What's the External Validity of a Study?

There's a concept in science that external validity is, if i do a study, it's like a controlled environment. That means that the controlled environment mirrors the real world. And this is why we had so many papers about how awesome s s our eyes are, and then decades later,. it's like, by the way, 70 % of is placeba, lik. How did that happen? I want you all to really think about this, right? The reason the effect sizes better is because we're removing flacibo (the placebo effect) Then the external validity of the study goes down. This is kind of what i see in terms of like, irvedan stuff too,