Psychology is the scientific study of human behavior and mind. That is not news. But what might not be so obvious is that most psychologists favor a certain portion of the lifespan, namely adulthood. The psychological literature is saturated with studies of adult participants, elegant theories about adult individuals and groups, and treatments for psychological disorders during adulthood. And most areas of psychology – such as social, cognitive, personality, and industrial/organizational – focus exclusively on describing, explaining, and predicting the behavior and mind of adults.Even many developmental psychologists – including those who populate their labs with infants and young children – focus on discovering connections between early experiences and adult behaviors. Some work to establish causal links between individual differences in early behaviors and certain adult characteristics. Others test interventions designed to have some future payoff during adulthood. Still others study adult reports of the past to catalog childhood experiences that can predict the potential for troubles or successes during the adult years.The same focus on adults pervades evolutionary psychology. Most theorizing and research in this area centers on understanding the evolutionary origins of traits seen in adulthood. The major difference between evolutionary psychologists and the rest of the field is that evolutionists are interested not only in proximal causes but also ultimate causes. So rather than studying merely what personality factors are linked to depression or what sorts of early relationships can result in later depressive symptoms, an evolutionist also might focus on understanding depression as an adaptation that brings benefits to adult functioning.To begin to understand why psychologists direct so much attention towards adults, we have to consider how most individuals view development. Most see it as a progression toward a target of adulthood. Underlying this belief is a tacit assumption that adulthood is the period of life where the real action of humanity takes place. For evolutionary psychologists, this focus on adulthood makes especially good sense because reproduction—the essential process in evolutionary explanations of behavior—goes on only among adults.One point, however, that is often missing from the thinking of evolutionary psychologists is that to become reproductive adults, we must first survive infancy and childhood. There is, therefore, every reason to believe that natural selection acts as much on the early portions of the lifespan as it does on adulthood. In fact, high levels of infant and childhood mortality for most of our evolutionary history suggest that selection may have had its greatest effects on the early stages of development.This view of adulthood as the goal of development also carries with it the assumption that development is a linear process of maturation, beginning with the simplicity and inefficiency of infancy and ending with the complexity and efficiency of adulthood. From this vantage point, children are merely unfinished and unsophisticated versions of adults, and childhood is simply a necessary period that individuals must get through on their way to adulthood.If you’ve ever spent an afternoon with a preschooler, children can seem like inefficient and inept adults. They can’t tie their own shoes, make their own sandwiches, or balance a checkbook. They are easily distracted, exceedingly irrational, and shamelessly sloppy. These everyday experiences can make childhood—the mayhem and mess of it all—feel like something that must be endured in order to get to adulthood.As the result of these sorts of conclusions about childhood, many professionals and laypeople feel the sooner that children get to adult levels of functioning, the better. This belief drives the push among parents to send their children to academic preschools or afterschool supplemental education programs, and the successes of marketers who sell toys, games, and other products designed to “boost brain growth.”From Caterpillar to ButterflyThere is, however, another way to look at development that does not conceptualize it as a linear progression from the immature to the mature. Consider the metamorphosis of caterpillars to butterflies. The caterpillar is not an immature form of butterfly. Rather the caterpillar is animal with a set of distinct traits and behaviors that are adapted to its present life as a caterpillar and not its future life as a butterfly. If you were to line these two animals up side by side and look closely, you’d see that the characteristics of the caterpillar are every bit just as refined and complex as those of the butterfly.Florida Atlantic University evolutionary developmental psychologist David Bjorkund argues that human development is better characterized as a metamorphosis, like caterpillars becoming butterflies, than as linear growth from the immature to the mature. Just as the caterpillar has its own complex organization adapted to the environment in which it lives presently, so does the child.In an evolutionary developmental framework, children are not amateur versions of adults focused on gaining adult perfection and complexity. Instead, children are a form of our species that are uniquely adapted to the physical, social, and cognitive demands of the environment in which they find themselves. Children and adults have different, though equally robust and sophisticated minds and behaviors, designed for different evolutionary functions.Childhood as an AdaptationWhen you look at childhood through this lens of evolutionary