If your communications director won't allow the use of a serif font in an appeal letter because the brand guidelines demand you use sans-serif Frutiger, here's a piece of research that might help you win them round.
Daniel Oppenheimer and two colleagues, Connor Diemand-Yauman and Erikka Vaughan, asked a group of college students between the ages of 18 and 40 to learn about two species of extra-terrestrials – the pangerish and the norgletti.
The pangerish profile was printed in a grey 12-point Bodoni font, whereas the norgletti profile was in a pure-black, 16-point Arial.
The researchers repeated the experiment with a study based at a high school in Ohio. For a complete term, 222 students were provided with learning materials in a selection of different fonts and the results were the same. The students reading Bodoni style fonts did better than those who were given the same information in Arial style fonts.
I say style, because the researchers used a selection of fonts in their experiments. They discovered that even the more complicated serif fonts such as Monotype Corsiva were better than Arial when it came to recall (but then again, so was Comic Sans).
When questioned in the Harvard Business Review, David Oppenheimer went as far as to suggest that recall could be improved by regularly changing fonts in a publication (e.g. in every chapter of a book or every story in a newsletter).
But before you decide to print your next appeal in six-point Desdemona, he also pointed out that there was a cut-off point where the use of difficult to read type would cause the effect to be reversed, particularly with uninterested or unmotivated readers.
Some charities don't seem that bothered about the choice of fonts in fundraising materials, leaving the decision on what they use down to the designer who created the brand guidelines - whether they understand the needs of donors or not.
That might well be a mistake, particularly when you take into account that a different font might cause your reader to remember far more about your charity.