Could it be psychological?

The fact that caffeine doesn't seem to play a role, it looks like the act/ritual of drinking coffee and probably expecting some effect from it has a psychological impact?

You could always run a double-blind experiment on yourself and find some objective ways to report nervousness (purely a state of mind or is it translated into physical factors, like heart rate or blood pressure?).

Why rush to put the effect on this person? There are over 1,000 compounds in coffee.

https://en.wikipedia.org/wiki/List_of_chemical_compounds_in_....

> Could it be psychological?

I think so. If you strongly associate the taste and smell of coffee with its stimulating effects, your body might react similarly to decaf coffee due to conditioned responses, even if the caffeine content is much lower

Interesting since I feel it should be the reverse, psychologically.

Personally, my sole reason for drinking decaf is to avoid the caffeine's stimulant effects...so, I'm mentally expecting zero effects.

Decaf coffee has less caffeine than regular coffee but it still contains a small amount of caffeine. This residual caffeine might be enough for you.

Since he wrote that Coke doesn't create this effect, I guess caffeine can be ruled out?

It could be the drink's affect on your stomach. I believe there's lots of evidence that anxiety can be caused by what's going on in the digestive track as much as what's happening in the brain.

Coffee is laden with beta-carbolines, some of which are known to be anxiogenic.

I think two reasons:

Caffeine is not the only stimulant in coffee. It’s just the most well known.

Some decaf coffee is more like “reduced caffeine”, not “no caffeine”.