Stephen Suomi, PhD, is chief of the Laboratory of Comparative Ethology at the National Institute of Child Health and Human Development in Bethesda, Maryland. Beginning in the 1970s as a University of Wisconsin graduate student and protegé of Harry Harlow, PhD, Suomi has devoted much of his career to expansion of that pioneer scientist’s world-famous work on early attachment in rhesus monkeys.

Q. What’s so important about early attachment between infants and their mothers?

A. It reflects millions of years of evolutionary history. The mother buffers the child from the big, scary world. How she does that can have profound impact on her youngsters’ ability to function socially, as well as on their basic biology. [11]

Q. Why study rhesus monkeys?

A. They’re probably the world’s second-most successful primate species, after us. They thrive within a wide range of climates and terrains. They also share about 94 percent of our genes and are born with the same basic emotions that human infants have. A generation matures in just three or four years. In one scientist’s lifetime, it is possible to see how events in infancy affect adolescence, adulthood, and even subsequent generations.

Q. What does the typical infant monkey get from its mother?

A. Much like humans, rhesus monkey infants need external nourishment and heat. They spend virtually all of their first few weeks of life in physical contact with their mother. They establish an attachment bond with her and train their sleep cycles to match hers.

Gradually they begin to spend time away from her, wandering farther and staying away longer. Throughout childhood they keep coming back to her, or at least looking back for reassurance. She serves as their secure base. Females grow up and stay in the same troop in which they were born, but once males reach puberty, they either leave their troop voluntarily or they’re forced out. They typically join all-male gangs and hang out together for anywhere from a few months to more than a year. It’s a rough time and nearly half don’t survive it. Those who do survive join a new troop and remain with it.

Q. Are all monkey infants born alike?

A. No. Just as human babies have individual temperaments, so do infant monkeys. Those that seem unusually shy and fearful have over-reactive hypothalamic-pituitary-adrenal (HPA) axes. This physiological system produces the stress hormone, cortisol. When they encounter mild stressors, their cortisol levels increase more and for longer periods than in less-reactive youngsters. Their hearts pump faster and longer under stress, and their immune systems appear to be more easily compromised.

In contrast, impulsive, aggressive youngsters have problems with serotonin metabolism. Serotonin is the neurotransmitter that slows or stops certain neurotransmissions. If you don’t have enough serotonin, you are less able to control behavior. This presents itself in young monkeys whose playful games escalate into fights. They also ignore social hierarchies, confronting older, dominant animals that clobber them for their efforts.

Q. If the differences are present at birth, what role does the mother monkey play?

A. A huge one. If there’s one thing I want to emphasize, it’s that genetic characteristics may be modified by experience. We have taken monkey infants away from their mothers and raised them almost completely among other monkeys their age. They form strong attachments with each other, show normal growth rates, and develop relatively normal behavior. Superficially, they appear to be completely normal.

But in novel situations, these peer-reared monkeys are more easily disturbed behaviorally and physiologically. They mirror their mother-reared counterparts who were born shy and easily frightened. As they grow up, these peer-reared individuals start showing problems with serotonin metabolism – particularly with respect to aggression. This is especially evident in males. We attribute many of their characteristics to the fact that peers aren’t as good at comforting each other as are mothers, and in new situations they become as scared as their partners.

Q. What other differences have you found in peer-reared monkeys?

A. They drink like fish, at least in comparison with mother-reared monkeys. For an hour a day, my colleagues in the alcohol institute have given both types of monkeys access to an alcohol-laced liquid, as well as a non-alcoholic one. Peer-reared monkeys consume more alcohol and develop greater tolerance for it. That is, it takes more alcohol to bring their blood levels up and to have the same effects. This seems to be related to serotonin turnover, and it’s true for humans, too. Individuals who have lowered serotonin metabolism are also at risk to develop patterns of binge drinking. In our monkeys, the serotonin deficits can stem from genetic background, early experiences, or both.

