The increase in stock-based compensation expenses of $5.6 million and $21.9 million during the three and nine months ended September 30, 2021, respectively, was primarily driven by the recognition of stock compensation expense related to issuance of awards to employees and board of directors along with the modification of certain performance restricted stock awards to become time-based vesting with a merger trigger, which was satisfied on December 21, 2020. See further discussion on the restricted stock awards in Note 12 of the notes to our accompanying financial statements.
Salary and related benefits expenses increased $12.8 million, from $11.0 million during the three months ended September 30, 2020 to $23.8 million during the three months ended September 30, 2021. Salary and related benefits increased $25.0 million from $33.9 million during the nine months ended September 30, 2020 to $58.9 million during the nine months ended September 30, 2021. These increases are primarily due to continued investment in personnel and contract employees to drive and reach our research and development goals.
We expect to continue to see an overall increase in research and development expenses to support our growth and initiatives related to the Lifestyle Vehicle, Multi-Purpose Delivery Vehicles, and Pickup which are expected to launch as early as 2022 and 2023.
Selling, General and Administrative Expenses, excluding Depreciation
Selling, general and administrative expenses increased by $37.1 million, or 441.5%, to $45.5 million in the three months ended September 30, 2021, compared to $8.4 million in the three months ended September 30, 2020. The increase was primarily due to increases of $13.0 million in professional fees, $13.2 million in stock-based compensation expenses, $7.2 million in salary and related benefits, and $2.6 million in occupancy costs. Other factors affecting selling, general and administrative expenses were individually immaterial.
Selling, general and administrative expenses increased by $128.2 million, or 806.3%, to $144.1 million in the nine months ended September 30, 2021, compared to $15.9 million in the nine months ended September 30, 2020. The increase was primarily due to increases of $66.8 million in stock-based compensation expenses, $29.1 million in professional fees, $11.9 million in salary and related benefits, $8.5 million in occupancy costs, and $5.9 million in marketing spend. Other factors affecting selling, general and administrative expenses were individually immaterial.
The increase in stock-based compensation expenses of $13.2 million and $66.8 million for the three and nine months ended September 30, 2021, respectively, was primarily driven by certain awards granted during the periods subject to time-based vesting. The remaining increase was primarily driven by the continued recognition of stock compensation expense, resulting from the modification of certain performance restricted stock awards to become time-based vesting with a merger trigger, which was satisfied on December 21, 2020. See further discussion on the restricted stock awards in Note 12 of the notes to our accompanying financial statements. Other factors affecting stock-based compensation expenses were individually immaterial.
Professional fees increased by $13.0 million and $29.1 million during the three and nine months ended September 30, 2021, respectively, primarily due to activities related to business development, legal fees, and consulting fees.
Salary and related benefits expenses increased $7.2 million to $9.3 million in the three months ended September 30, 2021, compared to $2.1 million in the three months ended September 30, 2020. Salary and related benefits expenses increased $11.9 million to $18.7 million in the three months ended September 30, 2021, compared to $6.8 million in the three months ended September 30, 2020. These increases were due primarily to investment in personnel to support our growth and achieve start of production in late 2022.
We expect to continue to see an overall increase in selling, general and administrative expenses to support our growth and initiatives related to the Lifestyle Vehicle, Multi-Purpose Delivery Vehicles, and Pickup which are expected to launch as early as 2022 and 2023.