Marx shines a light on class differences, understanding them as more important than race or gender. Where you are on the class ladder—that is, how much money you control—is the key determinant of your outlook on life. Class difference distorts human relationships, according to Marx.

Sally Rooney makes it clear that Marianne and Connell come from starkly different class backgrounds. They grow up in the same Irish town, but Marianne is the rich girl who lives in the big house, and we later find that her family also owns a spacious vacation home in Italy. Connell's mother cleans for Marianne's mother and, as the novel points out, is underpaid. Connell and his mother share a small townhouse.

A Marxist analysis would praise Rooney for noting the class differences between the two protagonists. Marxism says that the ruling class often tries to obscure or mystify (put a mist or fog around) class differences by pretending they don't exist or aren't very important. In Normal People, however, they drive a definite wedge between Marianne and Connell.

For example, Connell has to think about money all the time while at Trinity for university, whereas for Marianne, the money just appears as naturally as rain. One summer, the two break up when Connell loses his job in Dublin and has to go home to earn money. He is ashamed to ask Marianne if he can stay with her, because he is acutely conscious of not wanting to depend on her—but Marianne, to whom money is a given, misinterprets his leaving Dublin as Connell personally rejecting her.

This is a classic example of Marx's theory that money interferes with and distorts human relationships. As you read the novel, you can find many more references to how income disparity impacts the two main characters.