Douglas Medin and Andrew Ortony wrote a chapter in the 1989 book Similarity and Analogical Reasoning in which they called this effect of labels psychological essentialism. That is, when we describe something with a label, we are saying that the object is part of a particular category. For example, if we say that Susan has depression, we are saying that she is part of the category of people who are depressed.

A consequence of placing someone in a category is that we now think that there is some set of characteristics that are shared by everyone in that category. That is, we believe that the category has an essence. It is not important whether that particular category really has an essence. If we place an object in a category, people treat the object as if it has the essence of that category. Psychologically, then, a person suffering from the symptoms of depression is different than a person who is depressed, because we believe that the person who is depressed has some essence that the person who is just suffering from the symptoms does not.

Susan Gelman and Gail Heyman documented this effect in a 1999 study in the journal Psychological Science with children. They found that when you described someone as a carrot-eater, children thought that eating carrots was a deep part of who that person was. When you describe someone as a person who eats carrots, then they thought that eating carrots was less a crucial part of who they are.

I have always thought that you can use this effect as an insult as well. For example, someone might ask, "Is John a painter?" Dismissively, you could say, "Well, he paints..."

Psychological essentialism may be important for behavior change.

We use labels to describe things all the time. Often, we use these labels just to describe something in order to communicate it to someone else. The work on psychological essentialism suggests that after we use a label, we will treat the characteristics we label as less easy to change than characteristics we have not labeled. It seems easier to change the symptoms of depression than to change someone who is depressed. If we say that someone is aggressive, then it seems harder to expect them to change their behavior than if we just think that they have acted aggressively.