Difficult-to-read fonts make for better learning, according to scientists.
The finding is about to be published in the international journal Cognition.
Researchers at Princeton University employed volunteers to learn made-up information about different types of aliens - and found that those reading harder fonts recalled more when tested 15 minutes later.
They argue that schools could boost results by simply changing the font used in their basic teaching materials.
Hard to digest
The 28 volunteers in the Princeton study were given 90 seconds to try to memorise a list of seven features for three different species of alien.
The idea was to re-create the kind of learning in a biology class. Aliens were chosen to be sure that none of the volunteers' prior knowledge interfered with the results.
One group was given the lists in 16-point Arial pure black font, which is generally regarded to be easy and clear to read.
The other had the same information presented in either 12-point Comic Sans MS 75% greyscale font or 12-point Bodoni MT 75% greyscale.
The volunteers were distracted for 15 minutes, and then tested on how much they could remember.
Researchers found that, on average, those given the harder-to-read fonts actually recalled 14% more.
They believe that presenting information in a way that is hard to digest means a person has to concentrate more, and this leads to "deeper processing" and then "better retrieval" afterwards.
It is an example of the positive effects of what scientists call "disfluency".
"Disfluency is just a subjective feeling of difficulty associated with any mental task," explained psychology Prof Daniel Oppenheimer, one of the co-authors of the study.
"So if something is hard to see or hear, it feels disfluent... We'd found that disfluency led people to think harder about things.
"When we found that in the lab, we were very excited, because it has obvious implications for the classroom."
Keen to see if their findings actually worked in practice, the Princeton University team then tested their results on 222 students aged between 15 and 18 at a secondary school in Chesterland, Ohio.
Teachers were asked to send all supplementary learning materials to the researchers, who then changed the texts to the harder-to-read fonts Haettenschweiler, Monotype Corsiva, and Comic Sans Italicised.
Students were split randomly into two groups - with some of them receiving the amended versions, and the others the original, unchanged documents.
Teachers were not told about the hypothesis so as to avoid influencing the results.
Students given the harder-to-read materials scored higher in their classroom assessments than those in the control group. This was the case across a range of subjects - from English, to Physics to History.
The lead author of the study Connor Diemand-Yauman told the BBC that psychology is revealing all sorts of "counter-intuitive" results in the field of education.
"Everyday psychologists are showing that seemingly insignificant factors can have big effects on how we process and retain information.
"We're living in a recession. Now, more than ever, we must spend wisely on cost-effective teaching strategies."
Prof Oppenheimer also argues that their study has big implications for schools: "We all want to improve education. Most attempts involve curriculum reform which is both costly and time consuming.
"The fact that we can improve learning... with an intervention that costs no money and takes basically no effort to implement means that we can potentially improve education quickly and cheaply."
The researchers caution that their research was done with paid volunteers - who might be more willing to put in that extra bit of effort - and with students at a high-performing school.
And Dylan Wiliam, Emeritus Professor of Educational Assessment at the Institute of Education in London, told the BBC "the effects they represent are quite small".
"What really matters most when reading is mindfulness... it's not printing things badly that's needed, but more thoughtful reading".
For some types of learning "we need to slow the mind down," but according to Prof Wiliam this can be achieved very simply by getting students to read in groups, or just by following text with a finger.
The researchers stress that there are limits to how hard the font should be to read - following a roughly "U-shaped curve" after which the problems start to outweigh the benefits.
So what is that limit? "I wish I could tell you the answer" Prof Oppenheimer told the BBC.
"Obviously, if you can't read it at all, you can't learn it. At some point you may get so annoyed that you give up without trying! Different people probably have different thresholds.
"So, for the time being, its probably best to use small changes... rather than try to push the boundaries - at least until we know more."