You seem to have behavior and personality confused. Behavior is a response due to an internal/external stimulus while personality is the perception of oneself and everything else. Personality requires depth, built from experiences acquired during life. Behavior, knowledge, cultural, language, etc. affect personality at all times so to say a infant of only a couple months has a personality is ludicrous. At age 3, a child is beginning to develop personality. This is accomplished due to trying different behaviors and attempting interaction/communication with peers and older individuals. Although behavior can be an early indication of personality, one can easily manipulate an outgoing child (between ages 1 to 4) to a complete introvert by reinforcing certain behaviors. Instead of basing your argument off of an insignificant sample (3 children), do some research on cognitive and psychosocial development.
Sir you are correct but you have no kids...thus what is your experience beyond academia, experimental psych, and empiricism....have you ever spent time with infants/toddlers or even a dog or cat. By your line of thought the family pet has no personality and is just the sum of conditioning. And what about mothers that will tell you that infants do have personality. And yes I did purposefully manipulate my two younger kids cognitive perception of pain (not sensory perception) via perceptual set/perceptual expectancy. Btw you are using introvert in the popular sense and not the classical/clinical conceptualization. In all respect please go back in your Skinner air crib....or better yet....get out of the frickin thing...don't let school get in the way of your education.