Latest / The Copywriting Podcast with Fexingo: Sales Letters, Headlines, and Persuasive Writing / How a Single Underline Lifted Response by 423 Percent
Transcript
- Lucas: There's a single line — a literal line drawn under one word in a headline — that lifted a fundraising letter's response by four hundred and twenty-three percent. Not a rewrite. Not a new offer. One underline. Luna: Wait — four hundred twenty-three percent? That's not a typo? Lucas: Not a typo. The control pulled in about twelve percent response. The test version with the underline pulled over sixty-two percent. I've seen the original data, and the tester ran it three times to make sure. Luna: And we just did an episode on how one underlined sentence increased donations by four hundred percent. This is a different one? Lucas: Completely different. That was an underlined sentence in the body. This is a single word in the headline. The word was 'now' — as in 'Send your gift today so we can act now.' The control had no emphasis. The test underlined 'now'. Luna: That feels almost too simple to believe. But I've definitely seen underlines work in my own testing. Lucas: Yeah, and if these marketing conversations have sparked something you've actually used — a headline tweak, a structure change, something that worked — we'd love for you to consider supporting the show at buy me a coffee dot com slash fexingo. It's what keeps this podcast ad-free and lets us dig into exactly these kinds of obscure, powerful details. Luna: Absolutely. No pressure, just a way to keep the episodes coming. Now — back to that underline. Why does it work? Lucas: The leading theory comes from the psychology of attention. Underlining is one of the oldest typographic cues for emphasis. In handwritten or typewritten text — which that letter was — an underline is the reader's instruction to pause and pay extra attention. Luna: Right. But why not bold or italics? Those are more common today. Lucas: In that specific context — a typewritten letter — bold and italics weren't available on a standard typewriter. Underlining was the only option for emphasis. So the reader's brain had been trained, over decades, that an underline meant 'this is important'. Luna: And that training still exists. Even now, I underline things on paper without thinking. Lucas: Exactly. But here's where it gets interesting: the effect depends on scarcity. If you underline everything, nothing stands out. The original test showed that underlining more than one or two words per page actually reduced response compared to no underlines at all. Luna: So the magic is in the restraint. Just one word. Lucas: One word, placed strategically. In this case, 'now' — a word that already carries urgency. The underline amplified that urgency. Psychologically, it's like a visual exclamation point that's softer than an actual exclamation point. Exclamation points can feel shouty. Underlines feel like a nudge. Luna: I've noticed that in email subject lines, underlines don't exist — HTML doesn't support them in most clients. But I've seen people use asterisks or all caps to try to replicate the effect. Does that work? Lucas: Rarely as well. Asterisks are a convention from plain-text email, but they don't carry the same weight. All caps can trigger spam filters and feel aggressive. Underlining in digital copy — if you're on a platform that supports it, like some rich-text landing page builders — can still work, but you have to be careful. Luna: Careful how? Lucas: On screen, underlines are often confused with hyperlinks. If someone sees an underlined word and it's not clickable, they get frustrated. So you have to use it in contexts where hyperlinks aren't expected — like in printed materials, PDFs meant to be read on screen but not clicked, or in video sales letters where the text is displayed as an image. Luna: That's a huge practical takeaway. Don't underline in an email body unless you actually link it. Lucas: Right. But in direct mail, it's still a powerful tool. And there are other examples. One I love is from a magazine ad for a financial newsletter. The headline read 'How to Retire with More Money Than You Ever Dreamed Possible' — and they underlined the word 'More'. Luna: That's a smart choice. 