Our 2018 Equity Incentive Plan (the “Plan”) was approved by stockholders in May 2018, authorizing us to issue up to 4.35 million shares of common stock, along with non-qualified stock options, stock appreciation rights, restricted stock and performance units to our officers, key employees, non-employee directors and consultants. In May 2021, the stockholders approved an amendment to the Plan that, among other things, increased the authorized shares by 2 million. At June 30, 2021, there were 1.3 million shares available for future grants under this Plan. It is our policy to issue shares from authorized but not issued shares upon the exercise of stock options.

Under the terms of the Plan, each year eligible participants are granted time value restricted stock units (“RSUs”), vesting ratably over a three-year period, and performance restricted stock units (“PSUs”), with three-year cliff vesting. Upon vesting, each restricted stock award is exchangeable for one share of the Company’s common stock, with accrued dividends.

Restricted stock unit and restricted performance stock unit activity for the six months ended June 30, 2021 is summarized in the following table:

	Equity Incentive Awards
	Restricted Stock Units			Weighted Average Grant Date Fair Value		Performance Shares			Weighted Average Grant Date Fair Value		Options			Weighted Average Exercise Price
Balance at December 31, 2020		1,213,667		$	3.59		2,176,290		$	4.88		24,000		$	20.39
Granted		411,291			5.94		653,438			8.41		—			—
Settled		(548,504	)		3.37		(193,778	)		5.45		—			—
Forfeited or expired		(32,525	)		8.13		(151,369	)		12.61		(15,000	)		22.57
Balance at June 30, 2021		1,043,929		$	4.49		2,484,581		$	6.67		9,000		$	16.76

Awards estimated to vest in the future		987,615		$	4.55		2,200,512		$	6.78		9,000		$	16.76

Stock-based compensation expense for the three months ended June 30, 2021 and 2020 was $2.5 million and $0.9 million, respectively. Stock-based compensation for the six months ended June 30, 2021 and 2020 was $4.3 million and $0.7 million, respectively. The higher expense for the three and six months ended June 30, 2021 was primarily attributable to the modification of the 2019 and 2020 PSU awards, substituting budgeted amounts for actual performance for the second quarter of 2020 (one of twelve quarters in the respective performance periods), to offset the impact of COVID-19. This increased stock-based compensation for the three and six months ended June 30, 2021 by $1.2 million and $2.3 million, respectively. In addition, 2020 stock-based compensation was lower due to the first quarter 2020 reversal of $1.2 million of previously accrued expense as a result of lower expected performance achievement, in light of the global pandemic. Unrecognized stock-based compensation expense related to non-vested awards of $15.2 million is expected to be recognized over a weighted average period of approximately 1.9 years as of June 30, 2021.