User experience has many definitions. All are different but have one thing in common.
All definitions of user experience refer to the human element of the user experience. For example, here’s the definition provided by Wikipedia:
User Experience (UX) refers to a person’s feelings and attitudes toward using a particular product, system, or service.

According to Nielsen Norman Group:
User experience encompasses all aspects of the end-user’s interaction with the company, its services, and its products.

For this reason, UI UX design firms need to understand who uses it in order to design great user experiences. This is where the knowledge of psychology comes into play in UX design.

1. The Principle of Least Effort

The principle of least effort is very simple and self-explanatory.
People are looking for ways to get things done with as little effort as possible. This principle applies not only to humans but also to animals and machines. The simplest algorithms are always the most attractive. For example, if he has two routes to get from one place to another, you choose the shorter one, both of which are safe and convenient.

2. The principle of perpetual

The principle of perpetual habit states that people greatly rely on their memory and habit when doing this or that task. For instance, if you are a frequent flyer, you know that you go to check-in first and leave your luggage, then you go through a security check, then through the customs and duty-free, and then you approach the boarding gate. If for some reason, you arrived at the airport, and the whole process started not with check-in but with customs, you would grow confused, perplexed, and angry. The same applies to website and mobile app users — they want things to be where they should be.

3. The Principle of Socialization

The rule still applies in the twenty-first century, in the era of the Internet. It has translated into user psychology in UX. People want to be social, and they greatly rely on technology in order to achieve it. More than that, they use technology in order to feel accepted into society. Here is what you may make of it.

4. The Principle of Emotional Contagion

Have you ever noticed that emotions are contagious? If a person you like is laughing out loud, you will be smiling. If somebody in a very well-made movie is crying, you will feel sad. This is the psychology of UX design and of human beings — we subconsciously take over the emotions and behaviors of other people, especially those we like.

5. The Principle of Identity

All people need a sense of identity, a feeling of belonging, and an understanding of who they are. This is why applications and websites with wallpapers are so popular — people customize their devices to feel unique. This is why people collect awards and medals — to feel appreciated and recognized. This is why people make friends — to feel associated with somebody.

A similar thing happens with brands — people like associating themselves with brands because it gives them a sense of identity. “I prefer products from Google, so only Google Pixel and Google Home for me”. “I prefer coffee from Starbucks, so no Dunkin Donuts”. “I wear only Nikes”.

6. The Principle of Beauty

Numerous psychological studies prove: beautiful people and beautiful things are always more loved and popular than unattractive ones. People choose books by their covers. Given the opportunity to choose, people choose beautiful places to travel to, beautiful houses to live in, and nice clothes to wear. Few people can stay away from pursuing beauty. The concept of “beautiful” is often associated with the words “expensive”, “successful”, and “high-quality”.

7. The Magical Number Seven, Plus or Minus Two

The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information” is a psychological article published by George A. Miller — the cognitive psychologist at Harvard University. According to this paper, a person can hold from five to nine things in their short-term memory. The “Seven Plus or Minus Two” rule is also known as Miller’s rule.

8. The Psychology of Mistakes

All people make mistakes. Mistakes spark a wide range of emotions in people’s minds, chiefly negative ones — from sadness and anxiety to procrastination and apathy. Regardless of its severity, a mistake is still a mistake, and it is annoying. What are the mistakes in terms of user experience design psychology? These are counter-intuitive, unclear, or misleading scenarios, which prevent people from achieving their goals with the software. For instance, there is an online store of eco-products I truly love, but every time I make an order, I face a certain difficulty. Once I proceed to online payment, I have to include the order number in the payment description, but this number is nowhere to be found. It is neither on the payment page nor on the check-out page — I still have no idea where to find it. As a result, I am struggling to make a purchase and looking for a different store.

9. Focus, Attention, and Concentration

Normally, the attention span of an adult is from ten to twenty minutes. After this short time, people tend to lose focus and concentration and get distracted by their own thoughts or the things around them. The job of a UI/UX designer is to grab users’ attention and retain it for as long as needed. To do so, you have to reduce distractions and break the monotonous routine.

In conclusion,

Once again, UI/UX design is all about human psychology. You design not a product — you design interaction with the user. This is why you have to know your users, their human nature, their inclinations, weaknesses, strong points, and fears.
Remember the nine UI UX psychology principles described above, undertake careful user testing, and keep in mind that you are not a user.