The improvements in net interest income and non-interest revenue were partially offset by a $4.4 million increase in non-interest expense. The primary cause of the increase in non-interest expense was the acquisition and the costs associated with operating a larger organization for a full twelve months compared to nine months in 2011. Another significant contributor to the increase in non-interest expense was the cost associated with terminating the Maryland Bankcorp pension plan. During the fourth quarter of 2012, we completed the termination of the pension plan and recorded a one-time expense of $700,884 associated with the purchase of annuity contracts for the remaining participants. Our work associated with the pending acquisition of WSB Holdings, Inc., which has cost approximately $350,000, caused the increase in merger and integration expenses.
For the three month period ended December 31, 2012, net income available to common stockholders was $1.7 million or $0.25 per basic and diluted common share. This was $249,988 or 12.71% lower than the same period in 2011. During the three month period ended December 31, 2012, net interest income increased $703,592 or 9.14% primarily as a result of an approximately $60.0 million increase in average gross loans outstanding. The $60.0 million in average gross loan growth was a result of our business development efforts, expanded market area and increased name recognition. The provision for loan losses for the fourth quarter of 2012 decreased $400,000 from the same period in 2011. Compared to the same period in 2011, non-interest revenue increased $60,848 primarily due to a $279,904 increase in gains on sales or calls of investment securities and an $18,415 increase in other fees and commissions. This was partially offset by a decline in service charges on deposit accounts, earnings on bank owned life insurance and gains on sales of other real estate owned. During the fourth quarter of 2011, we recorded gains on sales of other real estate owned of $199,425. We did not sell any other real estate owned during the fourth quarter of 2012. An increase in other operating expenses of $1.6 million offset the improvements in net interest income, the provision expense, and non-interest revenue. Salaries and employee benefits and data processing expenses increased primarily because of our efforts to continue to enhance our infrastructure to support a larger organization. As previously mentioned, during the fourth quarter of 2012, we recorded a $700,884 one-time pension termination expense and approximately $350,000 in merger and integration expense associated with the acquisition of WSB Holdings, Inc. The addition of these expenses was also the primary contributor to the reduction of fourth quarter 2012 income tax expense compared to fourth quarter 2011.