighted-average period of 1.7 years for PSUs, 2.3 years for stock options, 2.4 years for RSUs, and 1.7 years for RSAs.