What role does an infant's attention to facial expressions play in his or her understanding of language?

We can figure out a lot based on microexpressions like their emotions or if they're lieing or not. so learning those as a infant will help in understanding how to communicate with someone and understand how to act based on their facial expression... like if a baby laughs which causes the person to smile, they'll learn that laughing will cause that reaction..

that make sense?