Mike Sewell
Chief Financial Officer at Cincinnati Financial
Thank you, Steve, and thanks to all of you for joining us today. Investment income continued to grow at a strong pace, up 8% for the fourth quarter and 7% for the full year 2021 compared with the same periods a year ago. Fourth quarter dividend income was up 14%, and net equity security purchases totaled $177 million for the year. Bond interest income grew 4% in the fourth quarter, while the pre-tax average yield of 4.05% for the year was down one basis point from a year ago.
The average pre-tax yield for the total of purchased taxable and tax-exempt bonds during the full year 2021 was 3.47%. Investing in fixed maturity securities continues to be a priority with net purchases during the year totaling $927 million. Valuation changes for our investment portfolio during the fourth quarter of 2021 were favorable for our stock holdings in aggregate, but unfavorable for our bond holdings. The overall fourth quarter net gain was nearly $1.4 billion before tax effects despite a decrease of $82 million for unrealized gains in our bond portfolio.
At the end of the fourth quarter, total investment portfolio net appreciated value was approximately $8 billion, including $7.2 billion for our equity securities. Cash flow was very strong in the fourth quarter as it has been all year. It contributes to investment income and was a major factor in the 5% increase in interest income we reported for the year. Cash flow from operating activities for full year 2021 generated just shy of $2 billion, a 33% increase compared with a year ago.
Expense management efforts in 2021 were very good, and we continue to carefully balance strategic business investments and expense controls. The full year 2021 property casualty underwriting expense ratio was 0.5 percentage points lower than last year even with an increase of 0.7 points from higher accruals for agency profit-sharing commissions. Regarding loss reserves, our approach to reserving remains consistent and aims for net amounts in the upper half of the actuarially estimated range of net loss and loss expense reserves. As we do each quarter, we consider new information such as paid losses and case reserves and then updated estimate ultimate losses and loss expenses by accident year and line of business.
During full year 2021, we experienced $428 million of property-casualty net favorable development on prior accident years. It favorably contributed to the combined ratio by 7%. On an all-lines basis by accident year, net reserves developed for the year was favorable by $283 million for 2020, $56 million for 2019, $44 million for 2018, and $45 million in aggregate for accident years prior to 2018. We believe overall reserves remain adequate.
During 2021, net loss and loss expense reserves in total increased by 8%. The IBNR portion increased by 6% in 2021, which followed an increase of 18% in 2020 for IBNR. Turning to capital management. We also follow a consistent approach, including share repurchases as part of maintenance intended to offset the issuance of shares through equity compensation plans.
We believe that our year-end financial strength remained in good shape and provides plenty of financial flexibility. During the quarter, we repurchased approximately 866,000 shares at an average price per share of $119.56. I'll conclude my prepared remarks as I typically do, with a summary of fourth quarter contributions to book value per share. They represent the main drivers of our value creation ratio.
Property-casualty underwriting increased book value by $1.26. Life insurance operations increased book value of $0.04. Investment income, other than life insurance and net of noninsurance items included $0.98. Net investment gains and losses for the fixed income portfolio decreased book value per share by $0.35.
Net investment gains and losses for the equity portfolio increased book value by $6.93, and we declared $0.63 per share in dividends to shareholders. The net effect was a book value increase of $8.23 per share during the fourth quarter to a record $81.72 per share. And now I'll turn the call back over to Steve.
Steve Johnston -- Chairman, President, and Chief Executive Officer
Thanks, Mike. As I said in my opening remarks, 2021 ended with many positives. We again achieved excellent premium growth and completed a tenth straight year of underwriting profit. We extended our record of annual dividend increases to 61 years and have already set the stage for a second -- 62nd year.
Earlier this month, A.M. Best recognized our capital strength and strong operating trends by affirming our A+ financial strength rating with a stable outlook. The key to our consistent results lies with our associates, who deliver -- who continue to deliver outstanding service to our agents and their clients, deepening our relationships with our agents and executing on our strategies for long-term success. Before we open the call for questions, I want to take a minute to recognize Steve Spray's recent promotion to President of the Cincinnati Insurance Company and all of our U.S.
Subsidiary companies. This promotion is a natural next step in the evolution of our executive leadership team that began several years ago. Because he's been in leadership roles across our organization, Steve has a deep understanding of what it will take to succeed far into the future. I'm confident that under his direction, our insurance subsidiaries will continue to grow deepening the products, services, and capabilities we have to support our agents and create shareholder value.
As a reminder, with Mike, Steve, and me today are Marc Schambow, Marty Hollenbeck, and Theresa Hoffer. Joanna, please open the call for questions.