Two similarly sized strangers, Ari and Taylor, get on a crowded subway train. They sit down in the last two available seats, which are adjacent. To begin, they take up the same amount of space. Thirty minutes later, though, things look very different. Ari is tight, contained, narrow—crossing their legs, hunching their shoulders, keeping their elbows close. Taylor, on the other hand, is expansive, relaxed, stretched out—spreading their elbows, shoulders, and knees wide.

This is a story about spreading out on the subway, and what we need to understand to explain it.

The phenomenon of taking up too much room on the subway as a man has gotten a lot of attention in the last few years under the label “manspreading.” The OED chronicled a huge rise in the use of the word “manspread” from November to December of 2014. New York City’s MTA started posting notices of subway etiquette reading “Dude… stop the spread. It’s a space issue.” Particularly egregious spreaders are shamed on social media. Academic analysts like Emma Jane use the public discussion of manspreading to take the pulse of contemporary feminism online.  

But why does this spreading happen? And what does it have to do with men in particular?

Easily Googleable answers often start with the fact that the spreaders are often men to explain the spread: men feel entitled; men are less comfortable in narrow positions; or men seem proportionately more attractive than women do when spreading out in expansive poses.

There’s probably truth in each of these. But we actually don’t need to posit anything that fancy just to explain the basic spreading-and-squeezing phenomenon.

I’m going to offer a different kind of explanatory story, one which starts with minimal explanatory resources. It’s a story that doesn’t begin with the fact that it’s mostly men who spread, but ultimately offers a way to explain why men spread more. The speculative story I offer will open space for more systematic, intersectional analyses of the (man)spreading phenomenon.

Start with something simple: preferences for personal space.

How will two people divide a squeezed space between them? If they’re the same size, and they have the same preferences for personal space, they may split the space evenly.

Roughly speaking, we can specify a person’s preferences for personal space in terms of a 3D buffer zone around their body that they prefer to keep empty. If this zone is evenly spread out around the body, we can imagine it as an invisible bubble of space with a human-like shape. (To a very unscientific approximation: this invisible space is shaped more like this than like this.)

Now here’s the simple move that explains Taylor’s spread and Ari’s contraction: assume that Ari and Taylor have differently sized personal space bubbles. If Ari has a bigger bubble (that is, Ari prefers to keep things farther away) than Taylor does, then when they share a limited space, they will allocate that space quite differently. Because Ari wants more space between them and other things, Ari will end up taking up less space than Taylor does, in order to maximize the distance between them and other things—including, in particular, Taylor.

Here’s how we can use this to explain spreading and squeezing behavior. When Ari and Taylor first sit down on a crowded train, Taylor is more likely to be in Ari’s personal space bubble than vice versa. Ari might thus shift slightly away from Taylor, to recover their personal space bubble. Then, either in the natural shift of subway motion or in virtue of intentional readjustment, Taylor might shift their body too. Taylor is likely to move closer to Ari than Ari would prefer, because Taylor’s personal space bubble is smaller—that is, Taylor is cool with being closer to things (including Ari) than Ari is.

But each time Taylor drifts towards Ari, Taylor might intrude again into Ari’s personal space bubble, before Ari ever intrudes into Taylor’s. Each time that happens, we would expect Ari to retract again. If this keeps up for long enough, Ari and Taylor might reach a natural limit, where Ari can’t squeeze any more, and Taylor is comfortably stretched out into the extra space that Ari’s self-contraction has opened up.

That’s a simple story of the spread. But it might seem too simple—too simple, that is, to explain the microaggression that is the manspread. Saying that Ari and Taylor have different preferences in this hypothetical case can seem reductive of all the systemic forces that contribute to the manspread as it happens in the complicated real world.

One particular worry goes like this: if men who spread into women’s personal space do so just because they have ‘less demanding’ preferences, doesn’t that put the burden on women to be ‘less demanding’ in their own preferences for personal space?

I don’t think it does. To understand why not, we need to understand more about the nature of personal space involved. We need to understand that it is fundamentally a protective space, encoded in primitive pathways of the brain. And we also need to see that this protective space primarily functions to link intrusions into this bubble with involuntary defensive movements like cringes and startles. Getting into this story will let us link this simple analysis with a broader understanding of the systemic forces that contribute to microaggressions like the (man)spread.

Tune in for Part II to hear how the brains of tiny monkeys can shed light on the situation you find yourself in on a crowded subway train.