Psychology
This is a science that studies how our minds work and how we behave. It investigates how our senses work and how attention and motivation affect our performance and behavior. It studies how we think and how we solve problems.
Psychology also researches how we develop from infancy to adulthood and how our emotions play a role in what we do and in the decisions we make. The ergonomists use psychological data to make sure what is designed does not overwhelm our minds and behavior.
For example, they design the backpack so that it is easy for a young child to understand how it is used. And, it is very important to make it look "cool!"