Experts say that simply being touched and held during the first few years of childhood may set up positive stress-response patterns that last a lifetime. Studies in rats have found that newborns who receive repeated touching, licking and grooming from their mothers have "all sorts of protective benefits against the negative effects of stress in adulthood."
The investigators discovered that pups whose mothers had been especially attentive during childhood had more of a certain kind of receptor on the surface of the structure in the brain called the hippocampus than relatively 'neglected' pups. These receptors responded specifically to the cortisol like hormone, which is secreted by the adrenal glands during stressful situations.
Production of this hormone is shut down when the hormone binds to receptors on the hippocampus, and a signal is sent from the brain back to the adrenal gland, effectively telling it to 'switch off' production. More corticosterone-receptors on the hippocampus of the 'well-fondled' rats means the brain is more sensitive to the hormone, and more efficient in sending back this signal to stop the stress response.
Humans also secrete cortisol, and while we don't lick and groom our newborns, most human parents do seem fond of holding and caressing them. The developmental effect seems to be time-specific. The stress-reactions of rats in adulthood seem to be 'programmed' by maternal touch during the first two weeks of life. This time period may be likened to the first 3 years of human development.
Harvard researchers are studying the hormonal levels of Romanian orphans who were simply "left alone in cribs or playpens, with no stimulation, no interaction" for the first few years of life. The Harvard researchers are finding similar measures. They're finding that their cortisol measures are really high, compared with similarly-aged children brought up in family homes. The plight of these orphans in later development has been well documented -- distant, highly-stressed, insular children who have difficulty coping with 'normal' human interaction and touch.