The trend has continued in the 2000s. Recess and free play time were reduced to make room for more standardized testing and academic work. Homework became common for even the youngest schoolchildren. After-school playtime morphed into structured activities overseen by adults.

Another trend that reduced outdoor free play is the grossly exaggerated fear of “stranger danger.” The spread of cable TV gave us more programming focused on rare but horrific cases of child abduction. Those fears, combined with the long, slow decline of trust in neighbors and fellow citizens, gave rise to a belief by the 1990s that persists today: Children who are not in sight of a responsible adult are at risk of abduction, so parents who allow unsupervised outdoor play are bad parents. The authorities should be notified.

The constant presence of adults is intended to keep children safe, but what are its likely effects? How might kids deprived of opportunities for free play, risk-taking and self-governance differ from previous generations when they leave the nest? We would expect two main areas of difficulty.

The first is that when these kids become adults, they are likely to be less resilient. Like the immune system, children are “antifragile,” as Nassim Taleb, a professor of risk engineering at New York University put it in his book by the same name. The immune system requires repeated exposure to dirt and germs in order to develop its protective abilities. Children who don’t get enough exposure are more susceptible to autoimmune diseases later on.

By the same logic, if we “protect” kids from the small risks and harms of free play, we stunt their ability to handle challenges and recover from failures. When such children arrive at college, we would expect them to perceive more aspects of their new environment as threatening compared with previous generations. We would expect to see more students experiencing anxiety and depression, which is precisely what is happening, according to national surveys and surveys of student counseling centers. These large increases do not just reflect a greater willingness to seek help; there has been a corresponding rise in self-harm, suicidal thinking and suicide among American adolescents and college students.

The second predictable consequence of play deprivation is a reduction in conflict management and negotiation skills. If there is always an adult who takes over, this is likely to create a condition sociologists call “moral dependence.” Instead of learning to resolve conflicts quickly and privately, kids who learn to “tell an adult” are rewarded for making the case to authority figures that they have been mistreated.

It’s easy to see how overprotection harms individuals, but in a disturbing essay titled “Cooperation Over Coercion,” the economist Steven Horwitz made the case that play deprivation also harms liberal democracies. He noted that a defining feature of the liberal tradition is its desire to minimize coercion by the power of the state and maximize citizens’ freedom to create the lives they choose for themselves. He reviewed work by political scientists showing that self-governing communities and democracies rely heavily on conversation, informal norms and local conflict resolution procedures to manage their affairs with minimal appeal to higher authorities. He concluded that self-governance requires the very skills that Peter Gray finds are best developed in childhood free play.