Consider the process you go through when you subscribe to a service. It might be a weekly food box service, a gym membership, or MasterClass video lessons. Either way, the process is easy: enter your information, choose a payment method, and voilà—you are subscribed. If you’re lucky, the first product or session might even be free. Now, consider the challenging process involved when attempting to cancel a subscription. Often, cancelling a subscription requires a tedious phone call, during which you have to explain why you want to cancel while the operator tries to convince you to stick around. This frustrating process is no mistake; it is an intentional design technique known as sludge.

Sludge is essentially the opposite of a nudge. While nudges try to push people to make better decisions by making certain choices easier than others, sludges make a process more difficult with the goal of creating friction, which makes the individual less likely to continue the process. Sludge, however, is not always used to get more money from people or disadvantage the consumer in some way—sometimes, it’s used ethically to encourage people to be thoughtful about their behavior instead of rushing into actions they might later regret. For example, privacy protection requirements can seem annoying, but this friction causes us to think twice about what data we are sharing online and can encourage us to be more careful with it.¹

Nudge: A behavioral science technique that guides people toward optimal decisions by gently directing behavior without restricting freedom of choice or changing incentives. Placing healthy foods at eye level in grocery stores or displaying positive social proof around recycling in cafeterias are just two common examples of nudges in everyday life.

Choice Architecture: The design of how options are presented to people. Subtly altering how options are presented—such as positioning the desired choice as the default option—can have a pronounced impact on individual behavior.

Framing Effect: The idea that information can be valued differently depending on how it is presented. Framing can play an important role in whether sludge-friction feels justified or manipulative.

Friction: An obstacle intentionally or unintentionally put in place to make a decision more difficult. Friction is an impediment that can be deliberately used to dissuade people from making certain choices, but it can also unintentionally appear in complex systems and processes.

Dark Patterns: Deceptive design practices that trick users into making unwanted decisions. Many of these misleading techniques rely on sludge by intentionally adding friction to steer behavior one way or the other. For example, making it difficult to cancel a subscription or hiding opt-out buttons are sludge tactics that take the form of dark patterns.

Although the terms “nudge” and “sludge” were coined relatively recently, the idea of an environment impeding or facilitating certain behaviors was theorized as early as 1890. At that time, American psychologist William James published a book, The Principles of Psychology, which suggested that people’s ability to get things done was impacted by the environments in which they were making decisions.¹

Two important behavioral scientists, Richard Thaler and Cass Sunstein, took James’ ideas a step further by suggesting that since people’s environments impact their decisions, governments and other authorities can use small influences within people’s environments to push individuals toward certain behaviors. While not wanting to limit people’s freedom by suggesting they should be forced to make particular choices, Thaler and Sunstein believed that nudges can help people make more rational choices themselves. Their vision for nudges is described in their book, Nudge: Improving Decisions About Health, Wealth, and Happiness:

“A nudge, as we will use the term, is any aspect of the choice architecture that alters people’s behavior in a predictable way without forbidding any options or significantly changing their economic incentive.” ³

The framing effect shows us that the way options are presented to us matters. By understanding the cognitive biases that influence people’s thinking, Thaler and Sunstein hoped that nudges could be used to reduce the fallacies people fall victim to when making decisions. These should be well-intentioned; however, if choice architects can identify which cognitive biases cause people to favor particular choices over others, the risk is that they use the information to their own advantage rather than helping consumers make the best choices for themselves. For example, companies may nudge customers toward more expensive products, using available information about consumer decision-making to advance their own profit.

While Thaler and Sunstein originally thought nudges were a behavioral science tool that could help individuals, they soon realized that nudges also had the potential to take advantage of people’s bounded rationality—the idea that an individual’s decision-making ability is limited by cognitive and environmental constraints. Ten years after publishing Nudge, Thaler published an article titled Nudge, not sludge, which described activities that were “essentially nudging for evil”; in other words, sludge.² There is a distinction here between sludges as “evil” nudges and sludges as a process—good or bad—that makes it more effortful to make a certain decision. The term is usually used in the latter sense, but you’ll see both crop up throughout the literature.

