The MIT Senseable City Lab recently teamed up with a few other research groups to investigate the relationship between human interactions and city size. If you happen to be a member of the Journal of the Royal Society Interface, you can download the full report here. But in true ATC fashion, I’m going to give you the Coles Notes version here.

What the study did was look at billions of anonymized mobile phone data in both Portugal and the UK in order to determine how our real life social networks change with city size. And what they found is a pretty consistent relationship:

[T]his study reveals a fundamental pattern: our social connections scale with city size. The larger the town you live in, the more people you call and the more calls you make. The scaling of this relation is “super-linear,” which means that on average, if you double the size of a town, the sum of phone contacts in the city will more than double – in a mathematically predictable way.

What’s interesting about this finding is that it starts to explain how cities–and the clustering of people–can act as fertile ground for the exchange of ideas and knowledge. The bigger the city the more people you probably know.

But what I’m curious about (I don’t have the report) is if there’s some kind of upper limit. Presumably this “super-linear” relationship tapers off after a certain city size, because there has got to be limits to the number of people we can maintain productive relationships with.

According to British anthropologist Robin Dunbar, that number was 150 people.

The City of Toronto recently started an initiative called “Comprehensive to the Core.” It’s a look at how downtown Toronto–which is growing at 4 times the rate of the rest of the city–should continue to grow moving forward so that it remains a great place to live, work, learn and play.

Here’s a presentation that was delivered last month by the city. It’s mostly infographics and so it’s a quick and fun read. And here’s an infographic that does a nice job of summarizing what’s happening in the core of Toronto.

What it’s saying is that downtown Toronto–which they consider to be bound by Bathurst Street in the west, the Don Valley Parkway in the east, the lake in the south, and Dupont Street in the north–is responsible for 51% of the city’s entire GDP. It’s also responsible for 33% of all jobs in the city and 25% of the city’s entire tax base. And yet in terms of size, it represents only 3% of the city’s land area.

That’s a powerful reminder of the economic potential of density and agglomeration economies. It’s also a reminder that we shouldn’t let politics deprive our economic engine of the services and investments it needs.