I don't have studies, but perhaps I can explain why people believe it and why I think it's plausible.
I am betting that the reason you doubt it is that it doesn't work for you, because you don't have the right associations with business attire. The effect is psychological, so it only works if you have the right prejudices. Personally, I associate suits and such with superficiality, paralysis, and unproductive rule-following. I associate relaxation with high performance and dishevelledness with focus. That reflects my background, though: I grew up in an academic household, idolizing rumpled intellectuals like Einstein and Turing. Apparently, it's more common to associate business attire with hard work, focus, and diligence and to associate relaxation and informality with goofing off. It might be strange to others, but wearing shorts automatically puts a lot of people in a mindset to slack off, and a tie does the opposite. I know one guy who puts on a tie every day for the same reason a programmer might dim the lights and put on headphones: to get focused and keep his head where it belongs. Taking the tie off is like the buzzer at the end of a game. He pushes hard and maintains focus until the tie comes off, and then poof, his mind is gone, moved on to family, dinner, and whatever games will be on TV that night. Even if I don't share that mentality, it sounds like it's useful to him.
Also, keep in mind that while some highly motivated people resent business wear because we dislike the conflation of social presentation and performance, many people hate business wear because they do conflate social presentation and performance, and business attire makes them feel obligated to focus and perform, which they do not enjoy doing. Even if we make common cause with such slackers, we should keep the difference in mind ;-)
That's funny. You know, thinking about it that way, I thought about what mode of thinking I get into when I put on ties. And to be honest, I can only think of ties for fancy parties these days. A good tie can make me feel like a million bucks paired with the right shirt and suit, whether I'm dancing to at someone's wedding, volunteering at some black tie charity event, or applauding speeches at some annual company bash. So when I put on a tie, I guess the theme song that would pop up is not Eye of the Tiger, but I Got a Feeling. :)
I am betting that the reason you doubt it is that it doesn't work for you, because you don't have the right associations with business attire. The effect is psychological, so it only works if you have the right prejudices. Personally, I associate suits and such with superficiality, paralysis, and unproductive rule-following. I associate relaxation with high performance and dishevelledness with focus. That reflects my background, though: I grew up in an academic household, idolizing rumpled intellectuals like Einstein and Turing. Apparently, it's more common to associate business attire with hard work, focus, and diligence and to associate relaxation and informality with goofing off. It might be strange to others, but wearing shorts automatically puts a lot of people in a mindset to slack off, and a tie does the opposite. I know one guy who puts on a tie every day for the same reason a programmer might dim the lights and put on headphones: to get focused and keep his head where it belongs. Taking the tie off is like the buzzer at the end of a game. He pushes hard and maintains focus until the tie comes off, and then poof, his mind is gone, moved on to family, dinner, and whatever games will be on TV that night. Even if I don't share that mentality, it sounds like it's useful to him.
Also, keep in mind that while some highly motivated people resent business wear because we dislike the conflation of social presentation and performance, many people hate business wear because they do conflate social presentation and performance, and business attire makes them feel obligated to focus and perform, which they do not enjoy doing. Even if we make common cause with such slackers, we should keep the difference in mind ;-)