Sunstein continued to examine sludge throughout his career. His 2021 book, Sludge: What Stops Us from Getting Things Done and What to Do about It, takes a look at all the administrative red tape people have to go through to meet their most basic needs. As a result, Sunstein suggests that sludges can actually prevent people from exercising their constitutional rights.

William James

A leading thinker, psychologist, and philosopher, James was the first educator to offer a course on psychology in the United States. His contributions to education established psychology as a formal discipline in the country, earning him the title “Father of American Psychology.” James laid the groundwork for the modern concept of sludge by exploring how environmental and situational factors affect human behavior.

Richard Thaler

Thaler is an American economist known for evaluating decision-making from a psychological perspective. In 2017, he was awarded the Nobel Prize in Economics for his contributions to the field. His work has been particularly influential thanks to his departure from the idea of humans as completely rational agents, exploring the economic anomalies through psychology-based behavioral insights. Thaler co-developed nudge theory alongside Sunstein, providing a theoretical foundation for our understanding of both nudges and sludges.

Cass Sunstein

A renowned scholar recognized for his work in law and behavioral economics, Sunstein is a best-selling co-author of the book Nudge, which he wrote with Thaler. In his work, Sunstein also wrote about the ethics of choice architecture, emphasizing that both nudges and sludges carry ethical implications, and that a focus on welfare, autonomy, and dignity is essential to avoid manipulative uses of behavioral design.

What is the difference between a nudge and a sludge?

While nudges are design elements that steer people toward a certain decision, sludge refers to design frictions that prevent people from engaging in a desired action.⁷ Nudges are most often used to help people make decisions that are in their best interest—like choosing a healthier meal option in a cafeteria or opting into a retirement savings plan. Sludges tend to have a much more negative connotation, often being used to dissuade people from canceling subscriptions or returning products.

That said, both sludges and nudges can have positive and negative effects. “Dark nudges,” for example, are intentionally designed to make harmful actions easier. Many subscription services use dark nudges to position the most expensive plans as the standard or default option, nudging consumers to choose these over cheaper plans. At the same time, some sludges can be positive. Introducing intentional frictions into cookie consent banners or payment platforms can encourage users to pause and consider their options carefully, increasing the likelihood that they make decisions that are in their best interest.

What are the cognitive mechanisms behind nudge and sludge?

The underlying cognitive mechanisms behind nudges and sludges are not well understood. Most literature in this area is geared toward solving practical problems, focusing largely on the “how” rather than the “why.” However, researchers have recently started exploring the cognitive mechanisms that contribute to the success or failure of a nudge or sludge intervention. 

In a large 2023 meta-analysis, researchers proposed a new cognitive framework for categorizing nudge and sludge interventions based on six main cognitive processes: attention, perception, memory, effort, intrinsic motivation, and extrinsic motivation.⁸ They found that both types of interventions have similar effect sizes. However, interventions that change effort—nudges by reducing effort and sludge by increasing it—are more effective than those targeting other cognitive processes. For example, reducing effort by making an option more convenient is more effective than using social norm messaging to boost motivation. This suggests that cognitive ease plays a crucial role in the effectiveness of nudges and sludges.

What are the psychological costs of sludge?

Sludge can create a variety of psychological costs for users who encounter behavioral frictions, including anxiety, frustration, and distrust, as well as the mental strain and uncertainty associated with difficult, complex decisions. The Organization for Economic Co-operation and Development (OECD) breaks down the main psychological costs of sludge into four overarching categories:⁹ 

• Search Costs: Psychological costs that occur when people encounter barriers while looking for information—such as uncertainty or discouragement—often due to frictions like unclear language or confusing systems.
• Decision Costs: Costs that arise when people are asked to make decisions in the face of ambiguous or overwhelming information, leading to decision-making barriers such as overload or decision fatigue.
• Cognitive Costs: The mental toll of navigating sludge, including understanding complex information or contacting support for assistance. This cognitive overload leaves fewer mental resources for other tasks.
• Emotional Costs: Costs that harm user well-being, including stigma, loss of autonomy, stress, confusion, anxiety, and disempowerment.

