Gross profit margin in the second quarter of 2018 of 40.9 percent was lower than the gross profit margin of 41.2 percent in the second quarter of 2017. Gross profit margin in the first six months of 2018 of 40.9 percent was equal to the gross profit margin in the first six months of 2017. The favorable impact of higher sales of water heaters and boilers, as well as pricing actions in the U.S., were offset by higher steel and other input costs in the second quarter of 2018.
Selling, general and administrative (SG&A) expenses in the second quarter and first six months of 2018 increased by $18.9 million and $28.6 million, respectively, as compared to the prior year periods. The increase in SG&A expenses in the second quarter and first six months of 2018 was primarily due to higher engineering and advertising expenses in China.
On March 21, 2018, we announced a plan to transfer water heater, boiler and storage tank production from our Renton, Washington plant to our other U.S. plants. The majority of the consolidation of operations occurred in the second quarter of 2018. As a result of the relocation of production, we incurredpre-tax restructuring and impairment expenses of $6.7 million in the first quarter of 2018, primarily related to employee severance and compensation-related costs, building lease exits costs and the impairment of assets. These activities are reflected in “restructuring and impairment expenses” in the accompanying financial statements.
We are providingnon-GAAP measures (adjusted earnings, adjusted earnings per share, and adjusted segment earnings) that exclude restructuring and impairment expenses. Reconciliations to measures on a GAAP basis are provided later in this section.
Interest expense in the second quarter of 2018 was $2.3 million compared to $2.5 million in the same period last year. Interest expense in the first half of 2018 was $4.6 million compared to $4.7 million in the same period last year. Higher interest rates in 2018 were offset by lower debt levels, primarily due to the repatriation of approximately $240 million of cash from outside of the U.S., which was used to pay down floating rate debt.
Other income was $4.6 million in the second quarter of 2018, equal to the same period last year. Other income in the first six months of 2018 was $10.4 million compared to $9.5 million in the first half of 2017. The increase in other income in the first half of 2018 was primarily due to higher currency and translation gains.
Our pension costs and credits are developed from actuarial valuations. The valuations reflect key assumptions regarding, among other things, discount rates, expected return on plan assets, retirement ages, and years of service. We consider current market conditions including changes in interest rates in making these assumptions. Our assumption for the expected rate of return on plan assets is 7.15 percent in 2018, compared to 7.5 percent in 2017. The discount rate used to determine net periodic pension costs decreased from 4.15 percent in 2017 to 3.65 percent in 2018. Pension income for the second quarter and first half of 2018 was $2.3 million and $4.5 million, respectively, compared to $2.2 million and $4.3 million in the second quarter and first half of 2017, respectively. The service cost component of our pension income is reflected in cost of products sold and SG&A expenses. All other components of our pension income are reflected in other income.
Our effective income tax rates for the second quarter and first six months of 2018 were 21.6 percent and 21.4 percent, respectively. Our effective income tax rates for the second quarter and first six months of 2017 were 27.8 percent and 27.5 percent, respectively. Our effective income tax rates in the second quarter and first half of 2018 were lower than the effective income tax rates in the same periods of 2017 primarily due to lower federal income taxes related to the U.S. Tax Cuts & Jobs Act (U.S. Tax Reform) which were partially offset by lower income tax benefits from settled stock based compensation awards. We estimate that our effective income tax rate for the full year 2018 will be approximately 21.5 percent to 22 percent, lower than 2017 due to U.S. Tax Reform.
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