We’ve all been there. You’ve rushed to catch the train to work. There’s no seat, and it’s stifling hot. But none of that matters, because you’ve managed to angle yourself and claim just enough space to slip a book out.

Then you feel it. A pair of eyes peering over your shoulder: a fellow commuter, sneaking a peak at your page. An immense irritation bubbles up inside of you, which naturally you suffer in silence. You flick the corner of your book, shuffle on your feet. But until they look in the opposite direction, you won’t be able to take in a single word.

Why is it so annoying when a stranger - or a loved one, for that matter - reads over your shoulder? What makes is more maddening than if they were taking a nosey at, say, a video you were watching on your phone, or just admiring your new pair of shoes?

The reading of all good books is like conversation with the finest people of the past centuries – Descartes

Carl Sagan likened reading a book to a “voyage through time”, while Descartes said it’s like having a “conversation with the finest people of past centuries”. If you believe either to be true, it makes sense that you wouldn’t want the process to be interrupted. But it goes deeper: that intense annoyance you feel at book-peepers is actually hardwired in our biology.

“The space surrounding you is known as ‘peripersonal space’, explains psychologist Madeleine Mason Roantree.

“Researchers found that when an object entered a monkey’s peripersonal space, they would reflexively show defensive mannerisms, including covering their face, closing eyes, crossing arms.”

Roantree likens the experience to someone “interrupting essential private moments of calm, which can lead to stress” – essentially, turning the reason why many of us read in the first place in its head.

Please, we beg, we pray, go throw your TV set away, and in its place you can install a lovely bookshelf on the wall. – Roald Dahl

There’s also a good reason why it winds you up more than being watched consuming other forms of entertainment like television or computer games.

“Reading is usually a solitary activity,” says Roantree, “whereas watching TV is often associated with socialising. You would go to the cinema in the first few dates of knowing someone, but you probably wouldn’t read a book aloud to them.”

The other crucial difference that sets reading apart is the fact we tend to project far more of our own experiences and opinions onto books.

“The visuals, and often the emotions, in television and films are generally spoon fed to us,” Roantree says.

“With reading, there is more interpretation involved. That’s why discussing books can often be more personally revealing than discussing movies, and leaves us feeling more vulnerable to social rejection if others interpret things differently.

Reading is a discount ticket to everywhere. – Mary Schmich

“Perhaps there is an element of not wanting others to read over our shoulders because it could in some way reveal what we are thinking or expose those raw and vulnerable parts of us. We feel ownership over the story – it’s ours.”

In other words: we’re protective of the world inside of our heads. Especially during rush hour.

Madeleine Mason Roantree is director of relationship psychology services at The Vida Consultancy. Written by Tom Ward.