Beyond these psychological impacts, sludge also imposes significant productivity and economic costs. For example, the U.S. government estimates that Americans spend a collective 10 billion hours filling out government forms every year. Worse than wasted time, sludge in public services like healthcare can even be life-threatening.⁹

What are sludge audits?

Sludge audits are methods for systematically identifying sludges—unnecessary frictions in a process or service that make it harder for people to achieve their goals.⁹ Governments around the world are increasingly using sludge audits to ensure services are easy for citizens to navigate while also streamlining internal operations for those working in the public sector. Sludge audits can take place on public-facing systems, such as those used to apply government services, or internal processes that create friction and hinder employee productivity.

Sludge audits go beyond basic system mapping—which focuses on the steps of a process—to examine the behaviors, decisions, and lived experiences of people as they interact with that process. Importantly, these audits consider how sludge can affect different groups in unequal ways. For example, sludges can have an even greater impact on people facing poverty, who often face chronic cognitive demands and have limited mental bandwidth to navigate friction-filled government systems.⁹

Sludge often uses friction to discourage people from making decisions that are in their best interest. The difficulty of cancelling subscriptions or memberships is likely an experience we’ve all had, but it may not be as serious as other forms of sludge, such as the challenges involved in applying for public assistance or the ways sludge can suppress voting participation through manipulation or confusion. Fortunately, studying sludge and how it impacts human behavior has opened the door to interventions designed to reduce its negative psychological, social, and economic impacts.

Understanding Sludge as a Barrier to Public Services

In the U.S., people applying for welfare often interact with three separate departments: one to take applications, another to process them, and a third to distribute payments.¹ The bureaucratic red tape people must navigate before receiving welfare support can serve as a significant barrier, preventing many from applying at all. Similar obstacles and frictions exist in accessing healthcare across different countries. People who need financial support or health services encounter sludge and might give up before they get the help they need. According to Sendhil Mullainathan, these obstacles contribute to the “bandwidth tax,” which helps keep poor people poor.

Fortunately, governments around the world are increasingly using insights from behavioral science to reduce these psychological barriers in public services.⁹ Research in the field of psychology has expanded our understanding of how various behavioral biases affect the choices people make and, ultimately, policy outcomes. Moving from a traditional view of policymaking that assumes people always behave perfectly rationally with access to the right resources and information. The study of concepts like sludge has prompted public officials to examine systems more closely for frictions that hinder access to public services and distort policy outcomes.

Sludge in Politics

Politics is another area where sludge can be used to influence outcomes. Nudges can be used to increase voter participation, such as by leveraging status quo bias to automatically register individuals who apply for a driver’s license. However, sludges can be used to restrict access to voting, for example, by purging people who haven’t voted recently from voter rolls or placing polling stations far from certain neighborhoods.² Again, this is another area where attention to human psychology and an acknowledgment of behavioral barriers can lead to improvements in accessibility, equality, and fairness in democratic participation.

How Sludge Shapes Consumer Behavior

The concept of sludge has also had a significant influence on the fields of marketing and digital design. When consumers encounter sludge in sales funnels or decision-making workflows, they’re more likely to make suboptimal choices.¹⁰ Reducing sludge—removing unnecessary friction so consumers can follow through on their intentions—can lead to greater satisfaction, increased retention, fewer refunds and returns, and less stress on customer support teams. In other words, understanding sludge and making an effort to remove frictions from consumer decisions is a win-win for individuals and organizations alike. These mutual benefits have encouraged designers to strive for simpler, sludge-free choice architecture that supports consumer satisfaction and autonomy.

Some key areas of controversy around sludge include its potential to manipulate and harm individuals, the overall ethics of choice architecture, and whether or not sludges and nudges actually work as intended once people become aware of them. There are also debates around the potential use of sludge to promote positive behaviors, like encouraging users to pause and think before engaging with fraud.

The Ethics of Choice Architecture

Understanding that sludge can be used just as easily as nudges can have serious implications for the ethical understanding of choice architecture. One criticism of nudges has always been that pushing people to make certain decisions limits their freedom. These criticisms are typically more concerned with the underlying ideologies of nudging, rather than the outcomes. That being said, when outcomes are intentionally designed to maximize profit rather than rationality, we have to wonder whether manipulating choice environments is ever ethically sound.⁴

It is almost as if choice architecture is a superpower, and architects have to decide whether to use their powers for good (in a ‘nudge’ form) or for evil (in a ‘sludge’ form). The issue is that it is often impossible to create choice environments that are neutral and grant people the freedom to make whatever choices they want. 

