social anxiety may be mostly in your gut. and can also be transmissible.

A bizarre experiment shows that your gut biome plays a critical role in anxiety disorders, and that you can (indirectly) pass it on.

Look, sometimes you come across a study so strange but so interesting that you want to dive into it, but don’t know how to set the scene. Since this is one of these studies, and this is my newsletter, screw it, I’m just going to blurt it our awkwardly. A group of scientists from Ireland found people with social anxiety, took samples of the bacteria in their gut, and transplanted it into mice who… well, also developed social anxiety in a matter of two weeks.

And yeah, that is pretty much the study. It really is that straightforward. Take six more or less socially adjusted adults and six with clinically diagnosed social anxiety, extract their gut biomes — i.e. have them poop in a cup, dehydrate it, and put it in a pill, just in case you’re wondering that actually means — then spread it between 72 lab mice you fed huge doses of antibiotics to prep their gut flora for the transplant, and monitor all their social interactions as they adjust.

Mice with the control gut flora behaved exactly how you would expect mice to behave around each other. The mice with gut flora from the socially anxious? They did their best mouse versions of calling their friends to say “I know we were supposed to do a thing tonight, but wouldn’t you know it, my cousin’s wife’s hamster-in-law needs me to help wash his hair! Maybe another time?” Meanwhile, all their non-social behaviors were the same as prior to the transplants, and like any of their social counterparts. So, basically, the scientists made introvert rodents with socially anxious poop.

Okay, what exactly does this all mean? Well, this very simple study comes with a lot of very interesting and exciting implications. First and foremost, it’s consistent with work dating back decades, which found that the 100 trillion microorganisms in our guts and all the other stuff with which we evolved play a significant role in our mental function. Secondly, we’ve actually compared gut biomes of introverts and extroverts and found appreciable differences between them, so we’ve effectively confirmed this in humans already and this study proves that we saw causation, not just correlation.

Even more importantly, note what the researchers did before the biome transplant. To make sure the new gut flora took hold, they effectively nuked the rats’ guts with huge doses of antibiotics such as ampicillin, vancomycin, and neomycin. If gut biomes are so important to mental health and social behaviors, this experiment clearly shows that using a lot of antibiotics can change our behavior and personality, especially early on in life. (Not a fun fact, childhood antibiotic overuse is also associated with obesity and hypertension by puberty, although the details are still being studied.)

So, while antibiotics are important and we need to prevent as much bacteria around us from evolving resistance to them because we need to do surgeries, treat diseases, and not get sepsis from bad cuts and then die of it — which is why we’re building AI models to help us find ways to invent new ones hopefully faster than resistance can evolve — they are powerful compounds and there are consequences of using them like they’re top shelf booze at 90% off for the night and it’s last call at the pub.

As this and similar studies show, we need to be really careful with our gut health for the sake of both our bodies and our minds, and we need to keep studying the details of our microbiomes so we can harness their power to resolve conditions we currently treat with a shrug, a few chemical band aids, and a wish for the best of luck varying from a half-hearted perfunctory pleasantry, to genuine well-wishing, depending on the doctor and their workload that day.

See: Ritz, N. L., Brocka, M., Butler, M. et al. (2023) Social anxiety disorder-associated gut microbiota increases social fear, PNAS 121 (1), DOI: 10.1073/pnas.2308706120

Johnson, K. (2020) Gut microbiome composition and diversity are related to human personality traits, Human Microbiome Journal, DOI: 10.1016/j.humic.2019.100069

Chu, Coco, et al., (2019) The microbiota regulate neuronal function and fear extinction learning, Nature 574, 543–548, DOI: 10.1038/s41586-019-1644-y