Seven-month-old lydia looked away and started to fuss when her mother's college roommate, who hadn't visited since lydia was born, got lydia up from her nap. lydia's reaction is most likely the result of

Stranger wariness/anxiety

Stranger wariness and anxiety among infants usually begins when he/she is 6 months old, and is a normal part of their cognitive development. An infant may no longer smile at friendly faces other than those of his/her parents or those he/she is familiar with. Rather, the infant may shriek or even burst into tears when a stranger or unfamiliar person makes eye contact or gets into close proximity with him/her, especially if done rather quickly. This stranger wariness in infants usually subsides after 2 years.