This category is one that most parents have some difficulty grasping. It encompasses the cognitive domain.
By two months a child should be following things with his eyes and getting bored if activities don't change. At 4 months a child should let you know if she is happy or sad, respond to affection and play peek a boo. By 6 months a child should enjoy playing with others, recognize strangers and enjoy looking at himself in the mirror. At 9 months a child should look at things as they fall and look for toys that she's seen you hide. At 12 months a child should be able to look at a picture of a common object when you name it.
I hope that this has helped to demystify things for you.
Kevin M. Windisch MD, FAAP
Sparks Pediatric and Adolescent Medicine
975 Roberta Lane # 101B
Sparks NV 89431
775-359-7111