https://slatestarcodex.com/2020/04/17/depression-the-olfactory-perspective/

Depressed people have worse sense of smell, and people with worse sense of smell are more likely to get depressed. Kohli 2016 tries to figure out what’s going on.

They review six studies testing how well depressed people can smell things. Most use something called “The Sniffin’ Sticks Test” (really!) where people are asked to say which of two sticks has an odor