This book uses case studies, natural experiments, and longitudinal data to identify and dig into a multitude of variations in human development. Its method is mainly prospective, gathering data in real time, as humans age, to be analyzed later, only using retrospective data to confirm results. It was co-written by four scientists, who primarily used the wealth of information from The Dunedin Multidisciplinary Health and Development Study, supplemented by the British Environmental-Risk Study and The NICHD Study of Early Child Care and Youth Development. The scientists begin by recognizing that human development is a complex phenomenon. “There are many factors and forces to consider that interact in complex ways over time and space…. We focus on cognitive, social, emotional, behavioral, biological, and physical development, including health, from birth to midlife, while considering the influence of nature and nurture…. It is probabilistic, not deterministic science.”
One thing the authors stress is that people have agency in how they develop, but also that relationships interact back and forth. “The child’s temperament evoking responses from others… contribute[s] to maintaining and even amplifying the child’s early disposition…. The child can be said to be a “producer of his own development.”” When further discussing the development of temperament, the authors parse out the difference between reactive and proactive person-environment processes. In a reactive process, “individuals with different psychological and behavioral inclinations experience, interpret, and react to the same situation differently.” A proactive process “involves individuals selecting or creating their own experiences and thereby maintaining, perhaps even amplifying, their early dispositions.” The authors refer to this as “niche picking.” This proactive process “challenges notions of children as wet clay, shaped exclusively by experiences that others generate.” Genes also play a formative role in temperamental development, even as they interact with and help shape environmental choices. “Development is a dynamic, ongoing process whereby things occurring within the child and/or within the environment in which the child develops after childhood serves as a pathway that accounts for how childhood functioning comes to predict adult functioning.” Human development is continuous. “Because the effects of early life and most experiences growing up are not deterministic but instead are likely to exert their effects in a probabilistic manner, in part because of the role of various mediators in maintaining or deflecting established development trajectories, we should not expect a single intervention effort at any point in time to get the entire developmental job done when it comes to promoting future well-being.”
This book is filled with numerous examples of how genes and environment together shape human development. One particular finding that I found intriguing was the difference between how mothers and fathers raise their children, at least early on in life. “How mothers in our study who had three-year-olds behaved towards their children systematically related to their experiences as children in their families while growing up, but in the case of study members who were fathers, there was, somewhat surprisingly, no such evidence of intergenerational transmission.”
Another issue the authors raise is the metrics by which traditional human developmental researchers measure success. “For most psychologists, sociologists, economists, educators, policymakers, and parents, life is about health, wealth, and happiness. This leads many to conceptualize development as healthy versus unhealthy or “optimal” versus “compromised.”” This does not necessarily make sense from an evolutionary perspective. “What succeeds in passing on genes to the next generation is best, and, critically, that depends on the context or circumstances in which a living organism exists, including those in which it develops…. What will prove most successful in passing on genes to descendants when growing up under one set of conditions, such as in a well-functioning harmonious family, should not be presumed to do so under different conditions, such as when there is much family conflict and limited attention paid to children. This evolutionary-biological view of life implies that the forces of Darwinian natural selection have adapted our species to respond to its experiences growing up in a manner that would increase the chances of the developing child surviving, maturing, and reproducing…. From this perspective, what has long been regarded as evidence of “compromised” development may be actually anything but…. What many today regard as compromised, troubled, or problematic development may well have evolved as a strategic response to adversity…. Our species, like many others, evolved the capacity to vary its development depending on early-life experiences and exposures…. Because evolution typically takes a great deal of time, we retain the responsiveness built into our species over the eons even though it is no longer as reproductively beneficial as it once was…. The fast [female] developer should… produce more children, even while “investing” less in them in terms of energy, time, and economic resources.” The authors suggest this is why, empirically, they found that girls who grew up in dysfunctional homes experienced puberty, on average, one to two years sooner than more advantaged girls. “The more parents regarded and treated their four-and-a-half-year-old harshly, the earlier their daughters had their first period…. By age fifteen, girls who matured earlier had engaged in more sexual risk taking than other girls…. They were not more likely, however, to engage in other risk-taking behavior, such as drinking and drug taking.”
