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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 with karate kicks in the air.

Oscar, his father, knows better. He asked me whether Asa really believes there are good guys and bad guys, or is that just an expression. I said that most young children think in simple 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—not that any of us is always a bad guy or a good guy, but that 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).

Complexity, twists and turns, dynamic unfolding, and endless variety of the human experience throughout life is fascinating to me, which is why I continue to study development and revise this textbook. The study itself is dynamic: New insights, new phrases, and new topics appear in every edition, and old topics require revision.

We all have echoes of Asa in us: We want life to be simple. Some aspects of development do not change—birth, death, families, attachment—and some old theories and perspectives are still insightful. They are detailed in this text. But life is not simple or stagnant. Learning about human development helps everyone respond to life’s variations and influences, not with imaginary kicks but with wisdom.

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 20 billion humans who are either alive now or 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 one of yours: Together we might learn how to limit the bad and increase the good in each of us as time goes on.