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For text-heavy pages on the web, is there evidence to suggest that serif fonts are a better option? The studies I've found seem very murky in pointing one way or another indicating that yes, users read serif fonts quicker but they're not more legible while other studies suggest there is no difference between the two.
Are there any conclusive studies that define serif fonts as being a better choice?
There is no evidence that serif or sans-serif significantly impacts readability. Alex Poole conducted a study on Which Are More Legible: Serif or Sans Serif Typefaces?. His conclusion:
What initially seemed a neat dichotomous question of serif versus sans serif has resulted in a body of research consisting of weak claims and counter-claims, and study after study with findings of “no difference”. Is it the case that more than one hundred years of research has been marred by repeated methodological flaws, or are serifs simply a typographical “red herring”?
It is of course possible that serifs or the lack of them have an effect on legibility, but it is very likely that they are so peripheral to the reading process that this effect is not even worth measuring ( Lund, 1999 ).
Indeed, a greater difference in legibility can easily be found within members of the same type family than between a serif and a sans serif typeface. ( Tinker, 1963 , Zachrisson, 1965 ). There are also other factors such as x-height, counter size, letter spacing and stroke width which are more significant for legibility than the presence or absence of serifs. Poulton, 1972 ; Reynolds, 1979 )
Finally, we should accept that most reasonably designed typefaces in mainstream use will be equally legible, and that it makes much more sense to argue in favour of serif or sans serif typefaces on aesthetic grounds than on the question of legibility. ( Bernard, 2001 ; Tinker, 1963 )
There is no evidence that one type of font should be used for either - serifs do not provide any guidance to the eye. From Alex Poole's study, again:
Serifs are used to guide the horizontal “flow” of the eyes; The lack of serifs is said to contribute to a vertical stress in sans serifs, which is supposed to compete with the horizontal flow of reading ( De Lange et al., 1993 )
These are the most common claims when trying to make a case for the utility of serifs. However, serifs cannot in any way be said to “guide the eye”. In 1878 Professor Emile Javal of the University of Paris established that the eyes did not move along a line of text in one smooth sweep but in a series of quick jerks which he called saccadic movements ( Spencer, 1968, p. 13 ; Rayner & Pollatsek, 1989, pp. 113-123 ). Unfortunately many graphic designers and typographers continue to use this rationale for the existence of serifs, due to a lack of communication and cooperation with the research community.
What you should be looking for is a font, serif or not, that has been designed for legibility. If you are dealing with text on the computer, finding a font that is focused towards computer display legibility would be preferred. For example Microsofts Segoe UI font (I use this simply as an example, not a specific suggestion - especially if we are talking web).
You asked which is "better" and the Evil Closet Monkey cited an excellent article on legibility.
But another interpretation of "better" concerns how the font will affect the reader's perception of the credibility of the information presented.
This article notes a study that found certain Serif fonts created a slight increase of confidence for the reader (1.5%).
That difference is so small, the various other factors on your page will no doubt have a more significant impact on your readers: layout, language tone, links and citations, other articles covered by the site, colour, images, the name of the author, and the past experience of each given reader.
So the conclusion is the same: choose any legible font that suits your site, and place greater attention on other concerns.
(If you know that a majority of your users come from a specific background, then you could make them more comfortable by selecting a font which they are already familiar with. But again, the effect will be fairly minor, and swamped by other aspects of the UI.)
The question is very general and sheds little light on a possible difference in readability between what are by far the most popular serif and sans serif font families, Times and Arial.
Counters (the "holes" in letters, e.g. in "o" and "p") and apertures (openings, such as in "c" and "t") are a significant factor, and they are much larger in Arial than in Times. This is partly because Arial is more rounded than Times (look at the letter "o" to verify), and partly because serifs reduce aperture size. The greater roundedness of Arial is one reason that for type of the same point size it will have far fewer characters per line than Times. (Have a look at this article.) So as point size decreases, we squint with Times before Arial. Of course you could then ask what is so special about point size and why don't we compare font sizes which have the same number of characters per line.
On the other hand, Times is considered more credible. In practice, this has a sizeable effect on readability. Encouraging readers to try harder than when they are reading a "Keep off the grass" notice to understand and reflect on what they are reading is often considered a good thing. Indeed some acts of reading are more enjoyable when more effort needs to be made, and thus texts in this category can become more readable for those who appreciate being encouraged to engage. If this consideration were left out, we might conclude that Comic Sans is a highly "readable" font, which in a sense it is but it hardly ranks high for credibility.
