Emotional development traces a detailed pattern across the lifespan. This pattern is comprised of periods of rapid change alternating with periods of consolidation and stability, self-amplifying individual variations on universal developmental themes, and progressive complexity in feelings, thoughts, personality, behavior, and self-regulation. We know that the patterning of emotional development is not a direct expression of some species-specific or genetic program. Nor is it simply a readout of socialization practices, family experience, or any other set of environmental contingencies. Despite our attempts to predict it, emotional development is indeterminate and malleable at almost any age. Despite our normative classifications, it is characterized by idiosyncratic and unique trajectories. In short, emotional development is organized and orderly without being prespecified or programmed. How do we account for its intrinsic organization? How does emotional development achieve its coherence, complexity, and patterning without design or instruction?

As in other developmental domains, the best answer to this question seems to depend on principles that extend well beyond developmental psychology. These principles express a new approach to change and novelty across the natural sciences, an approach variously called nonlinear dynamical systems theory (or dynamic systems theory, for short), complex systems theory, or chaos theory. At the heart of this perspective is the idea that natural systems behave very differently than the simple, idealized systems so well described by Newtonian mechanics.