Luckily they can't send different thumbnails to different viewers. Yet.
Veritasium actually did an interesting episode recently about how the major YouTubers optimize their titles and thumbnails using real time data now to figure out what works to go more viral. And apparently there's a convergence on "shocked" face.
LinusTechTips had mentioned that he did an A/B test with "no YouTube face" and "with YouTube face" finding it had a significant impact on the number of viewers.
I suppose we're hard wired to look for shocked expressions.
I was having a conversation about this with a friend last night. I saw a video on plastic CPUs on his smart TV's recommendations, and searched the topic instead of clicking on it because I didn't want to encourage what I called 'Reddit face'.
Veritasium actually did an interesting episode recently about how the major YouTubers optimize their titles and thumbnails using real time data now to figure out what works to go more viral. And apparently there's a convergence on "shocked" face.
https://www.youtube.com/watch?v=S2xHZPH5Sng