Table:
|  |  | Year Ended December 31, |  |
| --- | --- | --- | --- |
|  | 2019 | 2018 | 2017 |
|  |  | (dollars in thousands) |  |
| Research and development | $101,151 | $76,981 | $61,975 |
| Percent of revenue | 17.5% | 14.3% | 12.9% |
Paragraphs:
Research and Development
Research and development expenses increased $24.2 million, or 31%, in 2019 as compared to 2018. The increase was principally due to the 2018 Reallocation of headcount from sales and marketing to research and development, as well as investments to maintain and improve the functionality of our products. As a result, we incurred increased employee-related costs of $16.3 million and increased overhead costs of $3.4 million.
Research and development expenses increased $15.0 million, or 24%, in 2018 as compared to 2017. The increase was principally due to the 2018 Reallocation of headcount from sales and marketing to research and development, as well as investments to maintain and improve the functionality of our products. As a result, we incurred increased employee-related costs of $12.1 million.
Question:
Why did research and development expenses increase between 2018 and 2019?
Answer: The increase was principally due to the 2018 Reallocation of headcount from sales and marketing to research and development, as well as investments