Table:
|  |  | Years Ended December 31, |  |
| --- | --- | --- | --- |
|  | 2019 | 2018 | 2017 |
| Research, development and other related costs | $14,643 | $13,168 | $13,277 |
| Selling, general and administrative | 16,911 | 17,843 | 20,185 |
| Total stock-based compensation expense | $31,554 | $31,011 | $33,462 |
Paragraphs:
Stock-based Compensation Expense
The following table sets forth our stock-based compensation expense for the years ended December 31, 2019, 2018 and 2017 (in thousands):
Stock-based compensation awards include employee stock options, restricted stock awards and units, and employee stock purchases. For the year ended December 31, 2019, stock-based compensation expense was $31.6 million, of which $0.2 million related to employee stock options, $29.1 million related to restricted stock awards and units and $2.3 million related to employee stock purchases. For the year ended December 31, 2018, stock-based compensation expense was $31.0 million, of which $0.4 million related to employee stock options, $28.0 million related to restricted stock awards and units and $2.6 million related to employee stock purchases. The increase in stock-based compensation expense in 2019 compared to 2018 was due primarily to a higher volume of restricted stock unit grants.
Question:
What caused the increment in stock-based compensation expense in 2019 compared to 2018?
Answer: a higher volume of restricted stock unit grants