Back when I raised hens for (little to no) profit, and I had electricity to the coop, I kept a cheap radio out there dialed to AM Talk Radio.
They heard human voices all day, and it also kept predators at bay. The first raccoon attack I had was when I had to have the power disconnected as the guys that planted the crops all around me kept snagging that wire on their ever increasingly LARGE machinery and it was worth it not to have anyone ‘fried’ on my property.
I don’t know if they’ve done any ‘studies’ but it seemed to work for me. I usually had 50 hens at a time. They were calm and friendly. I don’t remember any chicken attacks, other than when ‘Lacy’ (Barred Rock) thought my earring was a bug as I was holding her, so she pecked me. ;)
When I raised some extra hens as meat birds, there were some that were mean enough to draw blood. They were raised the same way as the friendly ones.
With my friendly birds, I had one try to remove a large freckle from my leg. And I learned not to smile at them after eating in case there was something in my teeth, that was a painful lesson!