Occasionally, normal maps created or exported from Adobe Photoshop will have gamma values that are much too high - 2.2 in some cases. A gamma value of 1.0 is usually a good setting for normal maps.
With a little practice, you will be able to tell by looking at the normal map if this has occurred - it will be noticeably lighter and paler in color than usual.
The solution is to access the map's Adjustments dialog by clicking on the "Adjust..." button in the Image Map dialog. In the Adustments dialog, change the value in the Gamma field to 1.0, and exit the dialog. Your object should render as expected.