developmental psychology, it becomes clear that not all of the characteristic of infants and children are preparations for adulthood. Some function to adapt children to their immediate environment, not some future one. When such characteristics are no longer needed, they disappear. From this perspective, all of the mayhem and mess of childhood is not a necessary evil but rather play an adaptive role in children’s lives.We can easily look to research in developmental science to find illustrations of seemingly immature adaptations that fade when they no longer serve a function. The most obvious occur early in life. For instance, the placenta provides food and oxygen during the prenatal period. At birth, an extreme transformation occurs and newborns eat and breathe in an entirely different manner. Likewise, human infants have a set of automatic behaviors, such as the sucking and rooting reflex, that promote survival in the early weeks but would be terribly embarrassing if they persisted into adulthood.Our early motor and sensory limitations also are adaptive. Compared to the rest of the animal kingdom, human newborns are helpless. Halfway through the first year of life most animals can fend for themselves, while we humans cannot even crawl across the room. Developmental biopsychologists Gerald Turkewitz and Patricia Kenny suggest that our early inability to independently locomote is adaptive because it prevents infants from wandering far from their mothers and therefore enhances their chances of survival during this period. However, if crawling were our only form of locomotion during the later years most of us wouldn’t survive long enough to reproduce (or at least would have trouble attracting a mate).Likewise, newborn’s poor eyesight is adaptive because it works to reduce the amount of visual information that they have to deal with and consequently means that other developing sensory systems, such as hearing, do not need to compete for real estate in the brain. Supporting this claim are experiments from animal labs, such as Robert Lickliter’s, that demonstrates that earlier-than-expected visual stimulation can interfere with the development of the auditory system.There also are examples throughout childhood. For example, preschoolers often misattribute others’ actions as things they themselves did. This bias facilitates children’s learning because attributing their own as well as others’ actions to a common source (themselves) produces easily retrievable memories. Supporting this idea are findings that children who collaborate with adults on a spatial memory task (e.g., placing furniture in a dollhouse) later make more attribution errors (e.g., saying: “I put the table in the kitchen” when the adult had) but also remember more items in their correct locations compared to children who carry out the task independently.Another example is children’s common thinking that they are more skilled than they really are. For instance, 3- but not 5-year-olds who overestimate their imitative skills are more verbally advanced than better estimators. This is because children who overestimate their own abilities attempt more challenging tasks and persist longer than more realistic children, and this increased persistence boosts learning. If you’ve ever seen a preschooler on the dance floor at a wedding reception you know what I’m talking about. However, overestimating dancing skill becomes increasingly less charming with age and likely would be costly if retained at 3-year-old levels in adulthood.The major significance of this analysis for evolutionary psychology is the recognition that natural selection operates at all stages of the lifespan not just on adult traits. As such, evolutionary psychological explanations must include an appreciation for the possible adaptive value of any given behavior at a specific time in development. Childhood behaviors cannot be characterized as immature versions of adult behaviors; and adult-like behaviors may not always adaptive during childhood.The Role of DevelopmentIn addition to the idea that adaptations occur throughout the lifespan, a second significant insight that a developmental approach brings to the evolutionary table is the recognition that all human characteristics not only have an evolutionary history but also a developmental history. Adaptations do not simply materialize at various points in the lifespan but rather must develop from something else. More specifically, at the core of the evolutionary developmental psychology framework is the concept of probabilistic epigenesis: the idea that development emerges from continuous interactions between all levels of biological and environmental factors, from genes to culture, and at all times scales, from milliseconds to eons.This characterization of development as involving all levels of causation shifts a few things for mainstream evolutionary psychology. Most notably, it broadens the conception of the environment. Evolutionary psychologists most often discuss environmental influences in terms of inherited psychological mechanisms that trigger certain patterns of behavior. Generally these mechanisms are characterized as innate machinery programmed by evolution.Surprisingly even some development psychologists make claims of innateness. For example, Harvard University’s Elizabeth Spelke proposes that “humans are endowed with a small number of separable [core knowledge] systems that stand at the foundation of all our beliefs and values. New, flexible skills, concepts, and systems of knowledge build on these core foundations.”Claims of endowments, core knowledge, or