267 The following table provides information on the liabilities for guarantees that are included in the valuation of the host contracts, net of  the present value of the expected future premiums that are received to cover these guarantees:  The fair values of guarantees mentioned above (with the exception of life contingent guarantees in the United States) are calculated  as the present value of future expected payments to policyholders less the present value of assessed rider fees attributable to  the guarantees. For further details refer to note 3. For equity volatility, Aegon uses a term structure assumption with market-based implied volatility inputs for the first five years and  a long-term forward rate assumption of 25% thereafter. The volume of observable option trading from which volatilities are derived  generally declines as the contracts’ term increases, therefore, the volatility curve grades from implied volatilities for five years to  the ultimate rate. The resulting volatility assumption in year 20 for the S&P 500 index (expressed as a spot rate) was 23.9% at  December 31, 2013 and 24.4% at December 31, 2012. Correlations of market returns across underlying indices are based on historical  market returns and their inter-relationships over a number of years preceding the valuation date. Assumptions regarding policyholder  behavior, such as lapses, included in the models are derived in the same way as the assumptions used to measure insurance liabilities. Had Aegon used a long-term equity implied volatility assumption that was five volatility points higher or lower, the impact on income  before tax would have been a decrease of EUR 117 million or an increase of EUR 115 million, respectively, in 2013 IFRS income before  tax (2012: EUR 118 million decrease and EUR 104 million increase). These assumptions are reviewed at each valuation date, and updated based on historical experience and observable market data,  including market transactions such as acquisitions and reinsurance transactions. Aegon utilizes different risk management strategies to mitigate the financial impact of the valuation of these guarantees on  the results including asset and liability management and derivative hedging strategies to hedge certain aspects of the market risks  embedded in these guarantees. Guarantees valued at fair value contributed a net loss before tax of EUR 188 million (2012: gain of  EUR 289 million) to earnings. The main drivers of this loss before tax are positive results related to increases in risk free rates of  EUR 2,014 million (2012: EUR 714 million loss), EUR 514 million related to an increase in equity markets (2012: EUR 491 million  loss), EUR 67 million related to decreases in equity volatilities (2012: EUR 124 million loss) offset by EUR 302 million related to  decreasing own credit spread (2012: EUR 465 million loss). Hedges related to these guarantee reserves contributed fair value losses of  EUR 2,525 million to income before tax (2012: EUR 407 million gains) and DPAC offset and other contributed a gain of EUR 44 million  (2012: EUR 446 million gain). Guarantee reserves decreased EUR 2,341 million in 2013 (2012: increase of EUR 381 million).  Fair value measurement of guarantees in insurance contracts 1  Guaranteed minimum investment return in the Netherlands. 2  Balances are included in the insurance liabilities on the face of the statement of financial position; refer to note 37. 3  Incurred guarantee benefits mainly comprise the effect of guarantees from new contracts, releases related to expired out-of-the-money guarantees and fair value  movvements during the reporting year.  4  Account value reflects the liability value of the insurance contracts as a whole. 5  The net amount at risk represents the sum of the differences between the guaranteed and actual amount that is credited to the policyholders. For Individual  policies only positive differences are included, for Group pensions contracts carry forwards of negative differences are recognize.  2013 2012 GMI  1) ,  2) GMI  1) ,  2) At January 1 3,684 3,254 Incurred guarantee benefits  3) (1,222) 430 At December 31 2,462 3,684 Account value  4) 14,557 15,702 Net amount at risk  5) 2,756 3,841