Bias is a very difficult historical concept, one that troubles professional historians and students alike, so it is difficult to give a simple answer. One of the main difficulties is a simple, and very common, misunderstanding of the word's meaning. Saying, for example, that ‘Thomas Jefferson was biased’ is akin to saying ‘Thomas Jefferson breathed air’. It is true, but it is not particularly useful to state when discussing the Virginia Statute for Religious Freedom. All creatures possess biases.

First, it is important to understand that bias is not a loose or fuzzy term; it has a very specific epistemological meaning. Despite its very frequent misuse in the media (social and otherwise), it does not mean any of the following:

1. Lying
2. Having an opinion
3. Arguing a point
4. A non-specific synonym of racist, sexist or classist

Indeed, taken to their natural extreme, the use of the term bias is simply a way of discounting all of what has been previously written by the essayist. For example

"Thomas Jefferson believed that different races could never successfully integrate into a single society, but, he owned slaves, so he was biased."

On a basic, gut-reaction level, this sentence makes perfect sense. But let us examine the second clause more closely. Rephrased, all it argues is that because he owned slaves, he was racist, and because he was racist, we can ignore him. Most historians would agree that this is not a satisfactory way of dealing with historical biases, but there is little consensus on 'the way' to deal with them, as it appears to be an implicit understanding, developed over years of work, which is difficult to explain.

Having looked through a large stack of historical method guidebooks, I was shocked and indeed horrified by what I found. Many made no use of the word ‘bias’ at all. Sections on how to analyse primary or secondary material are brief, worryingly so, as most of the pages were spent discussing margin sizes and note taking. Mary Abbot’s essay on ‘sources’ in History Skills (Routledge, pp. 23-26), for example, briefly explains that ‘A good historian is a sceptic who takes nothing at face value, who is ever open to new ideas and who is prepared to modify her views in light of new evidence or persuasive argument’ before offering up a variant of the 5 Ws.

Jeremy Black and Donald Macraild, in their Palgrave Guide to Studying History are more thorough, offering 5 pages on bias (p. 12-15, 40-1), in which they explain to readers various possible ideologies espoused by historians, paying particular note of feminist and women’s history. This section offers some heady abstractions about historical conceptions of time, and some lambasting of ‘the first draft of history’ by newspaper journalists (by Orwell, not the authors of the guidebook). It provides interesting food for thought, and could open up fruitful discussions, but a useful guide to the practical implications of bias in student reading and writing it is not.

The late Richard Marius, who wrote Short Guide to Writing about History (Longman), which I used as an undergraduate, offers a lengthy account of ‘Questioning your Sources’ (p. 32-45) with special attention given to statistical enquiry. It offers a detailed look at each of the 5 Ws, in turn, and how they overlap and corroborate each other. It also explores the fallacies that often strike historians in determining causation and correlation. Overall, it is a very good look at how to examine material, but bias itself is only implicitly touched upon when he concludes that ‘Skepticism is one of the historian’s finest qualities. Historians don’t trust their sources, and they don’t trust their own first impressions. They question everything.’

Looking at these guides, all of which have slightly different audiences and aims, it would be hard to outright disagree with the material they put forth, but 'questioning sources' is simply not enough. It leads to being critical (if not petulant) for the sake of not looking naive, and rather mind-numbing lists of the answers to those 5 Ws, leading to no satisfying conclusion.

I turn now to the two guidebooks that discussed bias at greatest length. Jules R. Benjamin’s 2010 Student Guide to History introduces bias in the follow manner:

"One of your most important concerns when reading a primary source is determining its reliability. Primary sources can be fraudulent, inaccurate or biased [emphasis his]. Eyewitness accounts may have been deliberately distorted in order to avert blame or to bestow praise on a particular individual or group. Without intending to misinform, even on-the-scene judgements can be incorrect. Sometimes, the closer you are to an event, the more emotionally involved you are, and this involvement distorts your understanding of it. We can all recall events in which we completely misunderstood the feelings, actions, and words of another person. Historians have to weight evidence carefully to see whether those who participated in an even understood it well enough to describe it accurately." (p. 31-32)

This is a perfectly sensible way to ease into the idea. However, once his specific advice begins to flow, it becomes less in line with normal historical practice. When discussing bias within secondary sources, he explains that

"Any bias [emphasis his] on the part of an author or work that you are considering will tell you a lot about a work’s seriousness. Bias — an author’s perspective on a topic — can be modest or very strong. Strong bias is indicated by an unwillingness to consider or acknowledge other interpretations and by the use of harsh language to characterize authors with different perspectives. A book with a strong bias is unlikely to be a useful source, unless your research requires an examination of biased works, as would a paper on the anti-Semitism expressed in nineteenth-century French accounts of the Dreyfus trial." (p.110)

In all honesty, I was taken aback by the absoluteness of these statements. Bias as a determinant of seriousness (by which I believe he meant our ability to take it seriously, not its own intentions of serious argument)? All sources contain biases, and these can themselves be incredibly useful when teasing out information about the period, not just when discussing the bias itself. Arbitrarily discounting it as ‘unlikely to be a useful source’ based on ‘strong bias’ seems ludicrous and a very bad (even lazy) precedent to set. The second point, defining bias as ‘an author’s perspective on a topic’ is equally absurd (for reasons I will discuss at more length below), yet he continues this train of thought with

Modest bias, however, is a universal characteristic. Every author has a perspective on his or her subject and, like you, is making an argument in support of a thesis. A history of World War II written by an English author is likely to have a different viewpoint from one written by a German author. Many historical developments and their interpretations are topics of profound controversy, and it is almost impossible for a historian to investigate of these controversial areas without being affected by his or her own biases (p. 110).

