I would expect anxious/insecure parents to use placating behaviours (like device use) themselves, and I would expect their children to be anxious/insecure too.
So I would expect the study to find that the children of phone-overusers were more likely to be anxious/insecure.
Still, I would also expect that less phone use (subbed with more attention to kid) would help the kid with this.
I always wonder how much of the West's wealth(- holding capabilities) come from centralising and rationalising our superstitions through central authorities like the church.
You might play a G# note in the context of an E chord (where it's the third), and then you might play it in the context of a C# (where it's the fifth).
These are discernably different pitches, but the same "note", in the same key, in the same song!
While the other person replying is not technically wrong about why these things are grouped, it is kind of offensive to sufferers of Type 1.
In one case, a 3yo starts randomly getting sick one day, worse by the day, and will be dead if they don't get a diagnosis soon. From that day forth, their parents need to manage EVERY single bite of food they have, stab them with needles multiple times a day no matter what, and inject them with a insulin - where, if you miscalculate, will cause a seizure within an ~hour and death within a few hours. From a single typo.
Nothing will cure them, their life will be much shorter, filled with work and pain and expense with absolutely no relief, and nothing could've avoided it.
Now compare to Type 2, where you basically cannot get it if you maintain a reasonable diet and a reasonable weight.
Once you start showing symptoms, if you listen to your doctor and reform your diet (particularly with the 5% shock weight loss approach), you will almost definitely avoid it.
You will avoid it for the rest of your life just by eating well, which has the added benefit of extending your lifespan and healthspan and saving you money.
These things have nothing in common, for the sufferer or their family.
All the listed countries have low fertility rates, increasing screentime rates, etc.
I suspect if you cornered a parent of a 2yo in any of those countries, they would not say it is meaningfully more social and child-friendly TODAY that the USA is, or Australia (for which I can speak) is.
Last bit is not quite right: a lot of people want to be inside. That contributes strongly to the feedback loop you rightly identify.
(WHY they want to stay inside is another matter, but I suspect a large part is the stereotypical answer: unending seas of digital content highly optimised to hack the consumer's brain.)
But it's not new to me, I've seen hundreds of comments just like it.
It just stood out to me because it doesn't appeal to any facts, or anything you would expect in this commentariat - just a bunch of pretty low resolution, low-brow opinions.
I try not to get angry about "the bottom 20% eats 90% of the cheeseburgers" stats until I have a reasonable benchmark, or I have a really deep understanding of the "why" and "how" of a given disparity.
Two children raised in the same house with the same parents never have equal outcomes - I don't expect it anywhere else either (as a default assumption).
You can support the right to be a Nazi while strongly criticising it.
People can say that they really think eBay is doing the wrong thing, and maybe even boycott them, without necessarily saying they should be forced by law to stock children's books that became out of fashion thirty seconds ago.
So I would expect the study to find that the children of phone-overusers were more likely to be anxious/insecure.
Still, I would also expect that less phone use (subbed with more attention to kid) would help the kid with this.