Development is not a passive process - one doesn't just sit back and allow time and the environment to have an effect. We act on the environment, seeking stimulation and interacting with other people. One of the earliest behaviors studied in neonates is their ability to interact. At a very early age babies will smile and will engage in conversation-like interactions with their caregivers. Some developmental psychologists are quite interested in this exchange in infancy and in adulthood.
Looking at how two parties influence one another is the study of reciprocal interaction.
People don't just treat you in a generic manner - the way they react to you is often a reflection of how you behave.
I know we are sort of jumping around here, but the idea is to give you a general understanding to what is involved in the study of development. My goal is to provide you with some general ideas about what developmental psychology is and how developmental psychologists think about the changes seen across the life-span.
Development Occurs in Context