three months ended September 30, 2021 from $22.2 million for the three months ended September 30, 2020. The average cost of these advances decreased 30 basis points to 2.26% for the three months ended September 30, 2021 from 2.56% for the three months ended September 30, 2020.
Net Interest Income. Net interest income increased $109,000, or 37.8%, to $397,000 for the three months ended September 30, 2021 from $288,000 for the three months ended September 30, 2020. Average net interest-earning assets decreased $3.3 million period to period. This decrease was due primarily to a decrease in the average balance of other interest-earning assets, offset, in part, by increases in loans receivable and investment securities. Our interest rate spread increased 59 basis points to 1.43% for the three months ended September 30, 2021 from 0.84% for the three months ended September 30, 2020, and our net interest margin increased 45 basis points to 1.65% for the three months ended September 30, 2021 from 1.20% for the three months ended September 30, 2020. The increases in interest rate spread and interest rate margin were primarily the result of interest rates on average interest-bearing liabilities dropping more than interest rates on interest-earning assets during the three months ended September 30, 2021 versus the three months ended September 30, 2020.
Provision for Loan Losses. There were no provisions for loan losses for the three months ended September 30, 2021 or for the three months ended September 30, 2020. The allowance for loan losses was $858,000, or 1.16% of total loans, at September 30, 2021, compared to $850,000, or 1.22% of total loans, at December 31, 2020, and $850,000, or 1.24%, of total loans, at September 30, 2020. Classified (substandard, doubtful and loss) loans decreased to $0 at September 30, 2021 from $559,000 at December 31, 2020 and $562,000 at September 30, 2020. There were no non-performing loans at September 30, 2021, December 31, 2020 or September 30, 2020. There were no charge-offs or recoveries for the three months ended September 30, 2021 or for the three months ended September 30, 2020.
Noninterest Income. Noninterest income decreased $2,000, or 0.7%, to $269,000 for the three months ended September 30, 2021 from $271,000 for the three months ended September 30, 2020. The decrease was principally due to decreases of $4,000, or 1.8%, in fees on loans sold and $1,000, or 4.36%, in income from life insurance. The decrease was offset, in part, by an increase of $3,000, or 15.0%, in service charges and other income.
Noninterest Expense. Noninterest expense increased $52,000, or 8.3%, to $679,000 for the three months ended September 30, 2021 from $627,000 for the three months ended September 30, 2020. The increase was primarily due to an increase of $63,000, or 16.8%, in salaries and employee benefits, resulting primarily from an increase in commissions and related expenses on higher loan volume period to period, as well as increases of $19,000, or 65.5%, in accounting and consulting expense and $2,000, or 9.5%, in insurance expense. The increases were offset, in part, by decreases of $4,000, or 7.1%, in occupancy expense, $13,000, or 43.3%, in data processing expense, $13,000, or 54.2%, in legal expense and $2,000, or 2.7%, in other expenses.
In future periods, if we make grants of awards under our equity incentive plan which was approved by our stockholders, we would expect our noninterest expense to increase due to increased compensation expenses.
Income Tax Expense. There was no income tax expense for the three months ended September 30, 2021 or for the three months ended September 30, 2020, principally due to net operating loss tax carryforwards from prior years. The effective tax rate was 0.00% for the three months ended September 30, 2021 and 0.00% for the same quarter in 2020.
To provide financial assistance and liquidity to taxpayers during the COVID-19 pandemic, the CARES Act amended the federal income tax rules with regard to the usage of net operating losses (“NOLs”) for corporate taxpayers. The CARES Act allows for the carryback of losses arising in a taxable year beginning after December 31, 2017, and before January 1, 2021, to be carried back to each of the five taxable years preceding the taxable year of the loss. The CARES Act also temporarily repeals the 80% limitation for NOLs arising in tax years beginning after December 31, 2017 and beginning before January 1, 2021 and carried to another tax year. These NOLs are now permitted to fully offset the loss corporation’s pre-2021 taxable income.