What Is Nudging? The Complete Guide to Behavioural Nudges

In 1999, the maintenance team at Amsterdam Schiphol Airport etched a small image of a housefly into the enamel of the men’s urinals. No signs. No fines. No lectures about hygiene. Just a fly, positioned just above the drain - a visual target that men instinctively aimed for. Within months, spillage around the urinals had dropped by roughly 80%.^[1]

Thousands of kilometres away, researchers were studying a far weightier problem. In countries where organ donation was opt-in - you had to actively sign up - donor rates hovered around 15 to 20%. In countries that had simply changed the default to opt-out - you are a donor unless you actively withdraw - rates exceeded 90%. The transplant waiting lists were not shaped by people’s values. They were shaped by which box was pre-ticked on a government form.^[2]

Same psychology. Profoundly different stakes. Both are nudges.

What is nudging?

The word “nudge” entered the mainstream in 2008 when University of Chicago economist Richard Thaler and Harvard Law professor Cass Sunstein published Nudge: Improving Decisions About Health, Wealth, and Happiness. Their definition is precise: a nudge is any aspect of the choice architecture that alters people’s behaviour in a predictable way without forbidding any options or significantly changing economic incentives.

Three words in that definition do a lot of work.

Choice architecture refers to the way choices are presented and structured. Every form, menu, interface, physical space and communication is a piece of choice architecture - whether its designer intended it to be or not. Nudging is the deliberate act of shaping that architecture to guide better outcomes.

Predictable means nudges are not random. They exploit known and documented patterns in human psychology - our tendency to stick with defaults, to follow what others do, to choose the path of least effort. This predictability is what makes nudging a science rather than a guess.

Without forbidding or significantly changing incentives is what separates a nudge from a mandate or a tax. People retain full freedom of choice. The nudge simply makes one option marginally easier, more visible or more attractive than the others.

Thaler received the Nobel Prize in Economics in 2017, in part for this work. By then, governments from the UK to Singapore had already established dedicated nudge units - teams of behavioural scientists tasked with applying these insights to public policy at scale.

A nudge is not a trick. It’s a redesign of the environment that makes better behaviour the path of least resistance.

How nudging works: three core mechanisms

Most effective nudges operate through one of three psychological mechanisms. Understanding them is the foundation for designing interventions that actually change behaviour.

1. Defaults

The default is whatever happens when you do nothing. And humans, being cognitively efficient creatures, do nothing surprisingly often. We accept the pre-set options, leave forms as they come, and stick with the status quo. This is not laziness - it is the brain conserving energy.

Which means whoever sets the default holds enormous power. The organ donation research by Johnson and Goldstein demonstrated this with striking clarity: switching from opt-in to opt-out did more to increase donor rates than decades of awareness campaigns. In the Netherlands, automatic pension enrolment has produced similar results - participation rates that voluntary schemes could never approach.

The design lesson is simple: make the desired behaviour the default. Make the undesired behaviour require active effort. Do not rely on people’s motivation to overcome friction.

2. Social norms

Humans are social animals who look to others to calibrate their own behaviour, especially under uncertainty. This is social proof - one of the most powerful forces in behavioural science.

The UK’s Behavioural Insights Team (BIT), sometimes called the “Nudge Unit”, demonstrated this brilliantly in a tax compliance experiment. Letters sent to people who had not paid their taxes were redesigned to include a single sentence: “Nine out of ten people in your area pay their taxes on time.” No threats. No new information about penalties. Just a social norm. The result was a significant increase in on-time payments - generating millions in additional revenue without a single new enforcement action.

Hotels use the same mechanism: “Most guests staying in this room reuse their towels.” Not “please reuse your towels to help the environment.” What most people do matters more to us than what we “should” do.

3. Friction reduction and friction addition

Every extra step between intention and action is a moment at which people fall away. Reducing friction - removing barriers, pre-filling forms, simplifying interfaces - dramatically increases the likelihood that people follow through on their intentions.

