investment is commensurate with the Company’s future expansion into new markets, new product lines, and varied vertical integrations. The average number of R&D employees was 132 at September 30, 2020 compared to 227 at September 30, 2021.
Selling, general and administrative expenses. Selling, general and administrative expenses includes cash and non-cash compensation, benefits, amortization of intangible assets and related costs in support of our general corporate functions, including general management, finance and accounting, human resources, selling and marketing, information technology and legal services.
Selling, general and administrative expenses for the three months ended September 30, 2021, increased $25.2 million, or 146.5%, to $42.4 million from $17.2 million for the three months ended September 30, 2020. This increase was primarily related to increases in salaries and stock-based compensation due to increased headcount and branding expenses.
Selling, general and administrative expenses for the nine months ended September 30, 2021, increased $56.7 million, or 113.5%, to $106.7 million from $50.0 million for the nine months ended September 30, 2020. This increase was primarily related to increases in salaries and stock-based compensation due to increased headcount and branding expenses, in addition to costs associated with the restatement of our previous years’ financial statements.
Contingent Consideration. The fair value of the contingent consideration related to the Giner ELX, Inc. and United Hydrogen Group Inc. acquisitions was remeasured as of September 30, 2021, which resulted in a $8.5 million charge for the three months ended September 30, 2021 and a $8.8 million charge for the nine months ended September 30, 2021, both of which are reflected in the unaudited interim condensed consolidated statement of operations for the three and nine months ended September 30, 2021, respectively, primarily due to an increase in projected revenues associated with electrolyzers.
Interest, net. Interest, net consists of interest expense related to our long-term debt, convertible senior notes, obligations under finance leases and our finance obligations, as well as interest income. Interest decreased $11.9 million, or (68.9)%, from $17.2 million for the three months ended September 30, 2020 to $5.4 million for the three months ended September 30, 2021. Interest decreased $14.5 million, or (34.2)%, from $42.4 million for the nine months ended September 30, 2020 to $27.9 million for the nine months ended September 30, 2021. This decrease was primarily driven by the exchange and conversion during both 2020 and 2021 of the 7.5% Convertible Senior Note and 5.5% Convertible Senior Notes, and the adoption of ASU 2020-06 which reduced the noncash interest expense on convertible notes.
Other expense, net. Other expense, net consists of other expenses related to our foreign currency exchange losses, offset by interest and other income consisting primarily of interest earned on our cash and cash equivalents, restricted cash and available-for-sale securities. Other expense, net decreased $253 thousand for the three months ended September 30, 2021 in comparison to the three months ended September 30, 2020 and decreased $134 thousand for the nine months ended September 30, 2021 in comparison to the nine months ended September 30, 2020.
Realized loss on investments, net. Realized loss on investments, net consists of the sales and maturities related to available-for-sale debt securities. For the three months and nine months ended September 30, 2021, the Company had $254 thousand and $236 thousand, respectively, of net realized loss on investments. The Company did not have an investment portfolio in 2020, and as such there were no realized gains or losses.
Change in fair value of equity securities. Change in fair value of equity securities consists of the changes in fair value for equity securities from the purchase date to the end of the period. For the three months and nine months ended September 30, 2021, the Company had $607 thousand and $284 thousand, respectively, of change in the fair value of equity securities. The Company did not have an investment portfolio in 2020, and as such there were no realized gains or losses.
Loss on equity method investments. Loss on equity method investments consists of our interest in Hyvia, which is our 50/50 joint venture with Renault. For both the three and nine months ended September 30, 2021, the Company recorded a loss of $1.7 million on equity method investments as HyVia had not yet started commercial operations. The Company did not have any equity method investments in 2020, and as such there were no realized gains or losses.