respectively. The increase is largely driven by the sale of fixed maturity securities to generate cash required to fund the ceding of loss reserves associated with the proposed reinsurance transaction with Cavello Bay.
The following table summarizes the components of net investment income and net investment gains for the six months ended June 30, 2021 and 2020:
| | | | | | | | | |
| | Six Months Ended June 30 |
($ in thousands) | | 2021 | | 2020 | | $ Change |
Fixed maturity securities | | $ | 25,585 | | $ | 32,131 | | $ | (6,546) |
Other investments | | | 7,686 | | | 1,800 | | | 5,886 |
Gross investment income | | | 33,271 | | | 33,931 | | | (660) |
Investment expenses | | | (2,011) | | | (1,325) | | | (686) |
Net investment income | | | 31,260 | | | 32,606 | | | (1,346) |
Realized investment gains, net | | | 48,892 | | | 2,123 | | | 46,769 |
Total | | $ | 80,152 | | $ | 34,729 | | $ | 45,423 |
Average invested assets | | $ | 2,392,782 | | $ | 2,210,278 | | $ | 182,504 |
Interest and Other Expenses, Net
Our interest and other expenses decreased by $1.2 million to $7.9 million for the six months ended June 30, 2021 compared to $9.1 million for the six months ended June 30, 2020. The decrease is primarily driven by the refinancing of our long term debt at a lower effective interest rate.
Income Tax Expense
Our effective tax rate for the six months ended June 30, 2021 and 2020 were 21.4% and 21.9%, respectively. The decrease in the effective tax rate for the six months ended June 30, 2021 compared to the same period in 2020 was primarily due to the tax effect of share-based compensation.
Our income tax expense was $15.1 million and $6.7 million for the six months ended June 30, 2021 and 2020, respectively. The increase is due to the increase in income before income taxes compared to the same period in 2020.
On March 27, 2020, the President of the United States signed into law the Coronavirus Aid, Relief, and Economic Security Act (the “CARES Act”). The CARES Act, among other things, includes certain income tax provisions for individuals and corporations; however, these benefits do not impact the Company’s current tax provision.
Adjusted Operating Income
Adjusted operating income was $19.3 million for the six months ended June 30, 2021, a decrease of $5.6 million, or 22.6% from the adjusted operating income of $24.9 million for the six months ended June 30, 2020, primarily due to the decrease in underwriting income.
Adjusted Operating Return on Equity
Our adjusted operating return on equity was 8.1% for the six months ended June 30, 2021, an increase of 1.9% percentage points from 6.2% for the six months ended June 30, 2020 primarily due to the decrease in adjusted operating income combined with an increase in average book value.
Liquidity and Capital Resources
Sources and Uses of Funds
We are organized as a holding company with our operations primarily conducted by our wholly owned insurance subsidiaries, New York Marine and Gotham, which are domiciled in New York, and Southwest Marine, which is domiciled in Arizona. Accordingly, the holding company may receive cash through: (i) loans from banks; (ii) draws on a revolving loan agreement; (iii) issuance of equity and debt securities; (iv) corporate service fees from our operating subsidiaries; (v)