Book value per share and book value per share excluding AOCI, non-GAAP financial measures, are calculated as total stockholders' equity and total stockholders' equity excluding AOCI divided by the total number of shares of common stock outstanding. Since AOCI fluctuates from year to year due to unrealized changes in the fair value of available for sale investments, we believe these non-GAAP financial measures provide useful supplemental information.
Net income is also impacted by the change in fair value of derivatives and embedded derivatives which fluctuates from year to year based upon changes in fair values of call options purchased to fund the annual index credits for fixed index annuities and changes in interest rates used to discount the embedded derivative liability. Net income for the years ended December 31, 2017 and 2016 was negatively impacted by decreases in the discount rates used to estimate our embedded derivative liabilities, while net income for the year ended December 31, 2015 was positively impacted by increases in the discount rates used to estimate our embedded derivative liabilities.
Losses on available for sale fixed maturity securities were realized primarily due to strategies to reposition the fixed maturity security portfolio that resulted in improved net investment income, risk or duration profiles as they pertain to our asset liability management. Securities were sold at losses in 2017, 2016 and 2015 due to our long-term fundamental concern with the issuers' ability to meet their future financial obligations. See Note 4 to our audited consolidated financial statements for additional discussion of allowance for credit losses recognized on mortgage loans on real estate.
Salary and benefits expense increased in 2016 as compared to 2015 as a result of increase in salary and benefits of $6.6 million due to an increased number of employees related to our growth as well as an expense of $2.6 million related to assumption changes and the execution of an amended and restated retirement agreement with our Executive Chairman. These 2016 increases were partially offset by a decrease of $3.9 million related to expense recognized under our short-term incentive compensation program and other bonus programs during 2016 as compared to 2015 primarily as a result of the short-term incentive compensation program being paid out at a lower percentage of target in 2016 than in 2015.
Tax expense for the year ended December 31, 2017 was increased by $35.9 million related to the revaluation of our net deferred tax assets using the newly enacted federal tax rate as a result of Tax Reform. The effective tax rate for 2017 adjusted to exclude the impact of Tax Reform was 32.3%
The liability for lifetime income benefit riders is based on estimates of the value of benefit payments expected to be paid in excess of projected policy values recognizing the excess over the expected lives of the underlying policies based on actual and expected assessments including spreads and product charges and fees. The inputs used in the calculation of the liability for lifetime income benefit riders include actual policy values, actual income account values, actual payout factors, actual roll-up rates and our best estimate assumptions for future policy growth, future policy decrements, the ages at which policyholders are expected to elect to begin to receive lifetime income benefit payments, the percentage of policyholders who elect to receive lifetime income benefit payments and the type of income benefit payments selected upon election. The assumptions are reviewed quarterly and revisions to the assumptions are made based on historical results and our best estimates of future experience. The liability for lifetime income benefit riders is included in policy benefit reserves in the consolidated balance sheets and the change in the liability is included in interest sensitive and index product benefits in the consolidated statements of operations. See Results of Operations for the Three Years Ended December 31, 2017 in this Item 7. for a discussion and presentation of the actual effects of assumption revisions.