Sorry I don’t have any great sources on this. It’s rather speculation because how could you research this scientifically? Even if you could, an experiment like that would actually be unethical! And who would fund this, there is no way to talk in mainstream about advertising without running against massive financial interests. There are some search results but most of those articles look like mental garbage.

My guess is that because we’re constantly being told what to consume our minds work quite differently from what they would without advertising.

Our minds constantly have to resist intrusive advertising and psychological manipulation which means we constantly have to switch between and adversarial mindset and whatever content we were watching / reading. Or we become obedient and just “let the advertising wash through us”. And advertising constantly has to find new ways to activate our emotions.

Just as massive is the effect on content produced, there is a “natural selection” that any content that helps sell advertisement is more successful on the market. It’s not just that you can’t piss off your advertiser but that generally you want the consumer to be in a certain mood - or that content producers who do this naturally are more successful and grow.

Then there are privacy concerns which reduce humans to machines and creates a powerful system that can and is abused for political control (public relations).

How can any of that not have massive societal impacts, since it’s being done on a massive scale and is near ubiquitous? How can anyone assume these effects are not incredibly bad?

You could have a country banning advertising that has a kind of “content tax” that is funded publicly and administered independent from the government through separate elections. And that has strict mandates and distributes the money to news papers, websites, movies and video creators dependent on views - similar to music rights agencies. But none of this is even talked about. We’ve completely lost the ability to even think seriously about how to improve our society.