Do Nudges and Sludges Really Work?

Nudges have come under scrutiny for their ability to actually affect change. In one study, Cass Sunstein found that when people were told that they were being influenced by choice architecture, even when told it was in their best interest, they reacted negatively and acted against the nudge.⁵ This research suggests that the same might be true for sludges—if consumers are informed about the sludges companies use to influence them, they might be able to resist the impact and exercise more freedom. This might be especially true in situations where sludge causes people to lose money, because of loss aversion, a cognitive bias suggesting that we tend to avoid loss more than we attempt to achieve gain. Because of loss aversion, we are likely to be motivated to cancel our subscription, no matter the difficulty, for fear of losing money.

Using Sludges for Good

Sludge is typically discussed with the motive of advancing particular agendas rather than ensuring the good of the people. That being said, perhaps there is room for sludges to operate more similarly to nudges. For example, research suggests that short-term sludges prompting people to read contracts carefully or familiarize themselves with suspicious companies can help prevent them from investing in pyramid schemes.¹¹ 

As Thaler and Sunstein have claimed, it is impossible to avoid choice architecture altogether; options have to be presented in a particular way, and whatever presentation choice is made will influence decision-making processes.⁵ There may be no ‘neutral’ choice environment. Instead of getting rid of nudges and sludges, it might be more important to ensure that people designing them have the right intentions and can understand when to use one versus another to benefit people.

Personality Types and Sludges

Sludge works most effectively by preying on cognitive biases that turn people away from making the best decisions. When we try to perform an action and face friction—like jumping through hoops to cancel a subscription—these biases can frequently appear. For example, present bias describes our tendency to favor immediate payoffs over future payoffs, often leading us to procrastinate rather than endure short-term effort for long-term gain. That said, people tend to procrastinate to varying degrees, which might mean that different kinds of people are impacted by sludge differently.

University of Toronto professors Dilip Soman and Kim Ly conducted research into a broad range of studies to observe how people respond to choice architecture. They theorized that people could be split into three categories based on their responses: motivated enthusiasts, die-hard opponents, and naïve intenders.⁶ They proposed three different ways these groups of people would respond to behavior change, defined as “a change in decision or a change in a series of requirements and actions needed to accomplish a particular outcome.”⁶

Motivated enthusiasts are highly motivated individuals who are likely to engage in particular kinds of behaviors and are less likely to be influenced by tendencies such as procrastination. Die-hard opponents are those who are opposed to the behavior change being asked of them, meaning that if they are aware of nudges or sludge, they are unlikely to be influenced by them. Naïve intenders are individuals who believe in what is being asked of them and plan to do it, but they often fail to follow through. Naïve intenders are the most likely to be impacted by sludge because they tend to delay any behavior that requires overcoming friction, believing they will address it later, but ultimately fail to follow through.⁶

As Soman and Ly’s investigation suggests, more research into sludge is needed to fully understand how it impacts different types of people. Since sludge does not impact everyone in the same way, companies or public services might not know where it is present in their design and how they are impeding behavior. Having greater knowledge about how different people respond to sludge could help mitigate their negative effects, which is why Soman has helped develop a tool that can help organizations find areas where sludges are impacting end-user behavior.⁶

Welfare and Sludge

There are numerous government programs designed to alleviate the burdens faced by low-income populations. Although these programs are embedded within complex and messy politics, they are often put in place to help people. However, since a vast distance exists between policy- and program-makers and the intended recipients, government employees may not always consider how to make access to programs as easy and smooth as possible.

