Ever since human beings have thought about themselves, they have argued about the origins of human nature. First religion and then evolution have provided answers to the ultimate sense of this question, answers that have satisfied some and infuriated others. The question has also persisted in a developmental rather than an evolutionary time frame. Here it exists today as a riddle, a trick question like one posed in ancient times by the sphinx, to the despair of travelers who attempted to answer it. For us the question takes the form: ”Is our development shaped more by our genes or by our environment?” Despite all our attempts as developmentalists to answer this riddle by insisting that development always involves both elements and that it is the interaction of the two that should be the focus of our attention, the sphinx is not satisfied: the riddle persists—in the press, in public policy, and even in the back of our own minds. This persistence of the riddle is one of the reasons that I have found studying development so fascinating. In the course of the research I have done, my concept of development has been changed again and again. More recently, another question has gradually replaced the riddle in my mind and has led me to a different way to attempt to satisfy the sphinx, as I will relate below.