GWEN IFILL: In the eight years since Apple unveiled its first smartphone, an entire generation of young users has spring up who never knew a world without a device in hand.

That's the starting point for the latest addition to the "NewsHour" Bookshelf, "American Girls: Social Media and the Secret Lives of Teenagers," by journalist and author Nancy Jo Sales.

She recently sat down with Hari Sreenivasan.

HARI SREENIVASAN: Joining me now is author Nancy Jo Sales.

So, what are the challenges that American girls are facing today that are different from the ones they have always faced?

NANCY JO SALES, Author, "American Girls": Girls use social media lot. They're on their phones a lot. And I think most parents are aware of that.

So, really, the question is, what is happening there, and how are these kind of — some regrettable trends that we have seen happening for some time, such as the sexualization of girls, bullying, how are these moving on to phones and on to screens? And what is this environment doing to the life of girls and boys?

HARI SREENIVASAN: You paint a picture almost of these young girls at this kind of intersection of the existing angst that a teenage girl has, combined with this technological leap that we're all taking together, combined with pornography.

NANCY JO SALES: Yes, I think the kind of elephant, maybe dinosaur in the room right now is pornography. Pornography has been available online for some time.

But I don't think we have really begun to have a conversation about what that is doing to kids. And we know that they're watching it. They're teenagers. They're going through puberty. They're curious. They're watching pornography.

But this tends to be more and more violent pornography that is also, one could I think reasonably say, very degrading to women. And this is where a lot of teenagers now find their sex-ed. And porn is also — the porn aesthetic is also really engulfing social media. And social media is just full of really explicit sexual content.

And whether or not your child wants to see it or intends to see it, they are likely to see it.

HARI SREENIVASAN: What happens to all of us when there's, on the one hand, kind of hyper-sexualization, hyper-masculinization on the boys' side and girls' side? And this — if this is the new sex-ed, they have really reduced the female body down into this thing they can swipe across.

NANCY JO SALES: Yes.

I mean, I think it's really challenging for girls especially to have an expectation to produce images of themselves in order to get validation, the validation that everybody seems to want of likes and friends and followers.

It's very challenging to them, I think, to have an expectation to produce images of themselves which are sexualized in which they look — quote, unquote — "hot," and also to have to navigate through all these emotions that come through comments that might be on these pictures that are about how just hot they are or how not hot they are.

HARI SREENIVASAN: You capture a lot of different conversations, and there seems to be this conundrum, where you ask them, well, if this is all the stress that's associated with making sure you're checking up on your likes and your pictures and who liked you and what did they say, why are you on it? And they seem to say, well, I can't get off it.

I don't get that.

NANCY JO SALES: Yes, I talked to more than 200 girls in 10 states over 30 months.

And one of the things that struck me the most was how self-aware they are, how articulate they were, how analytical they were about this very thing they were engaged in so much of the time. And they're quite aware of the ways in which it's really toxic. And yet they can't leave it, because this is — this is the swamp they're swimming in. This is the world they're enmeshed in, where you have to be on it, or you're not part of the conversation, you're not part of what's happening.

It's — a girl in New Jersey described it as a second world that we live in. You know, there's the real world and then there's the second world that we're simultaneously in kind of all the time.

HARI SREENIVASAN: And this is also affecting just their relationships with one another, even in a non-sexualized way: Who's my best friend and what's my best friend allowed to say or do to me? Or, you know, it's just different than the conversations that I had growing up and deciding who to hang out with.

NANCY JO SALES: It's different to communicate from behind a screen, and this is true for adults as well. I mean, we know this from multiple, multiple studies that when you communicate from behind a screen, you're more likely to say things that you wouldn't face to face. You're more likely to become aggressive and even, some studies say, unethical.

Now, think about that. Like, we have children now growing up on phones, learning to communicate with each other and be people from behind a screen in an environment in which it's OK to do stuff that you wouldn't do if you were with the person.

I think that's really something that I think parents really need to think about when they think about how they're kids are growing up now.

HARI SREENIVASAN: You have a 15-year-old daughter.

How did these conversations go? She obviously was living with you at the time you were working on all this research. You go on all these trips. You talk to all these girls. You come back, and then what?

NANCY JO SALES: We talk about it all the time, like, on a daily basis, because to talk to her about what is going on in her life is to talk to her about what is going on, on social media, because that's part of what teenage life is about now.

So, I just try and have it be an ongoing conversation as if I were asking her about what happened in school today, you know, and just kind of find out what's going on with your friends. And sometimes really dramatic things come up, and I think a lot of parents have seen that.

HARI SREENIVASAN: What surprised me — and I'm wondering what surprised you during the reporting of this? But there were — you had stories about women — or young girls that were getting plastic surgery so they could look better in selfies.

But when you went across all these different young women, what shocked you?

NANCY JO SALES: The way that comments on pictures were so very sexualized, like, almost like catcalling, which if this happened in the street, we would say, well, that's really insulting.

But on social media, the culture of social media is such that you're supposed to say, oh, thank you, an emoji with heart eyes, like, you said I'm hot. And I'm just really — I'm really concerned as a parent and as a woman about what this is doing to a generation of girls.

For example, the book, it opens with a girl being asked to send nudes. A boy that she doesn't know very well says to her, really demands of her really, send nudes. She's not — he's not her boyfriend. And so, you know, all kinds of thoughts start to go through her head. Like, wow, should I be flattered? Should I be outraged? Should I be insulted? Is this — should I do it? Should I not do it? Well, if I did it, what would it look like?

And these are new kinds of things for anybody to have to think about, especially 13-year-old girls.

HARI SREENIVASAN: All right, the book is called "American Girls: Social Media and the Secret Lives of Teenagers."

Author Nancy Jo Sales, thanks so much for joining us.

NANCY JO SALES: Thank you.