Question: If someone is very sad, but you don't know that, and you root through that person's poop, will you be able to tell that they're sad? And could you tell what mood a person's in by examining they're poop only?

Ciarán O'Brien answered on 10 Nov 2014:

The human mind is way too complicated to tell what a person is feeling just by looking at their poop, unfortunately. We can’t tell if someone is happy or sad from their poop (Although if you see that they’ve been eating really nice foods like ice cream and chocolate, chances are good they’ve been enjoying themselves!)

What we can do right now is calculate the chances that someone with a certain number and type of bacteria in their poop is at risk of feeling very sad. There are some patterns being discovered in people suffering from depression, they tend to have larger numbers of some bacteria in their guts and lower numbers of other bacteria. But they’re only patterns, and we haven’t fully figured out what they mean yet.

The work I’m doing right now is attempting to find out if there’s a definite link between depression and the bacteria that live in your gut. We think there is, because we know that certain bacteria can or reduce depression in mice, but we can’t say for sure yet with people because our body and brain chemistry is quite different from mice, and we have different bacteria growing in us. The fact that it works on mice means that it should work on people, too, but we have to do a lot more testing to see which bacteria are involved.