My grandson, Asa, is in early childhood. He sees the world in opposites: male/female, child/grown-up, good guys/bad guys. He considers himself one of the good guys, destroying the bad guys in his active imagination and in karate kicks in the air.

Oscar, his father, knows better. He asked me if Asa really believes there are good guys and bad guys, or is that just a cliché. I said that most young children believe in simple, straightforward opposites.

Undeterred, Oscar told Asa that he knows some adults who were once bad guys but became good guys.

“No,” Asa insisted. “That never happens.”

Asa is mistaken. As he matures, his body will grow taller and become better able to sit with feet on the floor, not kicking. His thoughts will include the idea that people change as they grow older, a theme throughout this book. What Asa says “never happens” occurs every day—none of us is a bad guy or a good guy, but all of us keep developing, ideally for the better.

Oscar is not alone in his awareness. Many folk sayings affirm development: People “turn over a new leaf,” are “born-again”; parents are granted a “do-over” when they become grandparents; today is “the first day of the rest of your life.” We recognize that the past never disappears and that parents always influence children, as in the saying “The apple does not fall far from the tree,” but we also recognize many other genetic, biological, and social influences on each person, as detailed in the best-selling book Far from the Tree (Solomon, 2012).

The complexity, the twists and turns, the endless variety of the human experience at every age is fascinating to me, which is why I continue to study human development and revise this textbook, with new insights as well as new words and topics in every edition.

We all have echoes of Asa in us: We want life to be simple, for people to be good guys. But life is not simple. Learning about human development helps everyone respond to life’s variations and influences, not with imaginary kicks but with wise responses. Knowledge does that. In a vivid example, Stephen Pinker (2011) finds that humans kill each other less now than they did in previous centuries; he cites education as one reason.

Education occurs in many ways. This textbook is only one of them—an aid to understanding the complexity of your life, my life, and the lives of all the estimated 19 billion humans alive now or who once lived. Nonetheless, although life experiences and thousands of other books add to our education, writing this text is my contribution and studying it is yours. Together we might learn how to limit the bad and increase the good in each of us as time goes on.