By 2 to 3 months of age, most infants expect people to react positively when the infants initiate a behavior, such as a smile or a vocalization. this finding has been discovered using a method called the:

The method in question is called the still-face paradigm.
Infants like to see the reaction in other people when they are doing something - smiling, laughing, vocalizing, pointing at something. However, when this paradigm was used, people would just stare at the child emotionlessly. Suddenly, the child would grow anxious and desperately seek the person's attention, which tells us a lot about their expectations.