This paper from Harvard University describes the importance of 'serve and return' social interactions for infants and young children. Because responsive relationships are both expected and essential, their absence is a serious threat to a child’s development and well-being. If an adult’s responses to a child are unreliable, inappropriate, or simply absent, the developing architecture of the brain may be disrupted, and subsequent physical, mental, and emotional health may be impaired. The persistent absence of serve and return interaction acts as a “double whammy” for healthy development. Not only does the brain not receive the positive stimulation it needs, but the body’s stress response is activated, flooding the developing brain with potentially harmful stress hormones. Building the capabilities of adult caregivers can help strengthen the environment of relationships essential to children’s lifelong learning, health, and behaviour.