One example of a well-intended program that fell victim to sludge was the Canada Learning Bond. The bond is intended to encourage low-income Canadians to send their children to post-secondary education, as they would receive money that could help cover future expenses. To receive the bond, parents had to apply for a Social Insurance Number (SIN) as soon as their child was born, so that they could receive the $2,000 payment in their SIN account.¹

However, when the policy was first launched, the government found that less than 50% of eligible people were taking advantage of it. It later became known that the reason for the poor take-up rate was that parents had to apply for a SIN within three months of their child’s birth.¹ The life of a new parent is incredibly stressful, and applying for a SIN is not likely to be a top priority. Since these accounts had to be applied for in person, parents could not find the time to do so and forwent the bond as a result. Once again, this is a prime example of the bandwidth tax in action.

After this information came to light, the government attempted to simplify the application process for a SIN by making the forms available online and automatically sending them to families upon their children’s birth. While this effort helps alleviate the obstacles that prevent these new parents from taking the initiative, the required forms are still a burden. We might instead ask why any forms have to be filled out at all.¹ Can sludge be removed altogether, with children automatically receiving a SIN when they are born?

Do I Agree? Cognitive Bias and Terms of Service

In this article, our writer Tiago Rodrigo discusses the familiar tendency to quickly scroll through Terms of Service and click ‘Agree’ without reading the fine print. This occurs because people tend to avoid an overload of information. Companies know about and purposefully leverage our avoidance of information – through a sludge – to make consumers agree to things they  probably wouldn’t otherwise (such as waiving the right to data privacy).

Jesse Itzkowitz: Nudging Against Polarization

In this episode of The Decision Corner podcast, we sit down with Jesse Itzkowitz, a behavioral scientist at a leading global market research and consulting firm. Itzkowitz discusses the differences between nudges and sludges and dispels the idea that nudges are all about tricking people. He suggests that companies who use sludges are the ones that we should worry about, although he thinks people will soon see through their manipulations.

1. Civil Service College Singapore. (2019, November). Nudge and sludge: A conversation with Dilip Soman. ETHOS Digital Issue 05. Singapore Government Agency Website. https://www.csc.gov.sg/articles/nudge-and-sludge-a-conversation-with-dilip-somanThaler, R. H. (2018). Nudge, not sludge. Science, 361(6401), 431-431. https://doi.org/10.1126/science.aau9241
2. Thaler, R. H., & Sunstein, C. R. (2009). Nudge: Improving decisions about health, wealth, and happiness. Penguin.
3. O’Brien, H. (2019, May 22). Cass Sunstein and the rise and fall of nudge theory. NewStatesman. https://www.newstatesman.com/politics/economy/2019/05/cass-sunstein-and-rise-and-fall-nudge-theory
4. Kane, P. (2019, November 16). Playing politics: exposing the flaws of nudge thinking. New Scientist. https://www.newscientist.com/article/mg23231002-200-playing-politics-exposing-the-flaws-of-nudge-thinking/
5. Soman, D., Cowen, D., Kannan, N., & Feng, B. (2019). Seeing sludge: Towards a dashboard to help organizations recognize impedance to end-user decisions and action. SSRN Electronic Journal. https://doi.org/10.2139/ssrn.3460734
6. Madle, I. (2024, October 24). Nudges, Sludges And Dark Patterns: What Are They And Is There a Middle Ground? LAB. https://lab.co.uk/news-and-insights/nudges-sludges-and-dark-patterns-what-are-they-and-is-there-a-middle-ground 
7. Luo, Y., Li, A., Soman, D., & Zhao, J. (2023). A meta-analytic cognitive framework of nudge and sludge. Royal Society Open Science, 10(11), 230053. https://doi.org/10.1098/rsos.230053 
8. Organisation for Economic Co-operation and Development. (2024). Fixing frictions: ‘Sludge audits’ around the world: How governments are using behavioural science to reduce psychological burdens in public services (OECD Public Governance Policy Papers No. 48). OECD Publishing. https://doi.org/10.1787/5e9bb35c-en 
9. Kohler, T. (2024, July 26). Clean the sludge from decision-making workflows. Nielsen Norman Group. https://www.nngroup.com/articles/sludge-decisions/ 
10. Ryzhkova, M. V., & Kashapova, E. R. (2024). Sludges against Ponzi schemes. Journal of New Economy, 25(1), 50-68. https://doi.org/10.29141/2658-5081-2024-25-1-3