Willemijn Sterk - Behaviour Club

For many organisations, brand growth is still closely linked to visibility, differentiation and creative impact. But what if growth does not start with what brands say, but with what people actually do? For Willemijn Sterk, Adjunct Director at Behaviour Club, that distinction is crucial. From road safety to waste reduction and public space behaviour, her work focuses on one fundamental question: how do you design interventions that genuinely change behaviour – and what can brands learn from that?

Behavioural Science as a Growth Tool

At Behaviour Club, behavioural change is never approached in abstract terms. The agency works explicitly with scientifically validated behavioural techniques, translating insights from behavioural science into practical, real-world interventions. Their approach looks at behaviour through three interconnected lenses: the individual, the social environment and the physical environment.

In public spaces such as parks or streets, this often means looking beyond communication alone. Are bins visible and easy to use? What happens when they are full? What social cues influence people’s behaviour in that moment? “We strongly believe that measuring is knowing. If you want to achieve behavioural change, you need to test, measure and optimise continuously. That full trajectory is what we prefer to do,” says Willemijn.

Defining Behaviour

According to Willemijn, one of the most common pitfalls organisations face is defining their ambition too vaguely. Wanting “less litter” or “more sustainable behaviour” sounds good, but without clarity, it becomes impossible to design effective interventions. “Only once the behaviour is clearly defined, you can research when people do or don’t perform it,” Willemijn explains. “Do you want people to put rubbish in a bin, or to take it home with them? Those are two completely different behaviours, each requiring a different solution.”

However, she notes that measuring behaviour alone can be difficult, so looking at indicators such as reach, attitudes, intentions and social norms is also important to understand the full picture.

“We strongly believe that measuring is knowing. If you want to achieve behavioural change, you need to test, measure and optimise continuously.”

Purpose Marketing: Authenticity Over Trends

Purpose and corporate social responsibility play an increasingly important role in how brands position themselves. For Willemijn, the distinction between the two is clear: “Corporate social responsibility is about taking responsibility for contributing positively to society or the environment. Purpose marketing goes a step further: marketing driven by a higher purpose, where the product or service itself stems from that purpose.”

“Brands such as Dopper, Tony’s Chocolonely and Patagonia illustrate this well,” she continues. “Their purpose is not something added on top of the product; it is embedded in their DNA. Patagonia’s decision to actively encourage consumers to buy less clothing is a powerful example. Paradoxically, that stance strengthened the brand and drove growth.”

The danger, however, lies in inauthenticity. When brands jump on sustainability or social trends without structural changes behind them, consumers quickly sense it. Labels that were once strong quality markers for sustainable clothing risk losing credibility if they are adopted too broadly without clear differentiation.

For brands, the challenge is not to follow trends, but to claim something that is genuinely unique and meaningful for their audience – even if that means daring to invest without expecting immediate returns.

“CSR is about taking responsibility for contributing positively to society or the environment. Purpose marketing goes a step further: marketing driven by a higher purpose, where the product or service itself stems from that purpose.”

Designing Change With People, Not For Them

Looking ahead, Willemijn sees a clear shift that organisations need to make. Too often, policies and campaigns are still designed top-down, based on assumptions rather than real-life behaviour. This can lead to resistance and polarisation. Instead, she argues, organisations move from designing for people to designing with them - by engaging directly with their realities, needs and behaviour. By involving citizens, consumers or communities in the design of solutions, behavioural change becomes something people feel ownership over.

“Make people co-owners of solutions. Look at behaviour in real environments. For example, we tell people to live healthier lives, but the physical environment is full of fast food and unhealthy choices. If you want behavioural change, you need to address individual, social and physical factors together.” This approach also forces organisations to confront the inconsistencies of promoting healthy lifestyles in fast-food-dominated environments.

For brands, this perspective is also highly relevant. Purpose-driven growth is not about telling people what you stand for, but about enabling behaviour that aligns with those values. When purpose is translated into concrete actions, experiences and environments, it becomes credible – and powerful.

“Make people co-owners of solutions. Look at behaviour in real environments. If you want behavioural change, you need to address individual, social and physical factors together.”

A Broader View on Brand Growth

Behavioural change may not traditionally sit at the heart of brand growth discussions, yet it offers a valuable lens for the future. Growth does not only come from awareness or preference, but from relevance in everyday behaviour. Brands that understand this can move beyond messaging and become meaningful parts of people’s lives.

As Willemijn puts it, “Behavioural change doesn’t have to be big or loud. Often, it’s about subtle interventions, repeated over time, in the environments where people live their daily lives.” And perhaps that is where the real opportunity for brand growth lies: not in saying more, but in helping people do things differently – for the better.

“Behavioural change doesn’t have to be big or loud. Often, it’s about subtle interventions, repeated over time, in the environments where people live their daily lives.”