other traits described as innate, however, are untenable from an evolutionary developmental perspective because such claims devalue developmental processes. Core knowledge theorists, for instance, do not characterize their proposed systems as emerging from epigenetic processes but rather as adaptations that have been programmed by evolution to appear at a certain point in the lifespan (i.e., infancy). From an evolutionary developmental perspective, proposing that machinery is the product of evolution does not remove the need to explain developmental processes, because all characteristics are shaped through development and not programmed before development.Core knowledge theorists and others who make claims of innateness assert that such claims are justified when there seem to be no apparent prior experiences that could account for observed behaviors. To illustrate, NYU’s Gary Marcus argues that the ‘‘reason for believing that something is innate is that there may be no other satisfying account for how a given piece of knowledge could arise.” This is probably the most unsatisfying thing that you could say to an evolutionary developmental psychologist.The problem with the Marcus style of reasoning is that researchers who make claims of innateness don’t look very hard, or at all, for prior experiences that could account for their observations. This isn’t to say that their research isn’t rigorous. It often is. However, it is common for these investigators to study humans (or other animals) at one point in time (e.g., 4 months of age) and make claims about other points in time (e.g., the prenatal period and the first 4 months of life) without directly observing but merely thinking about these other points in time. This practice sounds to an evolutionary developmental psychologist like: “Four-month-olds can do x. I cannot think of any prior experiences that could account for x, nor have I carried out experiments at younger ages to try to determine whether certain early experiences are necessary for this behavior to occur. Therefore x must be innate.” It seems hard to argue with evolutionary developmental psychologists that it is not irresponsible science to make claims about time points never studied.A burgeoning body of research provides experimental illustrations of why claims of innateness are unjustified and demonstrates the value of an evolutionary developmental perspective. The most iconic example comes from studies of imprinting. In the 1930s, ethologist Konrad Lorenz showed that ducklings prefer the maternal call of their species immediately after hatching. Lorenz described this preference as an innate behavior programmed by evolution to occur without any sort of experience.Forty years later, developmental psychobiologist Gilbert Gottlieb demonstrated that Lorenz’ no-experience-necessary interpretation was wrong. Gottlieb developed a surgical procedure that prevented embryonic ducks from vocalizing while in the egg, and incubated the eggs in isolation. Therefore the ducks were deprived of all prenatal auditory experience (i.e., their own, maternal, and sibling vocalizations). When later tested, these devocalized and isolated ducklings did not show the species typical preference. Thus an adaptive behavior once thought to be innate is dependent on a specific experience, namely hearing embryonic vocalizations, at a certain point in the lifespan.Evolutionary developmental theory and research also works to dismantle one of the most common misunderstandings in evolutionary psychology – namely, that if a behavior is evolved then it must be the result of genetic determination. This idea that evolved behaviors are in the genes lingers in the air and turns off mainstream behavioral scientists to an evolutionary approach to behavior even though evolutionary psychology left behind the idea of genetic determinism a long time ago.An evolutionary developmental perspective swiftly knocks down misperceptions about the role of genes because it provides an appreciation for how genes and the environment interact across multiple levels to produce development, and a model for explaining how evolved machinery is translated into behavior.The probabilistic epigenetic model puts genes in their place. It does not grant genes a privileged role in development but rather conceptualizes genes as one part the developmental system that requires input from and interaction with other parts of the system to influence mind and behavior. In fact, this approach makes clear that no level of the developmental system is in control. Thus despite media reports of scientists who have discovered evolved genes that determine intelligence, sexual orientation, and athletic ability, an evolutionary developmental model describes exactly why genes in and of themselves never determine anything.Genes are in no position to determine behavior buried deep inside our cells and multiple levels away from behavior. Of course some of these genes have a lot to do with behavior and to a large extent in some cases. But genes are always expressed in an environment that plays a role in regulating their expression. This is how temperature can alter butterfly camouflage patterns, diet can change caterpillar shape, and social group composition can switch fish from female to male. In each of these examples, genetically identical animals develop differently based on differences in their environments.The takeaway message for evolutionary psychology here is that if someone is making the case that a certain adaptive behavior is innate or genetically determined, it is only because no one yet has figured out the developmental processes that underlie its emergence. This acknowledgement comes with a very broad conception of experience—a conception that is necessary given a growing body of research demonstrating that sometimes the experiences that shape development are the result of nonobvious processes (à la Gottlieb’s ducks) that do not conform to our intuitions or rational expectations.Most importantly, findings in the tradition of Gottlieb require evolutionary psychologists to rethink what is inherited from generation to generation, and what is involved in evolution. Such findings also help us understand how most humans (and other animals) develop in a species typically pattern if genes are not driving development. The answer is, in Gottlieb’s words, that evolution involves ‘‘selection for the entire developmental manifold.’’ Thus all animals inherit not only a species typical genome but also a species typical environment.Probabilistic Epigenesis as a Metatheory for Evolutionary Psychology (or How Probabilistic Epigenesis Will Save Psychology from Neuroscience) I’ve heard from colleagues in the biology department at my institution that the psychology department will become obsolete as soon as neuroscientists figure out the biochemistry of brain and behavior. I argue that the probabilistic epigenetic model illustrates exactly why such reductionism will never work if we truly want to understand behavior. Thinking that behavior can be reduced to brain activity or gene functioning devalues the role of the environment (e.g., parents, peers, culture, history) and developmental processes. There is no way to arrive at a deep understanding of behavior and development without taking into account all levels of the developmental system, which include things external to the organism and not in the realm of things that most neuroscientists are trained to study. Examination of behavior and development does not require an evolutionary developmental perspective, but a deep understanding of human behavior will emerge only from multidisciplinary research and from thinking about behavior, brain, and body as emerging from a probabilistic epigenetic process.Implications for Child DevelopmentAn evolutionary developmental framework also has practical implications for discussions of child rearing and education. The most salient is that if childish traits can serve important functions during childhood, then efforts to speed up development to reach maturity quicker might not always be a good idea. In fact, there is growing empirical evidence from multiple fields that faster is not always better. Here is one example: efforts to accelerate early intellectual development by putting preschoolers in academically oriented programs can increase later levels of anxiety about school and problem behaviors, and reduce later motivation for learning, expectations for success on academic tasks, and pride in accomplishments.An evolutionary developmental perspective helps us understand why findings like these emerge: When parents or educators push too hard and too soon to try to accelerate development, they are attempting to modify a delicate developmental system designed by evolution and firmly in place for millions of years. Just like the developmental system of a caterpillar would not appreciate attempts to manipulate it into producing wings, the developmental system of a child does not benefit from attempts to make to put it into a business suit or high heels. Rather than thinking about how we can speed up childhood we should ask: why are we bent on doing so?Parents and educators also should think through the implications of the degree of species atypical experiences in modern children’s everyday lives. Things like formal schooling, manufactured toys, manicured playgrounds, organized sports, and high technology are evolutionary novelties that historically have not been part of the human developmental manifold. An evolutionary developmental framework suggests that parents and educators should consider the implications in these shifts in early experiences in the decisions that they make for children. There is growing work that suggests that some parts of the species atypical lifestyle of modern children are having unexpected side effects, and that children do best in environments that gel with how their brains and bodies have been designed to grow. This perspective suggests that the solution to some common childhood problems might be to design life to work with how evolution has prepared their developing brains and bodies to grow not against it.ReferencesBjorklund, D. F., & Pellegrini, A. D. (2002). The origins of human nature: Evolutionary developmental sychology. Washington DC: American Psychological Association, 2002.Gottlieb, G. (1997). Synthesizing nature-nurture: Prenatal roots of instinctive behavior. Mahwah, NJ: Erlbaum.Lickliter, R. (1990). Premature visual stimulation accelerates intersensory functioning in bobwhite quail neonates. Developmental Psychobiology, 23, 15–27.Marcus, G. F. (2001). Plasticity and nativism: Towards a resolution of an apparent paradox. In S. Wermter, J. Austin, & D. Willshaw (Eds.), Emergent neural computational architectures based on neuroscience (pp. 368–382). Heidelberg: Springer.Kinzler, K. D. & Spelke, E. S (2007). Core systems in human cognition. In C. von Hofsten & K. Rosander (Eds.), Progress in brain research, Vol. 164 (pp. 257-264).Turkewitz, G., & Kenny, P. A. (1982). Limitations on input as a basis for neural organization and perceptual development: A preliminary theoretical statement. Developmental Psychobiology, 15, 357–368.
