Dynamic systems theory is a psychological theory of human development. Unlike dynamical systems theory which is a mathematical construct, dynamic systems theory is primarily non-mathematical and driven by qualitative theoretical propositions. This psychological theory does, however, apply metaphors derived from the mathematical concepts of dynamical systems theory to attempt to explain the existence of apparently complex phenomena in human psychological and motor development.
As it applies to developmental psychology, this psychological theory was developed by Esther Thelen, Ph.D. at Indiana University-Bloomington. Thelen became interested in developmental psychology through her interest and training in behavioral biology. She wondered if "fixed action patterns," or highly repeatable movements seen in birds and other animals, were also relevant to the control and development of human infants 
According to Miller (2002), dynamic systems theory is the broadest and most encompassing of all the developmental theories. This theory attempts to encompass all the possible factors that may be in operation at any given developmental moment; it considers development from many levels (from molecular to cultural) and time scales (from milliseconds to years). Development is viewed as constant, fluid, emergent or non-linear, and multidetermined. Dynamic systems theoryТs greatest impact has been in early sensorimotor development.
Esther Thelen believed that development involved a deeply embedded and continuously coupled dynamic system. It is unclear however if her utilization of the concept of "dynamic" refers to the conventional dynamics of classical mechanics or to the metaphorical representation of "something that is dynamic" as applied in the colloquial sense in common speech, or both. The typical view presented by R.D. Beer showed that information from the world was given to the nervous system which directs the body, which in turn interacts back on the world. Esther Thelen instead offers a developmental system that has continual and bidirectional interaction between the world, nervous system and body. The exact mechanisms for such interaction, however, remain unspecified.
The dynamic systems view of development has three critical features that separate it from the traditional input-output model. The system must first be multiply causal and self-organizing. This means that behavior is a pattern formed from multiple components in cooperation with none being more privileged than another. The relationship between the multiple parts is what helps provide order and pattern to the system. Why this relation would provide such order and pattern, however, is unclear. Second, a dynamic system is a dependent on time making the current state a function of the previous state and the future state a function of the current state. The third feature i
the relative stability of a dynamic system. For a system to change, a loose stability is needed to allow for the components to reorganize into a different expressed behavior. What constitutes a stability as being loose or not-loose, however, is not specified. Parameters that dictate what constitutes one state of organization versus another state are also not specified, as a generality, in dynamic systems theory. The theory contends that development is a sequence of times where stability is low allowing for new development and where stability is stable with less pattern change. The theory contends that in order to make these movements, you must scale up on a control parameter in order to reach a threshold (past a point of stability). Once that threshold is reached, the muscles will begin to form the different movements. This threshold must be reached in order for each different muscle to contract and relax to make the movement. The theory can be seen to present a variant explanation for muscle length-tension regulation but the extrapolation of a vaguely outlined argument for muscle action to a grand theory of human development remains unconvincing and unvalidated.
Esther Thelen's early research in infant motor behavior (particularly stepping, kicking, and reaching) led her to become dissatisfied with existing theories and moved her toward a dynamic systems perspective. Prior views of development conceptualized infants as passive and infantsТ motor development as the result of a genetically determined developmental plan. Thelen, in her work, contended that infants' body weights and proportions, postures, elastic, and inertial properties of muscle and the nature of the task and environment contribute equally to the motor outcome. None of these contentions have been scientifically validated due in part to the breadth and poor operational definition of the parameters used to represent the phenomena involved. It is theorized that infants can "self-assemble" new motor patterns in novel situations, but what this actually means awaits further and specific clarification. The theory contends that development happens in individual children solving individual problems in their own unique ways. Thelen used the proposition that because each child is different in terms of his or her body, nervous system, and daily experience, the course of development is nearly impossible to predict, and yet the theory does not account for clear trends and predictability in development for most children, despite there being multiple pathways to development. Development is supposedly not just the result of genetics or the environment, but rather the interweaving of events at a given moment. How such interweaving occurs is not specified by the theory in certain terms. Dynamic systems theory proponents claim to have had the greatest impact on early sensorimotor development.