92.3% for the three months ended June 30, 2021 and 2020, respectively. We attribute the increase in the combined ratio primarily to an increase in the loss ratio for the second quarter of 2021 compared to the second quarter of 2020.
Our interest expense for the second quarter of 2021 was $217,290, compared to $428,092 for the second quarter of 2020. We attribute the decrease to lower average borrowings under our lines of credit during the second quarter of 2021 compared to the second quarter of 2020.
We recorded income tax expense of $3.5 million for the second quarter of 2021, representing an effective tax rate of 17.9%. We recorded income tax expense of $5.3 million for the second quarter of 2020, representing an effective tax rate of 19.0%. The income tax expense and effective tax rates for the second quarters of 2021 and 2020 represented estimates based on our projected annual taxable income.
Net Income and Income Per Share.
Our net income for the second quarter of 2021 was $16.2 million, or $.53
per share of Class A common stock on a diluted basis and $.48 per share of Class B common stock, compared to $22.7 million, or $.79 per share of Class A common stock on a diluted basis and $.72 per share of Class B common stock, for the second quarter of 2020. We had 25.6 million and 23.5 million Class A shares outstanding at June 30, 2021 and 2020, respectively. We had 5.6 million Class B shares outstanding at the end of both periods.
Results of Operations - Six Months Ended June 30, 2021 Compared to Six Months Ended June 30, 2020
Our insurance subsidiaries’ net premiums earned for the first half of 2021 were $379.7 million, an increase of $8.1 million, or 2.2%, compared to $371.6 million for the first half of 2020, primarily reflecting the inclusion of the business of the Mountain States Insurance Group in the underwriting pool beginning with policies effective in 2021, as well as new business growth and renewal premium increases.
Our insurance subsidiaries’ net premiums written for the six months ended
June 30, 2021 were $425.5 million, an increase of $33.6 million, or 8.6%, from the $391.9 million of net premiums written for the first half of 2020. Commercial lines net premiums written increased $44.2 million, or 19.3%, for the first half of 2021 compared to the first half of 2020. We attribute the increase in commercial lines net premiums written primarily to the inclusion of the business of the Mountain States Insurance Group in the underwriting pool beginning with policies effective in 2021, as well as new business growth and renewal premium increases. Personal lines net premiums written decreased $10.6 million, or 6.5%, for the first half of 2021 compared to the first half of 2020. We attribute the decrease in personal lines net premiums written primarily to net attrition as a result of measures our insurance subsidiaries have implemented to improve underwriting profitability, partially offset by the impact of premium rate increases our insurance subsidiaries have implemented.
Our net investment income was $15.2 million for the first half of 2021, compared to $14.5 million for the first half of 2020. We attribute the increase primarily to an increase in average invested assets.
Net Investment Gains (Losses).
Net investment gains for the first half of 2021 were $6.7 million, compared to net investment losses of $4.2 million for the first half of 2020. The net investment gains for the first half of 2021 resulted primarily from unrealized gains within our equity securities portfolio at June 30, 2021. The net investment losses for the first half of 2020 resulted primarily from realized losses on sales of equity securities and unrealized losses within our equity securities portfolio due to a decline in equity markets at June 30, 2020. We did not recognize any impairment losses in our investment portfolio during the first half of 2021 or 2020.
Losses and Loss Expenses.
Our insurance subsidiaries’ loss ratio, which is the ratio of incurred losses and loss expenses to premiums earned, was 61.4% for the first half of 2021, an increase from our insurance subsidiaries’ loss ratio of 59.9% for the first half of 2020. We attribute this increase primarily to elevated fire losses that primarily impacted the loss ratio for our commercial multi-peril line of business. Weather-related losses of $18.6 million for the first half of 2021, or 4.9 percentage points of the loss ratio, decreased from $25.6 million for the first half of 2020, or 6.9 percentage points of the loss ratio. On a statutory basis, our insurance subsidiaries’ commercial lines loss ratio was 61.2% for the first half of 2021, compared to 60.6%