The authors next tackle the issue of genotype relating to phenotype. “There is a long chain of biological (and sometimes psychological and behavioral) events linking genotype with phenotype.” Environmental exposures and experiences are often necessary before a phenotype manifests. The authors refer back to niche-picking in suggesting agency is critical to human development. “Central to the niche-picking idea is an appreciation that individuals are often producers of their own development.” They use the particular example of intellectual development. “In the niche-picking case, genetics and intelligence may go together because those with certain genes find reading and learning more interesting, fun, and thus attractive than do others of a different genetic makeup. As a result, they end up more intelligent not simply because it is in their genes but rather because there is something about their genes that inclines them to do things—such as go to the library and be attentive in school—that foster intellectual development.” Again, the authors also stress the back and forth interplay with others in their environment. “How others react to a person’s behavior and how this feeds back to shape the person’s development and functioning” reinforce each other. “How genotypes can require certain contextual conditions to develop into phenotypes make[s] clear… that genes operate in an environmental context.”
The authors stress that any one individual gene plays a minimal role in almost all phenotype expression. “Because whole-genome research is identifying so many genes related to virtually all phenotypes investigated, there is ever more reason to believe that many genes exert only a very small effect on the phenotype in question, if it is actually causally influencing the phenotype [at all] (and not just correlated with it)…. Many, even most, genes are systematically related to many different phenotypes. Pleiotropy is the scientific term used to refer to this biological reality…. Given pleiotropy, even if specific genes are associated with a particular phenotype, it does not logically follow that all these genes are exclusively associated with that one phenotype.”
The book backtracks somewhat to discuss single gene-environment interaction (GXE). Particularly, they look at the effect of serotonin on depression as expressed through the gene 5-HTTLPR. “Not everyone reacts the same way to the same adversity…. Serious negative life events are a “risk factor” for depression…. Such risks are typically realized when other risk conditions co-occur…. Because of differences in their genetic makeup some people succumb to adversity whereas others do not.” This is called the diathesis-stress (or dual risk) model of psychopathology. “Instead of thinking about genes for this or that illness or psychopathology, we thought about genes for susceptibility to potential environmental influences…. Some individuals are more susceptible than others to both positive and negative environmental exposures and developmental experiences. Thus, imagine a science that sought to identify not just genes that supposedly “code” for some illness or malady but also genes that shape the way we respond to environmental insults—such as toxins or life stressors—and/or supports, such as high-quality schooling or positive life events.”
Finally, the authors sought to establish links between childhood socio-economic class origins and adult health. “Results revealed that three of four measurements of physical health—body-mass index, waist-hip ratio, and cardiorespiratory fitness, but not systolic blood pressure—showed a graded, dose-response relation with a child’s social-class origin…. The same proved true of dental health, indexed by amount of plaque on teeth, bleeding of gums, periodontal disease, and decayed tooth surfaces…. The fact that all these detected effects of childhood economic disadvantage on physical and dental health remained even when study members’ own occupational status at age twenty-six was controlled meant that the findings could not be the result of their social-class “destination.”” However, the authors caution that there could be confounding variables involved and, again, not to assume causation. “Even though adversity in childhood predicted poorer objective health in midlife, the possibility remains that other factors—third variables—might have influenced both the predictor and outcomes.”
The scientists also focused their analysis on the pace of aging versus chronological age. “Cells “turn over,” with existing ones eventually dying and being replaced by new ones through a process of cell division and thereby replication. Every time this occurs, that cap on the end of the chromosome—the telomere—shortens…. A cell line exists for only so long, and after enough cell divisions have occurred, the telomere no longer caps the end of the chromosome and cell death occurs…. [Therefore] the shorter the telomere, the older the cell and thus the older the individual…. It also turns out that they are shorter in adults suffering from age-related diseases than in their healthy counterparts of the same chronological age…. Children whose childhoods are characterized by adversity have shorter telomeres than those of other children of the same chronological age…. [Even] newborns whose mothers have experienced more stress during pregnancy have shorter telomeres than other newborns…. [Furthermore,] our study was the first to shed light on how adversity affects the development of telomeres…. Exposure to adversity in the form of psychological and physical violence predicts the actual shortening or erosion of telomeres over time…. Cells were aging, biologically speaking, faster than their agemates’ cells.” The authors conclude by cautioning, however, that, in general, although “individual differences in children within the first decade of life can forecast how they will function decades later, the power to predict later development from childhood measurements is often, though not always, limited…. The predictive power of any one force or factor will be limited most of the time.”
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