Galveston, TX — American National Group, Inc. (NASDAQ: ANAT) (formerly American National Insurance Company)(1) and subsidiaries (collectively, the “Company”) announced net income for the fourth quarter of 2020 of $306.4 million or $11.40 per diluted share, compared to $171.1 million or $6.37 per diluted share for the same period in 2019. The increase in net income for the fourth quarter of 2020 was primarily attributable to an increase of $77.0 million or $2.86 per diluted share in after-tax net gains on equity securities, an increase of $20.2 million or $0.75 per diluted share in after-tax net realized investment earnings, and an increase of $20.4 million or $0.75 per diluted share in after-tax earnings from our property and casualty business.
After-tax net gains on equity securities increased due to more favorable equity market conditions to $188.0 million or $6.99 per diluted share in the fourth quarter of 2020 compared to $111.0 million or $4.13 per diluted share for the same period in 2019. In addition, after-tax net realized investment earnings for the fourth quarter of 2020 were $25.7 million or $0.96 per diluted share, compared to $5.5 million or $0.21 per diluted share during the same period in 2019.
Net income for the twelve months ended December 31, 2020 was $467.5 million or $17.39 per diluted share, down from net income of $620.4 million or $23.07 per diluted share for the same period in 2019. The reduction in net income was primarily related to a $116.7 million or $4.34 per diluted share reduction in after-tax net realized investment gains and a $52.3 million or $1.94 per diluted share reduction in after-tax net gains on equity securities.
After-tax net realized investment losses increased during 2020 due to an expense of $82.8 million or $3.08 per diluted share for changes in estimated credit losses. The expense related to estimated credit losses primarily relates to certain of our mortgage loans which were negatively impacted by the economic downturn related to COVID-19 and is in accordance with our adoption of accounting guidance(2) which impacted 2020 but was not applicable in 2019. In addition, after-tax net realized investment gains were reduced due to fewer sales and lower gains on sales of real estate development entities in 2020 compared to 2019. The reduction in the net gain on equity securities was primarily driven by the first quarter downturn in financial markets resulting from the impacts of COVID-19.
After-tax adjusted net operating income for the fourth quarter of 2020 was $92.7 million or $3.45 per diluted share compared to $54.6 million or $2.03 per diluted share for the same period in 2019. The increase was mostly driven by an improvement in our property and casualty segment operating earnings.
After-tax adjusted net operating income for the twelve months ended December 31, 2020 was $208.0 million or $7.74 per diluted share compared to $191.9 million or $7.14 per diluted share for the same period in 2019. Increased earnings from our property and casualty business were partially offset by reductions in investment income and lower earnings from our life business due to unfavorable mortality experience.
For the twelve months ended December 31, 2020, total life insurance in force increased by $10.1 billion to $128.2 billion, and book value per share increased $17.46 to $240.20.