As children, we inhabit a land of imagination. Our bedroom is a forest, a snowbank is a fortress, and we are elves, warriors, princesses and superheroes. Most of us put away fantasy in adulthood but with an important exception (and I don't mean Comic-Con). As Felipe De Brigard of Duke University's Center for Cognitive Neuroscience explains in our cover story, “Why We Imagine,” we routinely unleash our imagination in reveries of how things might have gone differently. “We dip into alternative realities with a frequency and ease,” De Brigard writes, “that suggest this habit is core to the human experience.”

Why we engage in what cognitive scientists call “counterfactual” thinking is the question our article explores, drawing on research about the distinct logic of such musings and the impact on memory, emotion and motivation. In conjunction with the story, we asked visitors to our Web site to share their own what-ifs, which you can find at www.ScientificAmerican.com/WhatIfMoments.

In cities around the U.S., from Cleveland, to Ferguson, Mo., to Houston, communities have been pondering some more troubling counterfactuals: What if 12-year-old Tamir Rice had been playing with a ball instead of a toy gun in a Cleveland park last November? What if Sandra Bland had used a signal when she changed lanes while driving in Waller County, Texas, in July? More critically, what would have happened if the law-enforcement officers who confronted Rice and Bland had kept their cool? Surely these young people would be alive today. In a timely story, Rachel Nuwer reviews the factors that contribute to police violence and what we can do about it in “When Cops Lose Control.”

Parents of youngsters who have autism often torture themselves with what-ifs, wondering if there was something they could have done differently to prevent the disorder. Simon Makin's article, “What Really Causes Autism,” may offer some relief. In it, Makin sorts through the explosion of recent research showing that autism is primarily a genetic disease and discusses how these findings are beginning to open the door to new therapies.

Here's one more what-if. What if growing older made you happier, not just grumpier and creakier? Hmm. What if you clicked here to find out more?