Hick's Law predicts that the time and the effort it takes to make a decision, increases with the number of options. The more choices, the more time users take to make their decisions.

People tend to search for, interpret, prefer, and recall information in a way that reinforces their personal beliefs or hypotheses.

Subtle visual or verbal suggestions help users recall specific information, influencing how they respond. Priming works by activating an association or representation in users short-term memory just before another stimulus or task is introduced.

Cognitive load is the total amount of mental effort that is required to complete a task. You can think of it as the processing power needed by the user to interact with a product. If the information that needs to be processed exceeds the user’s ability to handle it, the cognitive load is too high.

People tend to make decisions unconsciously. Small cues or context changes can encourage users to make a certain decision without forcing them. This is typically done through priming, default option, salience and perceived variety.

The initial information that users get affects subsequent judgments. Anchoring often works even when the nature of the anchor doesn't have any relation with the decision at hand. It's useful to increase perceived value.

An interface is easier to use when complex features are gradually revealed later. During the onboarding, show only the core features of your product, and as users get familiar, unveil new options. It keeps the interface simple for new users and progressively brings power to advanced users.

Fitts's law is a predictive model which states that the time to acquire a target is a function of the distance to and size of the target. This is mainly used to model the act of pointing, either physically (e.g., with a hand) or virtually (e.g., with a computer mouse).

Users have learned to ignore content that resembles ads, is close to ads, or appears in locations traditionally dedicated to ads.

When we are choosing between two alternatives, the addition of a third, less attractive option (the decoy) can influence our perception of the original two choices. Decoys are “asymmetrically dominated”: they are completely inferior to one option (the target) but only partially inferior to the other (the competitor). For this reason, the decoy effect is sometimes called the “asymmetric dominance effect.”

The framing effect happens when your decision is influenced more by how the information is presented (or worded) than by the information itself. It's partly due to the fact that people evaluate their losses and acquire insight in an asymmetric fashion (see Loss Aversion and Prospect Theory, by Daniel Kahneman and Amos Tversky).

Social proof is a convenient shortcut that users take to determine how to behave. When they are unsure or when the situation is ambiguous, they are most likely to look and accept the actions of others as correct. The greater the number of people, the more appropriate the action seems.

While scarcity is typically invoked to encourage purchasing behaviors, it can also be used to increase quality by encouraging people to be more judicious with the actions they take. It can come in different forms: Time-limited, Quantity limited, Access-limited.

The curiosity gap is the space between what users know and what they want or need to know. Gaps cause pain, and to take it away, users need to fill the knowledge gap.

A mental model is an explanation of someone's thought process about how something works in the real world. It is a representation of the surrounding world, which might be accurate or not. What users believe they know about your product changes how they use it.

Users have an innate desire for things they're already familiar with. And the more we experience something, the more likely we are to like it. So, try to use common patterns when creating new experiences.

Skeuomorphism is where an interface object mimics its real world counterpart to facilitate transition to new technology. The digital object imitates reality by how it appears or how the user can interact with it. Skeuomorphism partly relies Familiarity Bias and on a usability concept called "Affordance" (the actions which users consider possible while interacting with an object).

Reciprocity is a social norm of responding to a positive action with another positive action, rewarding kind actions. In the context of digital product experiences, users are more likely to engage with your product if you first provide them value. They'll be more likely to trust you and reciprocate. That's even more important when you're about to ask for something big from your users (sign up, paywall, etc).

In the operant conditioning method, a variable-ratio schedule is a schedule of reinforcement where a response is reinforced after an unpredictable number of responses. This unexpected schedule creates a steady, high rate of responding.

The closer users are to reaching a milestone (e.g., completing a task, reaching a goal, etc), the faster they work towards reaching it. Interestingly, even artificial or estimated progress indicators can help to motivate users. That's why it's crucial that your experience provides a clear indication of progress to provide this feedback to your users.

Occam’s razor is a mental model which states that “it is futile to do with more what can be done with fewer”—in other words, the simplest explanation is most likely the right one. Be careful, simple does not mean ignoring important facts in an attempt to reduce the complexity. It requires open mindedness to seek a better solution with less complexity and less assumptions, exploring a better way of doing things.

When companies demonstrate genuine caring and social responsibility, they tend to be rewarded with increased brand loyalty, and greater profits.

The Hawthorne effect is a theory that describes a type of reactivity in which individuals modify an aspect of their behavior in response to their awareness of being observed.

Making users wait for something they requested while showing them how it is being prepared creates the appearance of effort. Customers are usually more likely to appreciate the results of that effort. This is also called the "KAYAK Effect" (based on the travel booking site that used that tactic).

Unless the incentive to change is compelling, people are more likely to stick to the default situation presented to them. This is also called the Status quo bias. It can be a powerful actor when trying to change behaviors.

People invest time, money, information, or effort into a product in anticipation of future benefits. It makes them more likely to return because of the increase in perceived value. When executed properly, user investments load the next trigger to use your product.

We hate losing or letting go of what we have (even if more could be had). Prospect theory says that a loss hurts more than an equal gain feels good. In other words, losing $1,000 will "hurt" more than the joy of gaining $1,000. Loss aversion can also lead to sunk cost fallacy.

When users are asked to do something, their brain instinctually perceive it as a threat. The smaller the initial ask, the smaller the fight or flight response and the more likely they are to agree to gradually bigger requests. Especially since the brain likes to be consistent with its previous actions. It's part of the reason why multi-step forms can perform up to 271% better than a big single-step form.