Rutgers University demonstrated this with printers: switching the default setting from single-sided to double-sided printing cut paper consumption by over 40%. Students could still choose to print single-sided. They just had to make an active choice to do so. Most didn’t bother.

Friction addition works in reverse: making undesired behaviour harder. Adding a confirmation step before an unsubscribe, placing unhealthy snacks on a higher shelf, or requiring a user to type “I understand the risk” before proceeding all use friction to slow down automatic choices and create space for deliberation.

Real-world nudge examples

The UK Behavioural Insights Team and tax letters

Perhaps the most methodologically rigorous nudge programme in history is the BIT’s tax compliance work. By testing different versions of letters sent to late taxpayers - varying the language, the social norms mentioned, the simplicity of instructions - the team identified combinations that consistently outperformed standard HMRC letters. The most effective versions used social norms and personalisation. The result was not just behavioural change but a measurable return on investment that dwarfed the cost of the programme.

Google’s cafeteria redesign

Google’s food team ran an extensive series of experiments in its New York offices. They moved salad bars to the entrance of the cafeteria, placed water at eye level and sugary drinks below knee height, served healthy options in larger bowls and unhealthy options in smaller ones. No food was removed. No employee was instructed to eat differently. The result: a significant shift in calorie consumption and a measurable increase in healthy food choices - achieved entirely through the redesign of the physical environment.

The Schiphol fly

The fly in the urinal at Amsterdam Schiphol Airport deserves its legendary status. It is the perfect example of a nudge: cheap to implement, invisible in its working, and extraordinarily effective. The fly exploits a hardwired human behaviour - the instinct to aim at a target - without any communication, any enforcement, or any change to what was permitted. The maintenance costs of the men’s toilets fell dramatically. It has since been replicated in airports and facilities around the world.

Pension auto-enrolment

In 2012, the UK introduced automatic enrolment in workplace pension schemes. Previously, employees had to opt in - and most did not, even when their employer would match their contributions. By changing the default to enrolment, with the option to opt out, participation rates climbed from around 55% to over 85% within a few years. In the Netherlands, similar default structures underpin the pension system that consistently ranks among the best in the world. The policy changed nothing about what people were allowed to do. It changed what happened when they did nothing.

Nudging and the SUE Influence Framework

At SUE, we situate nudging within a broader analytical structure: the SUE Influence Framework. The framework maps the four forces that determine whether someone changes their behaviour: Pains (what they want to move away from), Gains (what they want to move towards), Comforts (what keeps them doing what they already do), and Anxieties (what stops them from trying something new).

Nudges operate primarily on the Comforts and Anxieties side of this map. They reduce the friction and inertia that keep people in their current behaviour (Comforts) and lower the perceived effort or risk of the new behaviour (Anxieties). They do not typically address Pains and Gains - which is why the most powerful behavioural interventions combine nudge design with a deeper understanding of what people actually want and fear.

This is an important practical insight. A nudge that makes it slightly easier to sign up for a pension scheme will fail if the underlying Anxiety is “I don’t trust that I’ll ever see that money again.” The barrier is not friction - it is fear. No reduction in form-filling steps will address it. You need to work on the Anxiety directly.

Conversely, even a powerful message about the benefits of saving for retirement (a Gains intervention) will underperform if signing up requires navigating a complex 12-step process. Reduce the friction first. Then make the case.

When nudging works - and when it doesn’t

Nudging is not a magic wand. It has genuine power in specific conditions, and genuine limitations that are worth understanding before you design anything.