Q. So a nurturing mother can buffer a child from its genetic inheritance?

A. Exactly. One of the newest and most exciting findings has to do with a particular gene, called "5-HTT," that regulates serotonin turnover. There are different forms of the gene, just as you have different forms of genes that give you blue eyes instead of brown. To make a long story short, this serotonin transport gene comes in a long and short form. We have been able to characterize most of the monkeys in our colony with respect to whether they have the long or short 5-HTT.

We have found an interaction between the kind of gene and the experience the monkey infant had. Monkeys raised by good mothers showed fairly normal serotonin metabolism, regardless of which type of gene they had. But gene type made a huge difference in peer-reared monkeys. Those with the long version looked pretty good. Those with the short version had problems with aggression. This is among the first demonstrations of an exact interaction between a specific gene and the mother-infant relationship. I fully believe that this is just the first of many sorts of things we’re going to find.

Q. Are there things we can do to change the outcome?

A. Some monkey mothers naturally are more nurturing than others. We have taken highly reactive monkey infants – those most likely to grow up anxiety-prone – and given them to foster mothers that are extremely nurturing. These infants grow up with optimal outcomes. Most rise to assume dominant positions in their troops. Females become nurturing mothers themselves. They can calm themselves down much more quickly than other highly reactive individuals. They bring their heart rates down faster, lower their cortisol more quickly, etc. It seems that their experiences with nurturing mothers alter not only their behavioral propensities, but their physiological patterns as well.

Q. How do your findings apply to humans?

A. Good question. After all, these monkeys are not just furry little humans with tails. All these long-term effects are basically biological. Humans are presumably going through the same physiological experiences and the same emotional experiences, with the added influence of cognitive ability. So the basic ingredients for humans are there. We know this, because we see it in the monkeys.

We don’t expect everything to translate directly. It’s hard to believe, though, that these monkeys, without language and oral traditions, can be so sensitive to early experiences and be susceptible to lifelong consequences, and humans would not be.

Interview #2: 'The Brain-Growth Puzzle'

Charles Nelson, PhD, a professor at the Institute of Child Development and Department of Pediatrics, University of Minnesota in Minneapolis, has focused his research on early brain development and its lasting effects upon health and behavior. He heads the John D. and Catherine T. MacArthur Foundation Research Network on Early Experience and Brain Development.

Q. How do our earliest relationships affect us later in life?

A. This puzzle is as difficult as it is tantalizing. It's difficult, because there is no way on earth neuroscientists can subject human babies to emotional deprivation in order to study what happens to their brains. Only in animals can we do this. The human puzzle tantalizes us because children who have been emotionally deprived, for whatever unfortunate reason, are more likely to exhibit altered behavior. We infer that their brains have been affected because their behavior has been.

Q. Can you give an example?

A. Take the kids reared in Romanian or other institutionalized orphanages. They tend to be unusually vulnerable to stress. Precisely how their brains reflect this, we do not know. Nor can we be sure of exactly what deprivation they suffered. Many had mothers who were alcoholics, drug abusers, or mentally retarded. It's complex.

Certain experiences must occur within a set period of time. If the person is deprived of these, the brain cannot develop normally. [10] Two good examples of this are vision and hearing. The parts of the brain that govern these systems finish developing during the first few years of life. During this period, certain things must take place if the person is ever to see or hear normally. Babies born cross-eyed or with other problems that prevent them from focusing correctly must undergo corrective procedures early. Otherwise, the developing brain "learns" to function with the abnormalities. Optical corrections later cannot reverse this.

Babies born deaf, as well as those who, for whatever reason, are not exposed to normal speech sounds, can never develop normal language of their own. This remains true even if, as teens, they gain the ability to hear. That's because the relevant part of their brains finished developing before the sounds were introduced.

Q. Then how do experiences affect the brain’s physical development?

A. In three ways. First, experiences actually trigger anatomical changes in the brain. Rats, for example, when reared in environments with lots of other stimuli, actually form more connections between cerebral cells than do other rats.

Secondly, experiences can cause metabolic changes. Skilled musicians such as violinists need extra oxygen, and thus blood flow, in parts of their brains governing their specific motor activity. New blood vessels to deliver this blood will develop.