Here bias is established as being the same as ‘argument’, or even ‘focus’, as well as perspective. Although I would concede that bias does affect argument, focus and perspective, it is not the same thing (nor are these three things the same as each other) and confusing these terms is doing much harm. Such a loose definition of such a seemingly ubiquitous word leads us towards a society that you can discount another’s argument by crying ‘bias!’ and stomping away.

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On the other hand, I. W. Mabbett's Writing History Essays (Palgrave) has a more helpful definition. Unlike Black and Macraild’s more general history guide, Mabbett focuses specifically on writing essays, and therefore has more time to devote to bias itself, which he allots an impressive 11 pages (p. 44-55). He discusses problems of establishing authority, distinguishing between fact and opinion, the difference between prejudice and bias, and how to recognise and manage (rather than discount) authorial interest, tendentious language and that oft-dreaded concept, balance. I will not quote extensively from Mabbett, as to do so would be a disservice to the hard work he has done in creating an engaging the helpful flow to this section, but I would highlight this, his definition of bias.

"A bias is a built-in tendency to lean to one side, a preference that inclines one to favour one side in an argument. Again, what matters is whether this inclination prevents us from being influenced by evidence to the contrary. This amounts to much the same thing as being unwilling to take account of anything that conflicts with one’s prejudice. It is important to avoid confusing prejudice or bias with the mere possession of an opinion. We all have opinions; what matters is the extent to which we are ready to let our opinions be changed by examination of the evidence." (p. 49)

This is followed by a lengthy set of examples in which he explores how language implies (sometimes unsubtly) what the author’s opinion is, and then explains when this is prejudice, when it is bias, and when it an argument based on previously stated evidence and in which its one-sidedness is justified. Yet, as admirable as I find this analysis, my own views of bias stem not from history but cognitive science. Here, bias has a very specific meaning, with a host of sub-definitions for the various aspects and manifestations of it, and it is this specificity that has made the concept far more useful to me than the woollier idea of ‘perspective’. In brief, it is the systematic nature of cognitive biases that makes them an effective analytical tool. A bias is not simply a pre-conceived answer to a particular question, but a systematic mistake, an identifiable pattern, in logic or decision making across a length of time or sample group. Given the same criteria and the same context, the person will systematically make the same mistake, or come to the same incorrect conclusion. Why is this so helpful? The reason is twofold.

First, systematic mistakes can be tracked. A lie, an outright deception, is rarely systematic. People slip up, forget, or have different narratives in different contexts. Moreover, different source types, such as oral histories, photographs, letters, newspaper reports and so on, hide or highlight biases to different extents. If a person has made an error, in fact or logic, this conception of bias encourages me to look at a range of different materials, at different times and in different forms, across a long period or geographic base. I can then determine if something is appearing again and again. If it is systematic, knowing this can help me understand and analyse sources in which the bias is less visible, even invisible without outside knowledge. If it does not appear consistently, even within a very short frame of time or space, then I must consider if the mistake is a lie, and find evidence to that effect, or if the person has obtained misinformation in good faith. Both these answers lead to very interesting conclusions about the source and the individual.

Second, exploring cognitive biases prevents generalisation. Wider, all-encompassing terms such as ‘racist’ are very common in everyday speech, but fail to account for cognitive dissonance or different responses to race by the same person, other than to revert to the idea that they are lying. The clichéd argument that a person could have a genuine friendship with someone of a different race, but unabashedly condemn that race for economic distress (often unemployment), is difficult to reconcile under the wider term ‘racist’. With cognitive bias, however, it can be explained. A systematic mistake, such as Ingroup Bias, may affect a person differently in two different situations, leading to seemingly conflicting results. In one case, the outgroup is another race, who are seen to be ‘stealing jobs’ or ‘lowering wages’ or engaging in other behaviours that will ultimately destroy that person’s individual chance of employment success. In the other case, the outgroup of that same person may be individuals from another class or region, or even a different football team, and a person of a different race, who happens to work, play or live alongside that person, is now in the ingroup and the assessment of their behaviour is completely altered. The bias is technically the same, and all most humans to some degree, but has different outcomes in different situations. By framing bias in this way, I am not only prevented from discounting a source out-of-hand, but I am encouraged to look more deeply at the internal logic and external context of the source in a critical and ultimately useful way.

Of course, these brief examples are themselves very simplistic views of cognitive bias, but the core idea is also very simple. A bias is a consistent tendency (to use Mabbet’s language) or systematic mistake in logical deduction. It is not merely a reference to a person’s race, gender or educational background, though these things can affect which biases they hold. It is something measurable, and something that should encourage further, deeper and more careful research, rather than discount sources and arguments as irrelevant. This, I hope, will give you some idea on how historians deal with bias, to lesser and greater degrees of success.

This varies, but in general, I think historians deal with bias by accepting it and understanding what a point of view brings to or detracts from a historian's work. Every historian brings a bias to the archive and the keyboard. Heck, even the selection of a research topic is indicative of some sort of bias -- why would someone devote 5-10 years of their life to a project in which they had no interest?

You will likely never find any academic work that doesn't give at least some short shrift to the side(s) with which they disagree. I don't know if it's a Quixotic quest to find one that's perfectly even, but I do think it's pointless. Embrace diverse opinions and read many works on the same subject. This will help you not only understand something closer to an agreed-upon-truth, but it will also help you develop critical thinking skills.

If you'd like to read a book that does a much better job of explaining this than I do, Find a copy of Peter Novick's That Noble Dream. It's a little old and there's been quite a bit of ink spilled praising and reviling the book, but I think Novick does a good job of probing the question.