Yields calculated using annualized net investment income (excluding income related to equity securities, trading) divided by the monthly average invested assets at cost, or adjusted carrying value, as applicable, excluding equity securities, trading, repurchase agreement and dollar roll collateral, and consolidated variable interest entity non-controlling interests. Yield calculations for the year ended December 31, 2013 exclude assets transferred due to the sale of the Retirement Plans and Individual Life businesses. Yield calculations for all periods exclude income and assets associated with the disposal of the HLIL business. Yields by asset type exclude investment expenses.
The Unlock charge for the years ended December 31, 2013 and December 31, 2012 was primarily due to assumption changes in connection with the annual policyholder behavior assumption study, partially offset by actual separate account returns above our aggregated estimated returns during these periods. Assumption changes for the year ended December 31, 2013 were driven by an increase in lapses and partial withdrawals and a reduction in the spread between assumed investment income and contract crediting rates. Assumption changes for the year ended December 31, 2012 were driven by lower estimated gross profits due to increased unit costs and changes to benefit utilization assumptions.
The Company has a monitoring process overseen by a committee of investment and accounting professionals that identifies investments that are subject to an enhanced evaluation on a quarterly basis to determine if an other-than-temporary impairment (“impairment”) is present for AFS securities or a valuation allowance is required for mortgage loans. This evaluation is a quantitative and qualitative process, which is subject to risks and uncertainties. For further discussion of the accounting policies, see Note 1 - Basis of Presentation and Significant Accounting Policies of Notes to Consolidated Financial Statements. For a discussion of impairments recorded, see the Other-Than-Temporary Impairments within the Investment Portfolio Risks and Risk Management section of the MD&A.
The Company recorded a deferred tax asset valuation allowance that is adequate to reduce the total deferred tax asset to an amount that will more likely than not be realized. As of December 31, 2013 and 2012, the deferred tax asset valuation allowance was $0 and $53, relating mostly to foreign net operating losses. In assessing the need for a valuation allowance, management considered future taxable temporary difference reversals, future taxable income exclusive of reversing temporary differences and carryforwards, taxable income in open carryback years, as well as other tax planning strategies. These tax planning strategies include holding a portion of debt securities with market value losses until recovery, selling appreciated securities to offset capital losses, business considerations such as asset-liability matching, and the sales of certain corporate assets. Management views such tax planning strategies as prudent and feasible and will implement them, if necessary, to realize the deferred tax asset. Based on the availability of additional tax planning strategies identified in 2011 and as a result of the sale of the U.K. variable annuity business, the Company released $56, or 100% of the valuation allowance associated with investment realized capital losses. Future economic conditions and debt market volatility, including increases in interest rates, can adversely impact the Company’s tax planning strategies and in particular the Company’s ability to utilize tax benefits on previously recognized realized capital losses.
The Obama Administration has not yet released its proposed federal budget for FY 2014. Last year's proposal, entitled “FY 2013, Budget of the United States Government” (the “Budget”) includes proposals that if enacted, would have affected the taxation of life insurance companies and certain life insurance products. In particular, the proposals would have changed the method used to determine the amount of dividend income received by a life insurance company on assets held in separate accounts used to support products, including variable life insurance and variable annuity contracts, which are eligible for the dividends received deduction (“DRD”). The DRD reduces the amount of dividend income subject to tax and is a significant component of the difference between the Company's actual tax expense and expected amount determined using the federal statutory tax rate of 35%. If this proposal is included in the proposed federal budget for FY 2014 and subsequently enacted, the Company's actual tax expense could increase, reducing earnings.
For a discussion regarding Guaranty Fund and Other Insurance-related Assessments, see Note 12 - Commitments and Contingencies of Notes to Consolidated Financial Statements.