GALVESTON, TX — American National Insurance Company (NASDAQ: ANAT) announced net income for the third quarter of 2019 of $92.2 million or $3.43 per diluted share, compared to $173.5 million or $6.44 per diluted share for the same period in 2018. Net income for the third quarter of 2019 included after-tax net gains on equity securities of $6.8 million or $0.25 per diluted share compared to $99.9 million or $3.72 per diluted share in the third quarter of 2018. In addition, net income for the third quarter of 2019 also included after-tax realized investment earnings of $47.1 million or $1.75 per diluted share compared to $6.9 million or $0.26 per diluted share for the same period in 2018.
Net income for the nine months ended September 30, 2019 was $449.2 million or $16.71 per diluted share, up from $276.4 million or $10.26 per diluted share for the same period in 2018. Net income for the nine months ended September 30, 2019 included after-tax net gains on equity securities of $222.8 million or $8.29 per diluted share compared to $118.9 million or $4.42 per diluted share for the same period in 2018. In addition, net income for the nine months ended September 30, 2019 also included after-tax realized investment earnings of $89.1 million or $3.31 per diluted share compared to $16.9 million or $0.63 per diluted share for the same period in 2018. The increase in realized investment earnings was primarily due to the sale of real estate investments in joint ventures.
After-tax adjusted operating income for the third quarter of 2019 was $38.3 million or $1.43 per diluted share compared to $66.7 million or $2.46 per diluted share for the same period in 2018. The decline in the third quarter after-tax adjusted operating income was primarily due to lower margins in our annuity segment reflecting a decline in interest rates and to a lesser extent the impact of a significant contribution to our pension plan in the third quarter of 2018 that reduced tax expense and improved earnings for that period.
After-tax adjusted operating income for the nine months ended September 30, 2019 decreased slightly to $137.3 million or $5.11 per diluted share compared to $140.6 million or $5.21 per diluted share for the same period in 2018. In addition to the aforementioned 2018 tax reduction and lower annuity margins, we experienced stronger earnings from our life and property & casualty segments during the first nine months of 2019 compared to the same period in 2018.
A reconciliation of adjusted operating income, a non-GAAP measure, to GAAP net income is included in the table below.