Although traditional accounts of attachment theory attempted to partition the organism's attachment and separation responses into those that were instinctive and those that were the result of the developmental environment, recent findings from epigenetics are indicating that no such partitioning is possible, even in principle. Rather than assuming the expression of a given behavioral trait is based on some set of instincts (as Bowlby and many of his colleagues did for attachment and separation responses), behavioral development is now seen as a self-organizing, probabilistic process in which pattern and order emerge and change as a result of ongoing co-actions among developmentally relevant components both internal (e.g., genes, hormones, neural networks) and external (e.g., temperature, diet, social interaction) to the organism. Exploring the specific prenatal and postnatal features of the mother-infant interaction system is providing a new appreciation of the complexity of the origins and maintenance of early attachment and its long-term consequences.