The Science of Highlighting: Why Most People Do It Wrong (And How to Do It Right)

The Study That "Killed" Highlighting

In 2013, psychologist John Dunlosky and four colleagues published a landmark review in Psychological Science in the Public Interest. They evaluated ten common study techniques across four dimensions: ease of use, generalizability, effectiveness for different learners, and effectiveness across different learning conditions. Highlighting and underlining landed in the lowest tier: "low utility."

The media loved it. "Stop Highlighting Your Textbooks!" headlines spread. Education blogs repeated the verdict as gospel. Teachers told students to put away their highlighters. Within a few years, "highlighting doesn't work" became conventional wisdom.

But here's the thing: most people never read the actual paper. They read a summary of a summary, and the nuance got lost along the way.

The Dunlosky study evaluated highlighting as it is typically practiced, not as it could be practiced. And how do most students highlight? They drag a yellow marker across nearly every line on the page, creating a false sense of familiarity without any real engagement with the material. That's the version of highlighting that doesn't work.

The question nobody bothered to ask was: what happens when you highlight well?

---

What Dunlosky Actually Said

Let's look at the fine print. Dunlosky et al. (2013) explicitly stated that "highlighting and underlining led to no benefits for students on a range of different criterion tests." But they also noted that "most students report rereading and highlighting" as their primary study strategies, and that "these techniques do not consistently boost students' performance."

That word "consistently" matters. It means there were cases where highlighting did boost performance. The study wasn't a blanket condemnation; it was a warning that most students use the technique poorly.

The authors acknowledged a critical limitation: most of the studies they reviewed simply asked participants to read and mark text without providing any training on how or what to mark. Participants received no guidance on selectivity, no instruction on combining highlights with other strategies, and no framework for reviewing highlighted material.

Imagine evaluating the effectiveness of cooking by observing people who've never been taught to cook. You'd conclude that cooking doesn't produce good meals. But the problem isn't cooking; it's the lack of skill.

Dunlosky and his colleagues rated five techniques as "low utility" (highlighting, rereading, summarization, keyword mnemonics, and imagery use) and two as "high utility" (practice testing and distributed practice). The high-utility techniques shared something important: they forced active processing. The low-utility techniques, as typically used, were passive.

This distinction between passive and active is where the real insight lies. Highlighting can be passive, yes. But it doesn't have to be.

---

When Highlighting Works: The Research

Since 2013, a growing body of research has complicated the "highlighting is useless" narrative. Several studies have identified conditions under which highlighting becomes genuinely effective.

Yue et al. (2015) found that highlighting relevant information to a question predicts response accuracy. Their research showed that the act of deciding what to highlight forces readers to evaluate the importance of each sentence, creating a form of active processing. They also found that showing students highlights from an informed instructor improved learning outcomes, suggesting that pre-highlighted texts can serve as a form of guided reading.

Interestingly, Yue's team discovered that highlighting was more beneficial for students who didn't initially believe in the strategy. Students who were skeptical of highlighting but used it with proper guidance showed greater gains than enthusiastic but untrained highlighters.

A meta-analysis published in Educational Psychology Review (2021) examined the effects of both learner-generated and instructor-provided highlighting on learning from text. The findings suggested that highlighting improved learning when it was selective and targeted, rather than exhaustive.

Mason et al. (2024) investigated highlighting in digital reading environments and found that highlighting the content helped students decide what to focus on, promoting comprehension and learning performance.

What ties these positive findings together? In every case, highlighting worked because it was combined with a deliberate process: selectivity, annotation, or structured review. The highlighter wasn't the study tool. The thinking behind the highlighting was the study tool.

---

The Selectivity Principle

The single biggest predictor of whether highlighting helps or hurts your learning is how much you highlight.

Fowler and Barker (1974) published one of the earliest studies on this topic in the Journal of Applied Psychology. Their finding was blunt: the more text subjects highlighted, the worse their test performance. Students who highlighted everything effectively highlighted nothing. The visual differentiation that makes highlighting useful (separating important from unimportant) disappears when you mark 70% of the page.

Contrast this with selective highlighting. Research from the University of North Carolina's Learning Center recommends highlighting only after you've finished reading a paragraph or section. The process should be: read first, think about what matters, then go back and mark only the core idea. One sentence per paragraph is a good starting point. Two at most.

This aligns with how expert readers naturally engage with text. Skilled academics don't highlight as they read; they highlight during a second pass, after they've understood the structure of the argument. The highlight becomes a bookmark for retrieval, not a substitute for comprehension.

Here's a simple test for whether you're highlighting well: if you come back to a page a week later, can your highlights alone reconstruct the argument? If so, you've highlighted selectively. If your highlights just form a wall of color with no clear thread, you've fallen into the passivity trap.

Tools like Glasp's web highlighter support this kind of selective engagement. When you highlight a passage on any webpage, you're making a conscious decision about what matters. And because Glasp stores your highlights in a personal library, you can revisit them later for review, turning a one-time reading act into a spaced retrieval practice. Learn more about how to highlight text on web pages effectively.

