Developmental psychology is defined as the study of how behavior changes over the life span. Many studies and experiments have been conducted on the development of infants and children, and many theories have been hypothesized about how and why we develop the way we do mentally, emotionally, and physically. Many characteristics of the behavior of infants have been founded and studied such as stranger anxiety, attachment, and imprinting. One man named Konrad Lorenz went on to create one of the most famous studies on imprinting that ended up earning him a Nobel Prize! Lorenz's experiment involved the observation of geese. What Lorenz was about to discover was a mere accident. Lorenz stood by one day as goslings were hatching. Lorenz was "the first, large moving object" they goslings saw upon hatching. With this Lorenz essentially became Mother Goose. The goslings followed Lorenz around as if he was their real mother. This process became to be known as imprinting. Lorenz later discovered that something called a "critical period" exists in birds and other species that experience imprinting. A critical period is "a specific window of time during which an event must occur". If the Lorenz's goslings had not seen a large, moving object such as himself or a goose within about 36 hours, the goslings would not have imprinted to anything.
Here is a link to a video of Konrad Lorenz's geese in action.
Humans don't necessarily imprint as severely as geese, but we do in a sense form an incredible bond with our primary caregiver as an infant. All in all, at very young ages we are still able to form close bonds, or relationships, with those who provide us with nourishment and care.