Margen
You tap the rhythm of "Happy Birthday" on the table with your finger and it seems impossible that the person across from you doesn't get it. In 1990 a psychology doctoral student at Stanford named Elizabeth Newton staged exactly that scene 120 times, with pairs of strangers. The listeners got three songs right. Three out of a hundred and twenty: 2.5 per cent.
Newton had asked the tappers beforehand how many they thought would be guessed. They said half. They were off by a factor of twenty, and the reason is what makes the experiment great: while they tapped, they went on hearing the song. Her dissertation was called "Overconfidence in the Communication of Intent", which is a long title for the fact that nobody hears themselves from outside.
Do it once and you feel it in your body. While you tap, everything is playing inside your head: the melody, the words, the entrance of the chorus, even the tone of the voice singing it. You are not producing taps; you are accompanying a song that is already playing. The other person has a tabletop in front of them and a series of dry knocks with no pitch and no recognisable rhythm. What strikes you as unbearably obvious reaches them as Morse code.
That has a name: the curse of knowledge. It was pinned down in 1989 by three economists — Colin Camerer, George Loewenstein and Martin Weber — while studying why someone who holds a piece of information cannot reason like someone who lacks it. You know the other person has no idea which song it is. You know it, and even so your brain is incapable of simulating that ignorance: Newton's tappers knew it perfectly well and went on expecting half.
The message you send a colleague at 19:40: "Hey, the supplier thing, are you looking at it or am I?" Inside your head the whole email thread from last week is playing, plus Tuesday's phone call and the invoice that doesn't add up. They have three suppliers and eleven open conversations: what they receive is knocks on a table. And they reply what anyone receiving knocks replies: "the August one?", which strikes you as an absurd question, because in your head there is only one supplier in the world.
Or the discharge note they hand you when you leave hospital, written by someone who has spent twenty years naming the same things and for whom "non-specific findings" is a reassuring phrase.
What's missing is the title. A summary tells the same thing in fewer words and is still Morse. The title is what the other person needs to know before they can understand a single word of what follows. That 19:40 message, with a title in front, takes three seconds longer: "I'm writing because the Sanjurjo order got invoiced twice, in August. We have to claim it back before Friday. Do you call them or do I?" Your colleague knows whether it is theirs before the first line is over.
Hospitals have turned this into protocol. The shift handover is done in a fixed format — situation, background, assessment, recommendation — and in that exact order, precisely because the person coming on needs the title before the knocks, and because at three in the morning nobody is in a state to reconstruct a song from its rhythm.
It also explains why the person who knows most usually explains worst. The better you master something, the more compressed you hold it: where the other person needs four steps, you have one word. That's why the best manual for a program is rarely written by whoever programmed it, and why furniture assembly instructions are trialled on people who have never assembled anything in their lives: whoever designed the piece can no longer read them without already knowing where every screw goes. The expert who shines in an interview is almost always the one who first went and explained it to an outsider.
There is a cheap test for whether it's happened to you. Take the last thing you wrote — an email, a slide, a voice note — and read it to someone who doesn't work with you. Ask them to tell you what it was about. If the first thing they say is a question, you're missing the title.
Tomorrow, before you hit send on that message that explains itself, write a line in front of it starting with "I'm writing because" and give the song a name. Then tap all you like.