Yes James, the brain-physiology is important. However, I think there are some studies that demonstrate that the maturing is not totally a biological process. I cannot remember now any exact references, but there is, I believe, some evidence that environmental factors may act as boosters or brakes in the development of individual personality. I think that it was D.O.Hebb who made a number of interesting (albeit ethically questionable) experiments with apes that demonstrated that sensory deprivation may delay the development of brain functions. So if something is correlated with brain processes, it could mean that these brain processes are enhanced or delayed by environmental influences. If small babies grow up in a stimulating and friendly environment, this should have an impact on their brains. All this brings to my mind a slogan that the father of modern Finnish psychology, Eino Kaila, often used: structure determines function, but function determines structure as well. From him stems another slogan: my drives compel my will, but my will compels my drives as well!