Finally, neurochemical changes can occur. When a synapse breaks, perhaps through injury, the brain senses a shortage of some substance and may grow new axons to release a replacement supply.

Q. What chance does a child have who starts out in an unfavorable environment?

A. The fact that different parts of the brain develop at different rates means we have periods of opportunity, as well as vulnerability. [9] You hear people talk about the importance of exposing kids to all the right experiences early on, as though what happens later doesn't matter. Speech and visual experiences must take place early, but other parts of the brain – the frontal lobes, for example, that are involved in higher-level thinking – do not become adult-like until adolescence. It's up to us as a society to nurture children not only through infancy but long after.

Q. Does that mean "bad" mothers are to blame or are there other factors involved?

A. Bad outcomes, like good outcomes, are multifaceted, with no one person or event bearing full responsibility. Rather, a host of factors typically contribute to a child's future success or failure in life. For this reason we need to be careful in using the first few years of life to predict what will happen later on. Early emotional experiences have the potential to influence us for the rest of our lives. They may influence how we interpret other early experiences and how we see the world. But chances are many of the people committing atrocities in Kosovo and elsewhere had nurturing moms. Even if the first few years are wonderful, what they largely provide is a foundation, not a guarantee of lifelong health and happiness. Events that occur at any point in life can affect you, either for bad or good.

The Legacy of Romania's Orphanages

The longer that infants lived in Romanian institutions for abandoned or orphaned babies, where they received poor care and low stimulation and were malnourished, the higher their level of the stress hormone cortisol even six years or more after being placed in middle- to upper-middle class families.

More alarming, the elevated stress hormone levels of those who lived longest in these institutions were especially evident in the evening, when cortisol normally approaches its lowest level of the day. Failure to reduce cortisol to near zero at day’s end generally indicates dysregulation of this hormonal system.

Scientists normally rely on rats and monkeys for research into early deprivation because they can hardly deprive human children of nurturing in order to study its effects. There is at least one major exception: children raised in Romanian institutions experienced levels of deprivation and neglect of basic needs no animal experiment has duplicated.

Scientists are now studying the Romanian children closely, including 37 adopted by Canadian families, 21 of them after being institutionalized eight months or longer, and 16 adopted within four months of birth. They are matching these with 29 native-Canadian controls. Even after living with the families in British Columbia for six to seven and a half years, the Romanians who had been institutionalized at least eight months had higher cortisol levels than either the Canadian children or those adopted within four months.

Megan Gunnar, PhD, of the University of Minnesota, and Elinor Ames, professor emeritus, Simon Fraser University, British Columbia, say the difference can be attributed to the treatment the infants received after birth because those lucky enough to be adopted early had the same cortisol levels as Canadian children reared by families since birth, despite their presumed differences in prenatal and perinatal conditions. [4]

Rats Studies and Mothering:

Rats can teach a great deal about the health effects of infant-mother relationships and how they are passed from generation to generation.

The more frequently a mother rat licks and grooms her infant, the lower her offspring’s hypothalamic-pituitary-adrenal (HPA) response to stress will be. This means less exposure to the high levels of stress hormones over long periods that can lead to heart disease, diabetes, anxiety, depression, dementia, and other ailments. [7] Hyper-reactive animals – and humans – are thus vulnerable to these illnesses.

As adults, the offspring of nurturing mother rats also show substantially less fearfulness than do those of non-nurturing mothers. Examination of their brains reveals differences in neural systems that mediate fearfulness. Again, this means lower levels of stress hormones and healthier lives. [1]

Apparently, the mothers’ behaviors programmed these differences. The adult female offspring of nurturing mothers in turn become nurturing mothers themselves.

Much the same happens among humans, report Michael Meaney, PhD, of McGill University, Montreal, and doctoral student Darlene Francis. [3] "Conditions that most commonly characterize abusive and neglectful homes," they write, "involve economic hardship, marital strife and a lack of social and emotional support. Such homes, in turn, breed neglectful and even abusive parents."

