They rush up to him sometimes after a poetry reading, wanting to talk — not just because Billy Collins is a beloved former U.S. poet laureate, but because he is a poet who began his autobiographical poem “Only Child” with the simple declaration: “I never wished for a sibling, boy or girl.”

“I come across a lot of people who say, ‘Oh, I was an only child,’ or, ‘I have an only child,’ and they’re concerned about this,” Collins said. “There are certain anxieties that parents have about the only child being deprived of social abilities.”

Even the term feels tinged with melancholy, some undercurrent of not-quite-enoughness, but the various alternatives — “singletons,” “onelings,” “one-offspring” — never stuck. So we are left to ponder the only child.

More American parents are doing that, because more American parents are raising them: The proportion of mothers who had one child at the end of their childbearing years doubled from 11% in 1976 to 22% in 2015, according to Pew Research Center, and census data show the trend continuing to tick steadily upward.

Only-child families — the fastest-growing family unit in the United States — are in the midst of a sea change. Families are shrinking, and improvements in gender equality have made childbearing more of a question than a given. As Gen X and millennial women prioritize personal and career goals, as couples marry and start their families later in life, more parents find themselves mulling the logistical, financial and philosophical possibilities of a smaller family: What would it mean for them if they had only one child? What would it mean for their offspring?

When they ask Collins, he assures them that he found his circumstances, growing up in New York City in the 1950s, ideal.

“On weekends, I’d run around with this ragamuffin gang of friends, but at a certain point in the day I would break off and go and hide somewhere. I really enjoyed just being alone,” he said. “What’s wrong with being alone?”

At a child’s birthday party in Madison, Wis., a few months ago, Carrie Kilman found herself chatting with a man who’d brought his two kids to the celebration. He nodded toward Kilman’s 3-year-old daughter and asked: Did she have siblings?

“I said, ‘We’re happily a family of three,’ ” Kilman said.

“And he gave me this really knowing look that was more than a little patronizing, and he literally just said: ‘Trust me, you’ll change your mind. As she grows up, you’re going to want to give her a sibling.’ ”

Kilman sighed. “I’ve gotten this before, several times. It usually comes from men.”

A survey from 1896

Experts generally agree that we can trace the so-called “Only Child Syndrome” to Granville Stanley Hall, a pre-eminent child psychology expert who was appointed the first president of the American Psychological Association in 1892. It was Hall’s decidedly unscientific 1896 survey of “peculiar and exceptional children” that led him to famously declare that “being an only child is a disease in itself.”

“His study was done in an era where children were quite isolated, they lived on farms with great distances between them, they had great workloads, and they didn’t interact with other children the way children do today,” said Susan Newman, a social psychologist and author who has researched only children. “He concluded that only children are selfish, they’re lonely, they have more imaginary friends than other children — which is absolutely not true.”

Hall’s theories ultimately were debunked by an onslaught of credible research in the decades that followed. Studies going back as far as 1925 have concluded that only children are virtually indistinguishable from other children in terms of personality.

While having brothers or sisters can yield benefits — close, healthy relationships between siblings have been tied to happiness well into old age — the innumerable variables that shape any individual childhood make it especially difficult to draw clear conclusions about siblings vs. singletons as a whole. Overall, Newman, said, the existing research simply doesn’t show that only children are at any measurable disadvantage.

And yet.

“Hall’s ideas just stuck,” Newman said. “As a culture, we became so mired in the stereotype, and it’s very hard to change our thinking.”

But Newman believes its grip on our collective consciousness is finally loosening. “It’s changing because there are more and more only children, and people are seeing for themselves that only children are not lonely, they’re not odd, they’re not selfish.”

Choice and chance

The children might not have changed over the decades, but their parents have. Prospective parents — both now employed — have struggled to build their careers and to confront soaring costs of living. For those who live and work in cities or wish to preserve the freedom and flexibility they’ve enjoyed well into their 30s, having just one child can be an appealing solution.

“Purely from a financial standpoint, we live in New York City, in a two-bedroom apartment where my son’s bedroom is the size of a teapot,” Rachel Nobel Fields said of her 6-year-old son. Sometimes, she admitted, she and her husband struggle to keep up with just one child’s activities, to say nothing of adding more to the mix.

“Logistically, just to try to get to all of his end-of-the-year performances, it’s a lot,” she said.

Sometimes, one child is the result of both choice and chance, a result of a narrowing fertility window as parents wait longer to start their families.

“If I was younger, we probably would have had two. But being older, and then with the expense of a child, we are comfortable with one,” said Melissa Wilson, who lives with her husband and 4-year-old daughter in Minneapolis. “We can have fun, we can give her what she needs, we don’t need to worry about it as much as if we had two.”

People are often drawn to the idea of replicating a childhood model they enjoyed — or avoiding one they didn’t. Which means that people who are close to their own siblings — such as Wilson, who moved to Minnesota in part to be nearer to her sister — might feel a bit unsettled at first by the prospect of raising one child.

“I was a little worried about her being an only child,” Wilson said of her daughter, who happily bounces from swimming lessons to gymnastics lessons to quiet time at home, where she likes to paint. “But my brother-in-law is an only child, and he has lots of friends, and he did just fine. I just want to make sure she has enough friends that become close enough to be like family to her.”

‘It just never really came up’

Sometimes an only child isn’t a question at all.

“It wasn’t so much a conscious decision to not have two children as it just never really came up,” said Beth Carter, who lives in Tacoma, Wash., with her husband and 7-year-old son. “It just was never something that we felt like we needed to do; I just did not have the desire. The question is so often framed as a decision not to have a second instead of a decision to have a second, and I think that’s so interesting, as if that is the default — that you’ll have another.”

Still, she and her husband felt pressure to provide their son with a sibling — until they asked him if he wanted one.

He offered “a resounding no,” she said.

He isn’t the only one who feels that way. It’s an attitude that’s growing.

“The smaller family is definitely here to stay,” Newman said, adding, emphatically: “One child is a family.”