My husband a 'special' way of relaxing after a busy day. He loves to watch very realistic crime series. Favourites are Gomorrah and Narcos. I don't join in, but I get the picture from the visuals and the sounds in the background. It has honestly shaped how I feel about Colombia, for instance. To me it felt like a dangerous place to go. And I was right. Well, I used to be right, until I started reading about Medellín. Let me show you how that led to the answer of an important design question:
How do you design violence out of a neighbourhood? And how do we put these insights to work elsewhere in our lives?
In the 1990s, Medellín was one of the most dangerous cities in the world. Its hillside settlements, a two hour climb from the centre, were controlled by armed groups, and the instinct everywhere else would have been to seal them off, to treat them as the problem and keep the rest of the city away. Medellín did the reverse. In 2004 it opened the Metrocable, a cable car stitching those settlements to the metro below, and put libraries, schools and squares in what had been no go areas. It did not wall the neighbourhoods off. It connected them. It did not write them off. It invested in them. The point was never transport. It was access, visibility and the sense of belonging to the city.
The evidence is striking. A natural experiment compared 25 neighbourhoods the cable car reached with 23 comparable ones it did not, between 2003 and 2008. Violence fell across the city, but significantly more in the connected neighbourhoods, where residents also reported higher collective efficacy, the shared belief that neighbours will step in for one another (Cerdá and colleagues, American Journal of Epidemiology, 2012). Collective efficacy is the principle from behavioural psychology that makes the case make sense.
Communities stay safe through connection, not instruction, and connection is something the environment either enables or blocks.
This is Behavioural Design: safety built into the place, not preached into the people. You cannot lecture people out of isolation, you have to design it away. So how do you design isolation away? Here are three practical moves:
Find the isolated part of your system. The team that never sees how its work lands, the function tucked away where no one consults it. Isolation, not attitude, is often what produces the behaviour you dislike. Locate it first.
Build the connection that is missing. Before another campaign about collaboration, ask what physical or structural link would let these people see and rely on one another. A shared channel, a standing overlap, a reason to meet that did not exist.
Invest most where access is worst. Medellin put its best infrastructure in its poorest districts. Apply the same logic: put your effort into the group that is most cut off, not the one already well connected.
Where the behaviour you want is blocked by isolation rather than attitude, the strongest move is rarely a message. It is a connection that did not exist before.
And there you go. Who knew my husband's questionable approach to relaxing could lead to such useful insights? So, the next time someone tells you binge-watching is a waste of time, you have science on your side. You're welcome.
Behavioural Design Summer School
Two days in Amsterdam with Tom and Astrid, and a small group of people ready to be more influential by September.
Day one builds a working Behavioural Design × AI workflow you can use from the following week. Day two: two masterclasses based on the founders' books, one on designing for happiness and energy, one on finding the leverage points that unlock stuck change.
Dinner together on the first evening. A follow-up session three months later. Both books included.
🇳🇱 6 & 7 augustus 2026 · Dutch edition - waitlist only!
🇬🇧 20 & 21 August 2026 · English edition - only 8 spots left!
€1,749 excl. VAT · Only 16 spots per edition
The denomination effect: why a big note survives and small change vanishes
I don’t know if you still use cash (I most of the times use my debit card). But if you do and if you want to spend less, here’s something from behavioural science that might interest you. Researchers found that people are markedly more reluctant to break a single large note than to spend the same amount held as small coins and notes. The intact fifty feels like something to protect, while the loose change feels like it is already half gone. Behavioural scientists call this the denomination effect, and it quietly shapes how we save and spend without our noticing.
You can use this on yourself. If you want to hold onto money, keep it in larger, intact units, the single transfer to savings rather than the loose buffer in your current account, because whole, undivided amounts are psychologically harder to dip into.
And if you want to understand your own spending, watch where things come in small, easy denominations, because that is where money leaks.
The form money takes changes how easily it leaves.
This might also be useful if you give your kids pocket money. A single banknote is harder to break into than the same amount in loose coins, so a note may help them hold on to it.
Here's to the stubborn optimists, who unlock the power of Behavioural Design for optimising work, life, and everything in between.
See you next week!
Astrid Groenewegen
Co-Founder of SUE | Behavioural Design Academy, Author of 'The Art of Designing Behaviour' / 'De Kunst van Gedrag Ontwerpen' and ‘De Gelukscode’.
Source:
Cerda, M., Morenoff, J. D., Hansen, B. B., Tessari Hicks, K. J., Duque, L. F., Restrepo, A., and Diez-Roux, A. V. (2012). Reducing violence by transforming neighborhoods: a natural experiment in Medellin, Colombia. American Journal of Epidemiology, 175(10), 1045 to 1053. Policy background: Devlin, M., and Chaskel, S. (2010), Princeton Innovations for Successful Societies.
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