Bridging the gap: Monkey studies shed light on nature, nurture, and how experiences get under the skin

“Is it nature or nurture?”
“How does that work? How can social experiences actually change someone’s brain?”
“So early experiences matter, but how much?  Is it reversible? How long does it last? Is there a way to change the course?”

All of these are popular questions that I hear from students, community members, clinicians, and other scientists when I talk about my research with monkeys.  The nature vs. nurture question is one of high public interest.  It is one that is at the center of our understanding of who we are and how we come to be that way.  And it is a very old question.  Yet it is also one that continues to resonate and become even more intriguing as new discoveries rapidly change what we know about biology and genes, and illuminate with increasing specificity the ways in which nature and nurture together play dynamic roles in shaping the development of each individual.

For example, through research with humans, monkeys, rats, mice and other animals, we know that genes are not only involved in differences between individuals’ behavior, health, and biology, but also that an individual’s social environment and childhood experiences can actually change how genes behave and, in turn, have biological consequences.  In other words, those previous gray areas surrounding exactly how nature and nurture work together are now being filled in with a more specific understanding.

Why does this matter? There are many important reasons. Among them, it is this specific information that allows us to develop better prevention, intervention, and treatment strategies for those negative health outcomes that follow adverse experiences. One example of this can be found in our rapidly advancing knowledge of how brain neurochemistry, which plays a major role in mental health disorders, is affected both by genetic differences between individuals and also by early life experiences. This knowledge provides not only the basis for developing treatments that target the specific neurochemicals involved in a disorder, but also provides important clues for early identification and intervention for those at risk. At the same time, understanding that experiences have long-lasting consequences on biological pathways involved in lifetime health underscores the importance of public policies that work to promote better early environments.

I am one of the many scientists who are devoted to work aimed at better understanding how many different kinds of early experiences can influence a wide range of health outcomes during an individual’s lifespan. My own part of this work primarily includes non-invasive studies with monkeys and focuses on developmental questions about behavior, aspects of brain chemistry and development, and genetics. For example, I use neuroimaging (MRI) to look at how brain development can be affected by early life experiences and we have monkeys play videogames, solve puzzles, and respond to mild challenges so that we can better understand their learning, memory, cognition, and temperament.

Part of my work involves studying how middle-aged monkeys (15+ years old) who were raised in infancy with their mothers differ from monkeys nursery-reared in infancy with their peers. The two groups have the same experiences following the early life period, and during infancy and throughout their lives, both groups are housed in enriched environments with excellent diets, toys, and medical care. Although my current work is focused on a small number of nursery-reared animals, it does not involve creating new animals or a nursery. It depends on healthy animals who have been part of our work for many years and, as with all of our studies, we treat these animals humanely, with careful attention to providing them with healthy diets, environmental enrichment (e.g., a variety of toys, puzzles, fresh fruit and vegetables, and foraging opportunities), and excellent clinical care by veterinarians.  We do this because we care about the animals’ well-being and also because our studies depend upon healthy animals.

There are less than a handful of studies concerned with how monkeys’ early rearing influences their behavior and other aspects of health in middle- and older-age. As a result, although we have a strong platform of knowledge about the effects of early life experience in younger animals, we know very little about whether these effects persist into older age, about what systems are affected, and the degree to which individuals vary.

This study, like those of others who study the effect of different early life experiences on a range of health outcomes, is aimed at uncovering the biological basis of a key finding relevant to human health. We know from human studies that a wide range of early experiences, including not only childhood neglect and abuse, but also poverty and other types of adversity, are associated with negative health outcomes later in life. In humans, however, it is impossible to truly disentangle the effects of early adverse life experiences from differences in diet, environment, access to medical care, and other factors that vary across the lifespan. Animal studies allow us to control many of the factors that vary widely in humans and have consequences on health. For example, animals with different early experiences have the same environment and experiences afterwards, including healthy diets and excellent medical care. As a result, when we find significant differences in behavior, brain chemistry, brain structure, and immunology between animals with different early experiences we know that these differences are not due to disparity later in life.

Early experiences do not tell the whole story, however, as we know from the common observation that two individuals who experience the same early environment or challenging experiences, may wind up with very different health pathways.  Part of the obvious reason for this is genetic variation. Understanding how differences in genes contribute, however, and which biological pathways are affected or how permanent those effects may be, are now the real questions that remain to be fully answered. Animal studies provide one of the critical ways to view the interplay and roles of genes, environments, and experiences. This is because, unlike in human studies, animal studies can make use of strong experimental control and mechanistic approaches in order to compare the biological and behavioral responses of individuals who have similar genes and different environments, or individuals with different genes in the same environment.

Another part of my research involves studying how genes affect an individual’s response to the environment and how that occurs at a biological level.  The kinds of questions that we address include:  When two individuals experience the same stress, or the same environment, why are some relatively unaffected (resilient) and others more vulnerable?  What genes play a role in this difference?  What biological systems?  My work, along with that of my colleagues, has demonstrated that genetic factors play a crucial role in how individuals differ in terms of their resilience or vulnerability to early adversity. It is through studies with monkeys that my colleagues and I were able to first identify how interplay between specific genetic variation and early experiences together influence brain chemistry that influences a wide range of behaviors and aspects of health.  This finding in monkeys preceded and spurred subsequent similar studies in humans that continue to show that for most complex traits, genes do not always predict an individual’s destiny; environments have tremendous influence; and understanding individual differences requires consideration of both nature and nurture. As a result, we not only now know more about the genetic and biological underpinnings of individual differences in vulnerability to early life stress, but we also can move forward in identifying the specific ways that this occurs.

In all of these studies, our goal is to produce new understanding about how early experiences affect individuals throughout their lives.  Furthermore, like other biomedical animal research, our goal is to produce information that is relevant to human health and to address questions that are raised by challenges to human health but that cannot be addressed in studies of humans. In other words, aspects of similarity between human and nonhuman primate genetics and biological response to experiences are central to the rationale and success of the research. Studies with monkeys are a small, but important, part of the research aimed at uncovering how early experiences affect health.  As with most areas of research, new understanding and progress depend upon bridges between studies that use different populations (both human and other animal) and that draw from many different areas of expertise. Work in this area has progressed through the efforts of psychologists, neuroscientists, behaviorists, geneticists, molecular biologists, immunologists, physicians, population epidemiologists, sociologists, and others. In other words, the question is of interest from many perspectives and is addressed with interdisciplinary approaches that make it possible to build connections between findings so that the results of basic research can provide useful evidence to inform better health practices, clinical care, and public policy.

Why are these studies and findings important?  In short, because they provide us with a way to better understand the specific biological mechanisms by which early life events affect health.  As a result of decades of research in both humans and other animals, we know some of the specific biological, neural, immunological, and genetic pathways that are affected. These studies have informed progress in our understanding of the importance of early childhood experiences for lifelong health, the biological basis of mental health disorders, and the potential to change health trajectories through early identification of risk and appreciation of individual differences. Through the combined force of basic and clinical studies, we will continue to progress in understanding how genes, experiences, and biology interact. In turn, this understanding will continue to help in pinpointing mechanistic targets and shedding new light on those avenues for prevention, intervention, and treatment that improve human and animal health.

Allyson J. Bennett, Ph.D.