Does Highlighting Work? What the Research Actually Says

What does the research say about highlighting?

In 2013 a team led by John Dunlosky reviewed ten common study techniques and graded each one on how well it actually improves learning across subjects, ages, and test types. Highlighting and underlining were rated low utility — the bottom tier, alongside rereading and summarising. Practice testing and distributed practice were the only two techniques rated high utility.

That finding surprises people because highlighting is close to universal. It is fast, it feels organised, and it leaves visible evidence that you did the reading. But the review's conclusion was blunt: on its own, marking text does very little for retention, and in some studies it actively hurt performance on questions that required drawing inferences across a passage.

The review's method matters for how much weight to give it. Rather than testing one technique in one lab, Dunlosky's team pooled evidence across hundreds of studies and asked whether each technique held up for different kinds of learners, different materials, and different tests. Highlighting kept failing that generalisation test: whatever benefit appeared in a narrow setup tended to vanish when the test demanded more than recognising a marked phrase.

Why doesn't highlighting help you remember?

Because it asks almost nothing of you. Dragging a marker across a sentence is a decision about importance, not an act of remembering. Your eyes stay on the page, the answer stays visible, and your brain never has to reconstruct anything. Learning happens when you pull information out of memory, not when you push it past your eyes again.

There's a second problem. Highlighting fragments a text into isolated bright lines. If a question asks you to connect two ideas from different pages, a page of yellow stripes gives you no help — you marked the parts, never the relationships between them.

There's also a selection problem. Deciding what to highlight requires knowing what matters, which is precisely the thing you don't have yet on a first read of unfamiliar material. Students new to a topic tend to mark definitions and bolded terms because those look important, while the connective reasoning that exam questions actually target goes unmarked.

Why does highlighting feel like it's working?

This is the interesting part. Highlighted text is easier to read the second time. It stands out, your eye lands on it, and the meaning arrives instantly. That ease gets misread as knowledge.

Asher Koriat and Robert Bjork called this the foresight bias: while you're studying, the answer is sitting right there in front of you, so you cannot simulate what it will feel like to face the question without it. Your sense of how well you know something is measured under the easiest possible conditions, then applied to a test taken under the hardest. The gap between those two situations is where confident students get surprised.

This is why the habit is so stable. A technique that produced no feeling of progress would be abandoned quickly. Highlighting produces a strong feeling of progress and a visible artefact, so the feedback loop rewards it every single time regardless of whether any learning occurred.

Is highlighting ever useful?

Yes — as triage, not as study. Used on a first read to mark the handful of things worth coming back to, a highlighter is a reasonable filter. The failure is treating the marking itself as the learning, and treating a re-read of the marked bits as revision.

The useful version has two rules: mark less, and never stop at marking.

• Cap yourself at roughly one highlight per paragraph — if most of the page is coloured, you've marked nothing
• Highlight only after finishing a section, so you're choosing with the whole argument in view
• Treat every highlight as a to-do: it becomes a question, not a keepsake

What should you do instead of highlighting?

Convert. Take each thing you marked and turn it into a question you have to answer with the book shut. This is the switch from recognising to retrieving, and it is the difference the research keeps finding.

Henry Roediger and Jeffrey Karpicke showed the size of that difference directly: students who read a passage and then took practice tests on it substantially outperformed students who spent the same total time rereading. The rereading group felt more confident. They remembered less.

Note what the conversion does mechanically: it moves the answer out of view. That single change is the whole intervention. The content is identical, the time cost is small, and the difference is simply whether you're looking at the fact or reaching for it.

• "Mitochondria produce ATP through oxidative phosphorylation" becomes "Where is ATP produced, and by what process?"
• A highlighted date becomes "What happened in 1919, and why did it matter?"
• A marked definition becomes "Define osmosis without looking, then check"

How do you turn a highlighted textbook into something you can study?

Work in one pass, section by section, and don't try to capture everything. A chapter usually yields fifteen to thirty genuinely testable facts; the rest is context that supports them.

For each highlight, write the question first and the answer second. Writing the question first forces you to decide what the fact is actually for — and if you can't phrase a sensible question, that's a good sign the highlight wasn't worth keeping.

A worked example: a paragraph on the causes of the First World War might yield four questions — name the alliance blocs, explain why mobilisation timetables mattered, state what the July Crisis was, and describe how one assassination escalated. That's a page of highlights compressed into four things you can be tested on, and you'll know within seconds which of the four you actually have.

How often should you come back to it?

Not all at once. The same review that rated highlighting low utility rated distributed practice high, and Nicholas Cepeda's synthesis of the spacing literature found that spreading the same total study time across separate sessions produces markedly better long-term retention than massing it.

So the sequence that works is: highlight sparingly, convert to questions immediately, then answer those questions across several separate days. The highlighter earns its place only as the first step of that chain.

One practical warning: don't convert a whole textbook in one sitting. The conversion step is genuine work, and a hundred questions written in one evening is a hundred questions all due for review on the same schedule — which recreates the pile-up problem from the other direction. A chapter at a time, as you cover it, keeps the workload flat.