A dynamic process between brain and environment. See epigenetic notes and news.
Sex and cultural differences
Health and growth of premature infants improved by touch . (Tiffany Fields)
Also true for rats & and Harlow was on to this decades ago but the biochemistry was unknown.
Bowlby found psychosocial dwarfism in unstimulated human orphans probably due to lack of growth hormones.
Michael Meaney's recent work follows up on these isssues; see a description of that work and its relation to human stress response and mothering. Many now are looking at the epigenetics of these very early experiences--touch, nutrition, various stressors.
The mechanisms of mothering-effects are suggested in a recent paper where it was found that a lack of mother rats' licking and grooming of their infants turned off certain genes in their offspring -- a process called "methylation" which prevents those genes from producing hormones including estrogen and oxytocin. (Champagne, F. (2006) xxx Endocrinology)
Darwin probably knew this -- he commented on smaller brains of domestic vs wild versions of same species. Positive changes include thicker neocortex and more dendrites.
The hippocampus, relatively large in primates and especially so in humans, is involved in acquiring new memories, especially episodic memories. There can be an age and/or stress related loss of neurons here. This may be due to losses outpacing replacement -- apparently new neurons can be generated in the hippocampus to an extent.
Effects of social interaction on chicklid fish --size and color-- affected by testosterone levels. (discusses analogy to humans.)
(recall here the Sapolsky reading on orangutans male dominance--effects of long calls on growth.)
(similar stress related effects are likely in all primates including humans)
stress dwarfism in humans
See the 512 aggression notes, including Nisbett's "Culture of Honor" studies.
growth hormones may greatly affect body size.
Sapolsky's research on stress effects of baboon social life. Rank seems to relate to better bodies.
Baboons were once seen as good models for aspects of human behavioir and development, despite the fact that baboons are monkeys, not apes. Sapolsky and others have compared effects of stress in both species. Recently Sapolsky documented a profound change in a troop he had observed for year when the aggressive dominant males pillaged a human garbage dump, hogging all the food-- which turned out to be poisoned. Less aggressive males became the leaders with many consequences.
There are some informative video notes on an older baboon video; and "Murder in the troop." But see PBS notes and recent Sapolsy PBS special.