The prompt I gave to Caplan, of course, has no single correct response. There are multiple, sometimes conflicting, ways of evaluating the question of how many kids is best for one family: from the perspective of parents, of children, and of society. These various lines of inquiry warrant a tour of what’s known, and what isn’t, about how the size of a family shapes the lives of its members.

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A handful of studies have tried to pinpoint a number of children that maximizes parents’ happiness. One study from the mid-2000s indicated that a second child or a third didn’t make parents happier. “If you want to maximize your subjective well-being, you should stop at one child,” the study’s author told Psychology Today. A more recent study, from Europe, found that two was the magic number; having more children didn’t bring parents more joy.

In the United States, nearly half of adults consider two to be the ideal number of children, according to Gallup polls, with three as the next most popular option, preferred by 26 percent. Two is the favorite across Europe, too.

Ashley Larsen Gibby, a Ph.D. student in sociology and demography at Penn State, notes that these numbers come with some disclaimers. “While a lot of [the] evidence points to two children being optimal, I would be hesitant to make that claim or generalize it past Western populations,” she wrote to me in an email. “Having the ‘normative’ number of children is likely met with more support both socially and institutionally. Therefore, perhaps two is optimal in places where two is considered the norm. However, if the norm changed, I think the answer to your question would change as well.”

The two-child ideal is a major departure from half a century ago: In 1957, only 20 percent of Americans said the ideal family meant two or fewer children, while 71 percent said it meant three or more. The economy seems to have played some role in this shift. Steven Mintz, a historian at the University of Texas at Austin and the author of Huck’s Raft: A History of American Childhood, says that the ideal during the Baby Boom was in the neighborhood of three, four, or five children. “That number plummeted as the cost of rearing children rose and as more women entered the workforce and felt a growing sense of frustration about being reduced to childbearing machines,” he said.

The costs of raising children are not just financial. “As a parent who prizes his own mental and physical health,” says Robert Crosnoe, a sociology professor who is also at the University of Texas at Austin, “I had to stop at two, because this new style of intensive parenting that people feel they have to follow these days really wears one out.” (He added: “I am glad, however, that my parents did not think this way, as I am the third of three.”)

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