bonuses and sales commissions, contributed approximately two- thirds of the overall operating expense increase in this segment.  Bonus expense rose $4.2 million in 2010. Sales commissions and  employee benefits, including employee healthcare benefits and  matching contributions to our 401(k) plan in the United States, also  increased, but to a lesser extent. Operating expense in 2010 also  includes $2.0 million related to a separation agreement between  Morningstar and the former head of Morningstar Associates. A  slight reduction in product implementation fees related to our Advice  by Ibbotson service partially offset these increases. In 2009, the operating expense decrease was primarily because of  lower bonus and other compensation-related expense, partially  offset by additional operating expense related to the Intech acqui- sition. Bonus expense declined $10.4 million in 2009. Most of this  reduction reflects the changes we made to our bonus program for  2009 as part of our efforts to better align our cost structure with  revenue. The reduction in bonus expense also reflects a slowdown  in our financial performance in 2009 compared with 2008. Other  compensation-related expense was down, primarily because we  suspended matching contributions to our 401(k) plan in the United  States, reducing operating expense by $1.3 million in 2009. A slight  reduction in product implementation fees related to our Advice by  Ibbotson service also contributed to the decrease. The segment’s operating margin in the segment decreased 5.8 per- centage points to 51.5% in 2010. The margin decline mainly reflects  higher bonus expense as a percentage of revenue. The $2.0 million  separation payment reduced the margin by approximately 2.0 per- centage points. Salaries and employee benefits also contributed to  the margin decline, but to a lesser extent. In addition, acquisitions  had a negative impact on the margin of approximately 1.6 percent- age points. Operating margin was 57.3% in 2009, an increase of 3.3 percentage  points over 2008. Lower bonus expense as a percentage of revenue  was the main driver of the margin improvement. The decrease in  other compensation-related expense also contributed to the margin  improvement, although to a much lesser extent. These factors were  offset by incremental costs from the Intech acquisition and higher  compensation expense as a percentage of revenue. In 2010, corporate unallocated expense decreased $8.9 million, as  some expenses recorded in 2009 did not recur in 2010. Corporate  unallocated expense in 2009 includes $9.5 million of expense related  to adjusting the tax treatment of certain stock options and deposit  penalties. In 2010, expense related to liabilities for vacant office  space declined $2.2 million. In addition, in 2010 we capitalized  approximately $0.8 million of operating expense for software devel- opment. These factors, which favorably impact the year-over-year  comparison, were partially offset by incremental expense from  acquisitions, higher compensation and bonus expense, and higher  professional fees. After increasing $3.5 million in 2009, depreciation expense for cor- porate departments did not change significantly in 2010 from 2009.  We relocated to our new corporate headquarters in the fourth quar- ter of 2008, resulting in higher depreciation expense in the subse- quent years. Amortization of intangible assets increased $5.9 million in 2010 and  $2.4 million in 2009, mainly reflecting incremental amortization  expense related to acquisitions completed in 2010 and 2009. We  paid $281.9 million for acquisitions during the past three years. As  of December 31, 2010 and December 31, 2009, respectively, we had  $169.0 million and $135.5 million of net intangible assets. We amor- tize these intangible assets over their estimated lives, ranging from  one to 25 years. Based on acquisitions completed through December  31, 2010, we estimate that aggregate amortization expense for  intangible assets will be approximately $25.7 million in 2011. Some  of the purchase price allocations are preliminary, and the values  assigned to intangible assets and the associated amortization  expense may change in future periods. We do not allocate corporate costs to our business segments. The  corporate items category also includes amortization expense related  to intangible assets recorded when we allocate the purchase price  of acquisitions. The table below shows the components of corporate  items that impacted our consolidated operating income: Corporate Items ($000)  2010 2009  2008 Amortization expense  $ 24,850 $ 18,963  $ 16,648 Depreciation expense  7,244 7,386  3,902 Corporate unallocated  31,403  40,296   39,629 Corporate items  $ 63,497 $ 66,645  $ 60,179 % change  (4.7%) 10.7%  13.4% Morningstar, Inc. 2010 Annual Report 87