The result is that stress hormones kick in more often at high levels, leading to poorer health outcomes, greater stress, and less nurturing. The cycle is self-regenerating. Meaney and Francis draw two important conclusions:

"Variations in maternal care that fall within the normal range of the species can still have a profound influence on development. One does not need to appeal to the more extreme conditions of abuse and neglect to see evidence for the importance of parental care."

"Environmental demands can alter parental care and thus infant development…. Environmentally induced alterations in maternal care …(affect) development of specific neural systems that mediate the expression of fearfulness. Such individual differences in fearfulness, in turn, influence the parental care of the offspring, providing a neurobiological basis for the intergenerational transmission of specific behavioral traits."

The Research:

1. Caldji C, et al. (April 1998). "Maternal Care During Infancy Regulates the Development of Neural Systems Mediating the Expression of Fearfulness in the Rat." Proceedings of the National Academy of Sciences, USA, 95: 5335-5340.

2. Comery T, et al. (1995). "Rapid Communication: Differential Rearing Alters Spine Density on Medium-Sized Spiny Neurons in the Rat Corpus Striatum: Evidence for Association of Morphological Plasticity with Early Response Gene Expression." Neurobiology of Learning and Memory, 63: 217-219.

3. Francis D, et al. (1999). "Maternal Care and the Development of Stress Responses." Current Opinion in Neurobiology, 9: 128-134.

4. Gunnar M, (in press). "Early Adversity and the Development of Stress Reactivity and Regulation." in CA Nelson (ed.) The Effects of Adversity on Neurobehavioral Development, Minnesota Symposia on Child Psychology, Vol 31. Lawrence Erlbaum Associates, Inc., New Jersey.

5. Higley J, et al. (1997). "A Nonhuman Primate Model of Excessive Alcohol Intake: Personality and Neurobiological Parallels of Type I- and Type II-Like Alcoholism." in M Galanter (ed.) Recent Developments in Alcoholism, Vol. 13: Alcoholism and Violence, pp. 191-219, Plenum Press, New York.

6. Higley J, et al. (December 29, 1997). "Low Central Nervous System Serotonergic Activity is Traitlike and Correlates with Impulsive Behavior." Annals of the New York Academy of Sciences, 836: 39-56.

7. Liu D, et al. (September 12, 1997). "Maternal Care, Hippocampal Glucocorticoid Receptors, and Hypothalamic-Pituitary-Adrenal Activity." Science, 277: 1659-1662.

8. McEwen B, et al. (1999). "Corticosteroids, the Aging Brain and Cognition." Trends in Endocrinology and Metabolism, 10(3): 92-96.

9. Nelson C. (in press). "How Important Are the First Three Years of Life?" Applied Developmental Science.

10. Nelson C. (in press). "Neural Plasticity and Human Development." Current Directions in Psychological Science.

11. Suomi S, (in press). "Attachment in Rhesus Monkeys." in J Cassidy and P Shaver (ed.) Handbook of Attachment: Theory, Research, and Clinical Applications. New York, Guilford Press, 1999.

Facts of Life is prepared with assistance from:

Academy of Behavioral Medicine Research
Academy of Psychosomatic Medicine
American College of Neuropsychopharmacology
American Psychiatric Association
American Psychological Association
American Psychological Association-Division 38
American Psychosomatic Society
American Society of Psychiatric Oncology
College on Problems of Drug Dependence
International Psycho-Oncology Society
International Society for Traumatic Stress Studies
Society of Behavioral Medicine
Society for Developmental and Behavioral Pediatrics
Society for Public Health Education
Society for Research on Nicotine and Tobacco

The Center for the Advancement of Health, , a nonprofit institute, promotes the science that explores health as a complex and dynamic system of relationships among biology, behavior, psychology, and social context and works to integrate this knowledge into public awareness, health care policy, and health care practice. The Center was founded by the John D. and Catherine T. MacArthur Foundation and the Nathan Cummings Foundation, which continue to provide core funding.

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