---

Color-Coding: More Than Aesthetics

If selectivity is the first principle of effective highlighting, color-coding is the second. Using different colors to categorize information creates an additional layer of cognitive processing that single-color highlighting misses.

Research on color and attention shows that warm colors (yellow, orange, red) have a more significant impact on alertness than cool or neutral colors. This is likely tied to their effect on physiological arousal. Yellow, in particular, has been the default highlighter color for good reason: it stands out without obscuring text, and it activates attention.

But using only yellow is a missed opportunity. A study on color-coded note-taking found that students who used a color-coding system had significantly higher recall scores compared to students who used a single color or no color at all. The act of assigning colors to categories (key concepts in yellow, supporting evidence in blue, questions or disagreements in pink, for example) forces an additional classification step during reading.

Dr. Raul Pacheco-Vega, a researcher who has written extensively on academic reading strategies, recommends a four-color system for reading academic papers:

• Yellow: Main argument or thesis
• Blue: Methods and evidence
• Pink/Red: Points you disagree with or want to question
• Green: Connections to other readings or your own work

This system works because it transforms highlighting from a single action ("this is important") into a multidimensional one ("this is important, and here's why it's important"). The classification itself is a form of elaborative processing, one of the techniques Dunlosky's own research rates highly.

Glasp supports multiple highlight colors, making it easy to implement a color-coding system across all your web reading. You can assign meaning to each color and maintain consistency across articles, papers, and blog posts.

---

Digital vs. Paper: What Changes?

One of the most surprising findings in recent highlighting research involves the difference between digital and paper contexts.

A study comparing highlighting across print and digital environments found an unexpected pattern: on paper, reading comprehension decreased as highlighting frequency increased. But on digital texts, the reverse was true. More highlighting correlated with better comprehension. The researchers suggested that digital highlighting may be more efficient because students on paper often highlighted portions outside of the important parts of the passage, while digital tools allowed more precise selection.

This finding challenges the assumption that paper reading is always superior to screen reading. At least when it comes to highlighting, digital tools offer several advantages:

Precision. Digital highlighting lets you select exact phrases and sentences, while paper highlighters often bleed into surrounding text. There's no accidental over-marking.

Searchability. Paper highlights are useful only when you physically return to the book. Digital highlights can be searched, tagged, organized, and exported. With Glasp, all your highlights are automatically saved to your profile and can be searched across your entire reading history.

Portability. Your highlighted passages travel with you. If you highlighted an article on your laptop, you can review those highlights on your phone during a commute. Glasp even supports Kindle highlight imports, bringing your physical reading into your digital knowledge library.

Integration with AI. This is a capability that paper simply can't match. Glasp's AI Summary feature can analyze your highlights and generate summaries, identify themes, or suggest connections between passages you've marked across different articles. This turns your highlights into inputs for deeper thinking, not just passive bookmarks. You can read more about how AI is reshaping the way we learn.

That said, digital reading comes with its own challenges. Screen fatigue is real. The temptation to skim is stronger on screens. And some research shows no significant difference between digital and paper comprehension overall. The key takeaway isn't that digital is universally better; it's that digital highlighting tools, when used intentionally, remove several friction points that make paper highlighting less effective.

---

Social Highlighting: The Multiplier Effect

Here's where things get interesting. Everything we've discussed so far treats highlighting as a solo activity: you read, you mark, you review. But what if you could see what other people highlighted in the same text?

This is the idea behind social highlighting (sometimes called collaborative annotation), and the research behind it is compelling.

Cornell University's Center for Teaching Innovation describes social annotation as a practice where "students collaboratively read, analyze, and annotate digital texts, fostering engagement, critical thinking, and discussion." Studies on social annotation tools show improved comprehension and retention, because students don't just passively absorb information; they actively reflect on their reading in conversation with a community.

The benefits break down into several categories:

Distributed attention. No single reader catches everything important in a text. When you see highlights from dozens or hundreds of other readers, you discover passages you might have skimmed over. It's like having a study group that reads everything for you and flags the best parts.

Social proof of importance. If 200 people highlighted the same sentence, that's a strong signal that the sentence captures something essential. This form of crowd-sourced curation helps readers prioritize, especially in unfamiliar subjects where they might not know what to look for.

Exposure to different perspectives. Other readers' annotations reveal how people from different backgrounds interpret the same text. A data scientist might highlight the methodology section of an article that a designer would skip entirely. Seeing both perspectives enriches your own understanding.

Accountability and motivation. Knowing that others can see your highlights creates a subtle motivation to read more carefully and highlight more thoughtfully. It's the same principle behind why people exercise more consistently when they have a workout partner.

This is the core of what Glasp offers. Glasp's community feed lets you see what other readers have highlighted across the web. You can follow people whose reading tastes align with yours, discover new articles through their highlights, and build on each other's knowledge. It's highlighting as a social learning practice, not just a personal study hack.

Glasp also organizes highlights by topic, making it easy to explore what the community has found most valuable in any subject area. Whether you're researching machine learning, philosophy, or product design, you can browse topic-based collections of highlights from readers around the world.

