To understand why, consider what we know about the basic differences between the sexes. And yes, with all due respect to women astronauts, tech-company CEOs, and army generals, there are a number of settled pink/blue distinctions. Most will come as no surprise to parents or kindergarten teachers, but here’s what cognitive and neuroscientists now agree on. On average, boys are more physically active and restless than girls. They have less self-control and are more easily distracted. They take longer to mature. They have a harder time sitting still, paying attention, and following rules, especially in the early years of school. Not surprisingly, then, they are three times as likely as girls to be diagnosed with ADD. They make up 70 percent of K–12 suspensions and 67 percent of special-education students. In Lord of the Flies terms, we might say that boys need more “civilizing” than girls. They require more cues, more reminders, and more punishment to learn to control their aggression and to mind their manners. Boys—not girls—often require remedial education to sit still, to look at the person speaking to them, to finish the task they were working on. These days, experts might put it this way: boys come into the world with less natural human capital than do girls. This doesn’t hold true in terms of cognitive ability, which doesn’t vary in ways that matter to boys’ difficulties. It’s the “soft skills” that are the issue. “Success in life depends on personality traits that are not well captured by measures of cognition,” the Nobel Prize-winning economist James Heckman has concluded. “Conscientiousness, perseverance, sociability, and curiosity matter.” For at least the first three of these, boys are just naturally slower.

The implications for family life are profound. Lone parents tend to have a tougher time providing the predictability and order that help boys become capable students and workers. Poverty undoubtedly worsens the problem: in general, low-income children have poorer “executive function,” such as self-control and cognitive flexibility, than do middle-income children, according to a 2011 study by a group of Berkeley neuropsychologists. But poor children in single-parent families still came out worse in the study than kids with poor married parents. This is probably because unmarried parents tend to break up more frequently, go on to new relationships, sometimes serially, and bring stepparents and half- and step-siblings into their children’s lives. Cohabiting unmarried parents are three times as likely to break up before their child reaches his fifth birthday than are married parents. Various studies have shown that each of these “transitions” leads boys to exhibit more behavior and attention difficulties and to become more aggressive.