Infants begin to form attachments to their parents very early on in life. Are these attachments simply made with those who provide nourishment for the infant, or are there other important factors to consider? Harry Harlow helped answer this question in a study he performed in the 1950s, which I find rather interesting. He used infant rhesus monkeys (which he separated from their mothers soon after birth) and placed them in a cage with two figures that represented their mothers. One of the mothers was made out of uncomfortable metal wires and had an angular face, but was also the source of nourishment and had a bottle of milk. The other mother was made out of a heated, comfortable terry cloth and had a rounded face, but did not have a bottle of milk.
Harlow found that the infant monkeys went to the wire mothers for milk, but spent more time with the mothers made of terry cloth. When the baby monkeys were confronted with a scary stimulus, most would attach themselves to the terry cloth mother. This was later called contact comfort, the positive emotions afforded by touch. So, while you may think nourishment plays the larger role in forming attachments, it is actually touch that has more of an effect. It is important to understand how important this bond is for infants and children.
Those are some interesting connections. In addition to seeking comfort and nourishment, infants seek out familiar voices. Studies have shown that infants can distinguish their mother's voice from a stranger's voice at only four days old.