'More' is the core benefit. Lucas: Exactly. They tested it against a version with no underline, and the underlined version pulled forty-eight percent more leads. Same layout, same copy, just a line under 'More'. Luna: So the principle is: identify the single most important word in your headline — the one that carries the benefit or the urgency — and underline only that. Lucas: That's the rule. But you have to test it. Some audiences, especially younger ones who grew up with hyperlinks, might not respond the same way. I'd always run an A/B test before rolling it out at scale. Luna: Have you ever tested it yourself, Lucas? Lucas: I tested it on a landing page for a client — a high-ticket coaching program. The headline was 'Start Building Your Business Today' and we underlined 'Today'. The test ran for two weeks. The underlined version had a conversion rate of three point two percent versus the control at two point one percent. That's a fifty-two percent lift. Luna: Fifty-two percent from one line under one word. That's consistent with the classic data. Lucas: It is. And interestingly, the lift was even higher on mobile — four point one percent versus two point three. Possibly because on a small screen, the underline stands out more against the surrounding text. Luna: That's a counterintuitive insight. Most people think mobile requires bigger, bolder elements. But a subtle underline might actually work better because there's less visual noise. Lucas: Exactly. The underline doesn't take up extra space. It's a minimal intervention. And minimal interventions often win because they don't disrupt the reading flow. Luna: So what's the biggest mistake people make when they try this? Lucas: Underlining too many words. I've seen landing pages where every other word is underlined — 'Get Your Free Report Now' — and it looks like the page is trying to scream at you. The brain quickly learns to ignore it, and response drops. Luna: Or they underline the wrong word. Like underlining 'the' in 'Get the free report' instead of 'free'. Lucas: Right. The word you underline has to carry the emotional payload. 'Free', 'now', 'you', 'today', 'guaranteed' — these are high-impact words. Underlining a neutral word does nothing. Luna: I want to ask about a different scenario: what about handwriting? In a handwritten note, underlining feels natural. But in a typed letter, does it feel too deliberate? Lucas: That's a great question. In the original fundraising letter, the underline was done by hand with a pen — not typed. That was deliberate. The writer wanted it to feel like a personal emphasis, not a mechanical one. In typed letters today, you can simulate that by using a handwritten font for the underline or adding it as a graphic element. Luna: So authenticity matters. If it looks like a machine did it, it loses some of the effect. Lucas: Absolutely. The underline should feel like a human touch. That's why in digital copy, I sometimes use a different colour — like a red underline — to signal that it's intentional, not a broken hyperlink. Luna: That's clever. It distinguishes it from the blue underlined link. Lucas: Yeah. But again, test it. I've seen cases where a red underline outperformed a black one by twenty percent, and other cases where it bombed. Context is everything. Luna: Let's talk about the downside risk. Can an underline actually hurt response? Lucas: Yes, if it triggers a negative association. For example, in some European markets, underlining in advertising is associated with aggressive used-car sales tactics. There's a cultural stigma. So if you're mailing to a sophisticated audience, an underline can feel tacky. Luna: So it's not a universal booster. You have to know your audience. Lucas: Exactly. The golden rule of copywriting applies: know who you're talking to. For a conservative, older donor base — like the one in that original test — an underline feels familiar and respectful. For a startup founder audience, it might feel dated. Luna: Are there any other typographic tricks that have similar outsized effects? Lucas: Handwritten P.S. sections. We've covered that before. Also, circling a key number with a red pen — I've seen that lift response by thirty to fifty percent in direct mail. And a handwritten margin note like 'This is the part most people miss' can do wonders. Luna: All of those are about creating a sense of personal attention. The reader feels like someone took the time to highlight what's important for them. Lucas: Exactly. That's the deeper principle. The underline isn't magic. It's a signal of human intentionality in a medium that often feels impersonal. When you underline one word, you're saying, 'Out of all these words, this one matters most. I'm telling you directly.' Luna: And that's why it works even today, despite all the digital noise. Lucas: It does. And the best part is, it costs nothing to test. Take your next headline, pick one word, draw a line under it, and see what happens. You might be surprised. Luna: I'm going to test it on my own email this week. I'll let you know how it goes. Lucas: Please do. And if listeners try it, I'd love to hear the results. That's the kind of real-world data that keeps this show interesting.