subordinated debentures due to the issuance of $50.0 million of subordinated debentures during the second quarter of 2020.
Six Months Ended June 30, 2021 Compared to Six Months Ended June 30, 2020
Net interest income was $51.7 million for the six months ended June 30, 2021, an increase of $10.2 million, or 24.7%, compared to $41.4 million for the six months ended June 30, 2020. The increase in net interest income was primarily due to growth in average interest earning assets, lower rates paid on deposits, and the recognition of PPP loan origination fees, offset partially by declining yields on loans.
Net interest margin (on a fully tax-equivalent basis) for the six months ended June 30, 2021 was 3.56%, compared to 3.48% for the six months ended June 30, 2020, an increase of 8 basis points. Core net interest margin (on a fully tax-equivalent basis), a non-GAAP financial measure which excludes the impact of loan fees and PPP balances, interest, and fees, for the six months ended June 30, 2021 was 3.33%, a 4 basis point increase from 3.29% for the six months ended June 30, 2020.
Average interest earning assets for the six months ended June 30, 2021 increased $529.2 million, or 21.8%, to $2.95 billion from $2.42 billion for the six months ended June 30, 2020. This increase in average interest earning assets was primarily due to strong organic growth in the loan portfolio, the funding of round 2 PPP loans, as well as continued purchases of investment securities. Average interest bearing liabilities increased $332.4 million, or 19.9%, to $2.01 billion for the six months ended June 30, 2021, from $1.67 billion for the six months ended June 30, 2020. The increase in average interest bearing liabilities was primarily due to an increase in interest bearing deposits and the issuance of subordinated debentures in the second quarter of 2020, offset partially by a decrease in federal funds purchased, FHLB advances and the payoff of the Company’s notes payable.
Average interest earning assets produced a tax-equivalent yield of 4.24% for the six months ended June 30, 2021, compared to 4.66% for the six months ended June 30, 2020. The average rate paid on interest bearing liabilities was 1.00% for the six months ended June 30, 2021, compared to 1.70% for the six months ended June 30, 2020.
Interest Income. Total interest income on a tax-equivalent basis was $62.0 million for the six months ended June 30, 2021, compared to $56.1 million for the six months ended June 30, 2020. The $5.9 million, or 10.5%, increase in total interest income on a tax-equivalent basis was primarily due to continued organic growth in the loan portfolio and PPP loan income.
Interest income on the investment securities portfolio on a fully-tax equivalent basis increased $383,000, or 8.1%, during the six months ended June 30, 2021, compared to the six months ended June 30, 2020, due to a $97.8 million, or 33.8%, increase in average balances between the periods, which was offset partially by a 62 basis point decrease in the aggregate portfolio yield.
Interest income on loans, on a fully-tax equivalent basis, for the six months ended June 30, 2021 was $56.7 million, compared to $51.1 million for the six months ended June 30, 2020. The $5.6 million, or 11.0%, increase was primarily due to a 19.9% increase in the average balance of loans outstanding, which was offset partially by a 36 basis point decrease in the average yield on loans. The increase in the average balance of loans outstanding was due to strong organic loan growth. The decrease in yield on the loan portfolio was primarily driven by lower market interest rates and lower loan fee recognition, offset partially by increased recognition of PPP loan fees.
Interest Expense. Interest expense on interest bearing liabilities decreased $4.3 million, or 30.2%, to $9.9 million for the six months ended June 30, 2021, compared to $14.2 million for the six months ended June 30, 2020, due to lower interest rates paid on both deposits and borrowings.
Interest expense on deposits decreased to $7.2 million for the six months ended June 30, 2021, compared to $10.9 million for the six months ended June 30, 2020. The $3.7 million, or 34.1%, decrease in interest expense on deposits was primarily due to a 74 basis point decrease in the average rate paid, even as the average balance of deposits increased 29.8%. The decrease in the average rate paid was primarily due to the impact of lower market interest rates.