If an historian studies events that his own country was involved in, it is likely that he will be at least somewhat biased (see reference 4). Not all historians are nationalistic; however, it is very difficult to be objective in discussing a world conflict when most of your sources on it come from your own country. Historians should be conscientious about choosing research from multiple sources, in order to avoid bias from national or linguistic identity.
Historians, like all academics, tend to have relatively narrow research specialties. There are exceptions to this rule, but getting ahead in academia generally requires finding a topic narrow enough that genuinely novel contributions can still be made. Many academics tend to see their subject through the narrow lens of their area of specialty, a tendency that can lead to misjudgment (see reference 2). Historians who have an interest in a particular area, such as Marxist theory, may view historical events in a way that is biased toward intellectual frameworks in their area of specialty.
The tendency for information to be "lost in translation" from one language to another is well documented (see reference 5). Historians who only read in one language may have a slight cultural bias resulting from terms that cannot be translated properly from one language to another. Historians must be careful when dealing with translated source materials, to ensure that substantive material is not lost in translation.
Sometimes, a historian's objectivity can be compromised by excessive exposure to biased sources. All the factors that can lead to an historian being biased in primary source research can further contribute to "second-hand" bias among historians who rely heavily on the initial historian's work. Historians must draw on multiple sources when doing secondary source research, to ensure that the claims of any one historical research source are backed by others.