The idea that only children are precocious persists and may, as Ms. Blake suggests, be connected with the fact that only children are often raised in richer verbal environments and share meals and other activities with adults. (I love it that an artist friend still brags that my daughter was 2 when she insisted that a crayon was “magenta, not pink.”)

My research suggests that only children experience more intensely emotional family lives. The parental gaze is more focused; the love more concentrated. This intensity can be enriching, and also suffocating. Many adult only children told me that they wanted their first child to have a sibling precisely because this kind of intensity was too much for them.

At the end of their parents’ lives, only children are sometimes said to be burdened in ways that children with siblings aren’t. Data from the National Alliance for Caregiving show that, in fact, the closest living sibling most often shoulders responsibility for elder caretaking. Still there is something existentially troubling about the idea of facing one’s parents’ mortality alone; in my interviews with hundreds of only children, I found that this was the issue people felt most viscerally about when deciding whether they wanted to have one or more children.

Given that about one in five American families now have just one child, this seems like a good time to question the misconceptions about only children and the dangers of raising a child without siblings. For one thing, one-child families make obvious sense in a time of diminishing resources. This may explain recent studies showing that parents who have one child tend to be happier. (In a recent study at the University of Pennsylvania, for example, Hans-Peter Kohler surveyed 35,000 sets of twins and found that of those women who had children, the happiest ones were those who had just one child.) Call me selfish but, as the mother of one child, I enjoy more time, energy and resources than I would if I had more children. And it is hard to imagine that this isn’t better for my family as well as for me.

Most people say they have their first child for themselves and the second to benefit their first. But if children aren’t inherently worse off without siblings, who is best served by this kind of thinking? Instead of making family choices to fulfill breeding assignments we imagine we’ve been given, we might ensure that our most profound choice is a purely independent, personal one. To do so might even feel like something people rarely associate with parenting: it might feel like freedom.