---

A Practical Highlighting Protocol

Based on the research, here's a step-by-step protocol for highlighting that actually improves learning:

Step 1: Read First, Highlight Second

Never highlight on your first pass through a paragraph. Read the entire section, understand the argument, and then go back to mark the key point. This prevents the most common mistake: highlighting before you know what's important.

Step 2: Limit Yourself

Aim for one highlight per paragraph, two at most. If everything feels important, that's a sign you need to re-read with a clearer question in mind. Ask yourself: "If I could only remember one thing from this section, what would it be?"

Step 3: Use Color with Purpose

Assign meaning to each color before you start reading. A simple system:

Stick with this system consistently. Over time, your brain will automatically start categorizing information as you read.

Step 4: Add a Note

For every two or three highlights, write a brief annotation. This can be a one-sentence summary, a question, or a connection to something else you know. The annotation is what transforms passive highlighting into active processing. Glasp lets you attach notes to any highlight, keeping your thinking right next to the source material. For a complete guide to annotation across all media types, see our guide to annotation.

Step 5: Review and Retrieve

Highlights are useless if you never look at them again. Schedule a weekly review of your recent highlights. Tools like Glasp make this easy by collecting all your highlights in one place, organized by article, date, or topic. During review, try to recall the context of each highlight before re-reading the source. This acts as a form of retrieval practice, one of the highest-rated learning techniques from Dunlosky's own research. Active recall is one of those high-utility techniques. Learn more about how active recall works.

Step 6: Share and Discuss

Share your highlighted passages with others. Post them to your Glasp profile, discuss them with colleagues, or use them as starting points for writing. Teaching and explaining what you've read is one of the most effective ways to solidify understanding.

---

Passive vs. Active vs. Social Highlighting

Here's how the three approaches compare across key dimensions:

The progression from passive to active to social represents a shift from marking text to engaging with ideas to learning in community. Each level builds on the previous one.

---

Frequently Asked Questions

Is highlighting really a bad study strategy?

No, but passive highlighting is. The 2013 Dunlosky study that popularized this claim specifically evaluated highlighting without selectivity, annotation, or structured review. When students highlight selectively (one to two sentences per paragraph), use color-coding, and add marginal notes, highlighting becomes a genuinely effective learning technique. The tool isn't the problem; it's how most people use it.

How much should I highlight on a page?

Less than you think. Research from Fowler and Barker (1974) found that test performance decreased as the amount of highlighted text increased. A good rule of thumb is to highlight no more than 10-20% of any given text. For a typical paragraph, that means one sentence, possibly two. If you find yourself highlighting more than a third of the text, step back and re-read the section with a specific question in mind.

Does the color of my highlighter matter?

Yes, but not in the way you might expect. Individual colors have modest effects on attention (warm colors like yellow and orange tend to increase alertness). The real benefit comes from using a system of colors, where each color represents a category of information. This forces you to classify what you're reading as you read it, which is a form of elaborative processing that strengthens memory encoding.

Is digital highlighting better than paper highlighting?

Research suggests that digital highlighting has certain advantages. One study found that increased highlighting frequency on digital texts correlated with better comprehension, while the opposite held true for paper. Digital tools also offer searchability, portability, and integration with AI. However, the most important factor isn't the medium; it's whether you're highlighting selectively and combining it with annotation and review.

How does social highlighting improve learning?

Social highlighting works through several mechanisms. First, it exposes you to passages you might have overlooked, expanding your attention. Second, seeing that many other readers highlighted the same passage serves as a signal of importance. Third, reading others' annotations introduces you to different interpretations and perspectives. Research from Cornell University and studies on social annotation tools like Perusall consistently show improved comprehension and retention in collaborative annotation settings.

Can I use Glasp for academic research?

Absolutely. Glasp lets you highlight any web page, attach notes, and organize your highlights by topic. For academic research, you can use color-coding to categorize findings (evidence, methods, counterarguments), export your highlights for use in papers, and use the AI Summary feature to identify patterns across multiple sources. The community feed also helps you discover relevant articles through the highlights of other researchers in your field.

---

Conclusion: Highlighting, Done Right

The "highlighting doesn't work" myth is one of the most persistent oversimplifications in learning science. What the research actually shows is more nuanced and far more useful: mindless highlighting doesn't work, but strategic highlighting is a powerful learning tool.

The evidence is clear. Selective highlighting forces you to evaluate what's important. Color-coding adds a layer of categorization that strengthens memory. Adding annotations transforms a passive mark into an active thought. And sharing your highlights with a community creates a social reinforcement loop that benefits everyone involved.

These aren't complicated techniques. You don't need to overhaul your entire study routine. You just need to shift from "mark everything that looks important" to "read first, think second, highlight third, annotate fourth." For strategies on reading more deeply, see our guide to deep reading.

Glasp was built for exactly this kind of intentional reading. It's a free web highlighter that lets you highlight any page on the internet, organize your highlights with colors and notes, review them in a personal library, and share them with a community of curious readers. Whether you're a student, researcher, or lifelong learner, Glasp turns your reading into a lasting knowledge asset.

Stop highlighting everything. Start highlighting what matters. And let others see what you found.

Get started with Glasp for free →