When you’re born, you don’t have a shadow, but it quickly builds up as you grow and start to identify with things (like name, gender, and appearance) as this is me, and that is the rest of the world.
As children, we learn that we need to act in a certain way to be accepted. We see the world as good and evil and must suppress the bad. For example, if you grew up in a home that valued humbleness, you may find it hard to voice your opinions.
Gender roles influence the shadow. For example, men who struggle with expressing emotions appropriately can become aggressive because they were told to be tough. A woman who was taught to be reserved can find it challenging to be assertive at work.
Basically, the shadow is formed by judgment in general. We often label traits, people, and things as right and wrong.
Luckily, as adults, we have the option to embrace our dark aspects and become whole again.