A child who throws a tantrum in the supermarket, demanding a treat, is clearly not a grown-up. This is obvious to most people older than that child; such ‘childish’ behaviour stands in stark contrast to mature, grown-up conduct. But what happens when you’re supposed to be a grown-up—by age, at least—but find yourself caught up in your own life? How do you recognise that your thoughts, beliefs, actions, and reactions may not be particularly mature? Most of us assume that real grown-ups want to be ‘grown up’—for their own good, and for everyone else’s. There’s a collective belief that if every adult acted like a grown-up, the world—and the individuals in it—would be better for it. To understand what it means to be grown-up, we must start by understanding what makes behaviour childish. And that part is simple. Take the child in the supermarket who desperately wants an ice cream. They see a fridge full of options and a parent nearby with money. They sense the possibility that the wor...