Nudging works best when:

• The behaviour is habitual or low-attention - System 1 is in charge
• People are essentially indifferent between options and just need a gentle steer
• The desired behaviour is easy to perform once the path is cleared
• Social context can be mobilised - there is a clear “what most people do” to reference
• The stakes for any individual choice are relatively low

Nudging is less effective when:

• The behaviour involves strong existing habits, values or identity
• The stakes are high and people are deliberately deliberating - System 2 is engaged
• The nudge conflicts with a powerful Anxiety (fear of loss, distrust of the organisation)
• The desired behaviour requires sustained effort over time, not a one-time choice
• Cultural or social norms run counter to the nudge direction

A word on dark nudges

The same psychological mechanisms that make nudging a powerful tool for good can be weaponised against people’s interests. Dark patterns in digital design - pre-ticked boxes for unwanted subscriptions, confusing unsubscribe flows, artificially manufactured scarcity - are dark nudges. They exploit the same cognitive shortcuts but in the opposite direction: steering people towards choices that benefit the organisation and harm the user.

The ethical test for a nudge is straightforward: would the person whose behaviour you’re shaping thank you for it if they understood exactly what you were doing and why? If yes, it is a nudge. If no, it is manipulation.

Frequently asked questions about nudging

What is nudging in simple terms?

Nudging means designing the environment around a choice so that people are gently steered towards a better option - without being forced or given financial incentives. The Schiphol fly is the most famous example: a small image that gave men a target to aim for and cut spillage by around 80%. Nothing was forbidden. No one was instructed. The environment was simply redesigned.

Who invented nudging?

The concept was formalised by American economist Richard Thaler and legal scholar Cass Sunstein in their 2008 book Nudge: Improving Decisions About Health, Wealth, and Happiness. The book drew on decades of research in behavioural economics and cognitive psychology, synthesising it into a practical framework for policy and design. Thaler was awarded the Nobel Prize in Economics in 2017, partly for this work.

Is nudging manipulation?

Not when it meets three conditions: transparency (the intervention is not hidden and would be recognised if pointed out), freedom of choice (all options remain available and no option is removed), and alignment with the person’s own interests (the nudge helps them do what they actually want to do). A nudge that works against someone’s interests or removes their ability to choose freely is a dark pattern, not a nudge. The distinction lies in intent and execution, not in the psychological mechanism.

What are the three main types of nudge?

The three most powerful nudge mechanisms are: (1) Defaults - making the desired option the one that happens when you do nothing, as with pension auto-enrolment; (2) Social norms - showing what most people like you already do, as with the BIT tax letters; (3) Friction - reducing the effort required for good behaviour and increasing it for harmful behaviour, as with printer defaults and healthy food placement.

When does nudging not work?

Nudging works best on habitual, low-attention behaviour where System 1 is in charge. It is less effective for high-stakes, deliberate decisions like buying a house or choosing a medical treatment. It also fails when the nudge conflicts with strong existing habits, values or social norms in the target group - or when the underlying barrier is an Anxiety that friction reduction alone cannot address.

What is the difference between nudging and Behavioural Design?

Nudging is one technique within the broader discipline of Behavioural Design. Where nudging focuses on subtly steering choices through the environment, Behavioural Design encompasses the full process: diagnosing why behaviour is or isn’t happening, mapping the four forces of the Influence Framework, designing interventions across all those dimensions, and testing their effectiveness. Nudging is a tool. Behavioural Design is the method.

Conclusion

Nudging is one of the most consequential ideas to have moved from academic research into mainstream practice over the past two decades. It works because it takes seriously what we actually know about how humans make decisions - not how we imagine they do. People are not rational calculators carefully weighing options. They are cognitive efficiency machines, following the path of least resistance, doing what others do, sticking with what was set for them by default.

Understanding that is the beginning. Designing with it is the practice. And knowing when nudging is not enough - when you need to address deeper Pains, Gains and Anxieties - is the craft.

Want to develop that craft? The Behavioural Design Fundamentals Course teaches you the SUE Influence Framework and a structured method for diagnosing and redesigning behaviour. Rated 9.7 out of 10 by more than 10,000 professionals from 45 countries.

PS

At SUE, our mission is to use the superpower of behavioural science to help people make better choices - for themselves and for society. Nudging at its best is not about manipulation. It is about designing environments that make it easier to be the person you want to be. The fly in the urinal. The pension default. The fruit at eye level. Small design decisions with consequences that reach further than any awareness campaign ever could.