June 30, 2021. Increases due primarily to our continuing investment in personnel and contract employees to drive and reach our research and development goals.
We expect to see an overall increase in research and development expenses to support our initiatives related to the Lifestyle Vehicle, Multi-Purpose Delivery Vehicles, and Pickup expected to launch as early as 2022 and 2023, respectively.
Selling, General and Administrative Expenses, excluding Depreciation
Selling, general and administrative expenses increased by $41.2 million, to $44.6 million in the three months ended June 30, 2021, compared to $3.4 million in the three months ended ended June 30, 2020. The increase was primarily due to increases of $16.9 million in stock-based compensation expenses, $10.2 million in professional fees, $4.8 million in marketing spend, $3.3 million in salary and related benefits, and $2.9 million in occupancy costs. Other factors affecting selling, general and administrative expenses were individually immaterial.
Selling, general and administrative expenses increased by $92.8 million, to $100.3 million in the six months ended June 30, 2021, compared to $7.5 million in the six months ended ended June 30, 2020. The increase was primarily due to increases of $54.7 million in stock-based compensation expenses, $16.1 million in professional fees, $5.9 million in occupancy costs, $5.6 million in marketing spend,and $5.0 million in salary and related benefits. Other factors affecting selling, general and administrative expenses were individually immaterial.
The increase in stock-based compensation expenses of $16.9 million and $54.7 million for the three and six months ended June 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 during three and six months ended June 30, 2021, 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 11 of the notes to our accompanying financial statements. Other factors affecting stock-based compensation expenses were individually immaterial.
Professional fees increased by $10.2 million to $11.2 million in the three months ended June 30, 2021, compared to $1.0 million in the three months ended June 30, 2020. Professional fees increased by $16.1 million to $17.5 million in the six months ended June 30, 2021, compared to $1.4 million in the six months ended June 30, 2020. The increase was primarily due to activities related to our business development, legal fees and miscellaneous support activities.
Salary and related benefits expenses increased $3.3 million to $5.4 million in the three months ended June 30, 2021, compared to $2.1 million in the three months ended June 30, 2020. Salary and related benefits expenses increased $5.0 million to $10.1 million in the three months ended June 30, 2021, compared to $5.1 million in the three months ended June 30, 2020. Increases were due primarily to increase in headcount.
We expect to see an overall increase in selling, general and administrative expenses to support our initiatives related to the Lifestyle Vehicle, Multi-Purpose Delivery Vehicles, and Pickup expected to launch as early as 2022 and 2023, respectively.
Interest Expense
Interest expense decreased by $3.6 million and 9.4 million in the three and six months ended June 30, 2020. The decrease was primarily due to interest expense on related party convertible notes and the amortization of debt discount during three and six months ended June 30, 2020. The related party convertible notes were repaid during Business Combination.
(Loss) gain on Fair Value Change in Contingent Earnout Shares Liability
The Company has a contingent obligation to issue 15.0 million of its common stock to certain stockholders and employees (the "Earnout Shares"). We recognized a non-cash loss on fair value change in contingent Earnout Shares