1.2 Themes of Development

Continuous versus Discontinuous Development

The first theme that helps organize the way we think about the nature of development over time is the shape of developmental change (Figure 1.5). Theories about this developmental change are often called stage theories: a stage theory attempts to explain why we may observe brief periods of rapid development followed by longer stretches of stability. Stage theories give us an idea about the nature of change as well. Relatively abrupt change is likely to be more apparent and cause us to wonder about its source or mechanism.

When we observe a gradual day-by-day or week-over-week progression of change, we see continuous development. Many topics in the field of lifespan development show a continuous pattern of development. For example, vocabulary growth across the toddler and early childhood years shows a dramatic but smooth progression of growth in the number of words a child produces and comprehends (e.g., Pan et al., 2005).

However, early psychological theorists, such as Jean Piaget and Erik Erikson (whose work you’ll learn about throughout this course), noted that certain cognitive and personality characteristics seem to develop in intermittent fits and spurts—or stages. For a period of time, the developing individual has one set of capabilities, such as a toddler who communicates in two-word utterances, and then, rather abruptly, a reorganization takes place, and the individual is transformed. Seemingly overnight, that toddler begins speaking in complete sentences. This is discontinuous development—a change in developmental kind, form, or degree that does not directly follow from what came before. Some developmental progressions may appear to be discontinuous, such as the sudden changes of puberty. However, when researchers consider the biological changes that prepare a body for the growth spurts of puberty, such as hormone changes, we begin to see that continuous development patterns are also present.

Figure 1.5 In contrasting theories, development can be viewed as continuous or discontinuous. Continuous development is conceptualized as a gradual, ongoing process, whereas discontinuous development is marked by distinct, stepped stages. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Whether a characteristic appears to follow a continuous or a discontinuous pattern, an underlying assumption in developmental psychology is the idea that development is progressive and proceeds from basic forms of a characteristic in early life to more advanced forms later. That is, development moves forward. From this perspective, moving backward to an early stage or lower capability, a process termed regression, is a sign that something may be wrong. In order to promote early child development, major organizations like the United Nations International Children’s Emergency Fund (UNICEF) and the World Health Organization (WHO) have made it a priority to educate caregivers about the importance of psychosocial development (Lucas et al. 2018). For example, poor nutrition can lead to delayed physical growth and poorer long-term health. However, interventions that provide nutrition supplementation in early childhood can help children catch up on physical growth and improve health outcomes (Lucas et al., 2018).

Sources of Developmental Change

We now turn to the theme for which developmental psychology is best known: the relationship between nature and nurture. You may also hear this referred to as the nature versus nurture debate (Angoff, 1988). Suppose we observe a developmental change (such as a growth spurt in height) or a state of stability in an individual (such as having a consistent outgoing personality), and their genetic blueprint appears to be the reason for it. In that case, we argue that nature or biological forces are directing the changes or working to keep the characteristics consistent. On the other hand, if the impetus for change seems to come from outside the individual, whether it be a physical or a social aspect of the environment (such as engaging in a new hobby or growing up in a certain family dynamic), then we argue that nurture (the environment) is the reason.

For most physical and psychological characteristics, the source for developmental change or stability is rarely nature or nurture alone. Instead, developmental psychologists focus on the complex and fascinating ways that nature and nurture combine and work together to direct change and stability across the lifespan (Figure 1.6).

Figure 1.6 A father and son, side by side, mirror the growth of the trees they walk through. Development and change are influenced by both biological and environmental factors. (credit: “Man in Black Jacket Beside Boy in Pink Jacket Holding Plush Toy during Daytime” by Quaries Official/Quaries.com, CC BY 2.0)

Heritability

Until a few decades ago, the field of psychology often explored the question of what caused developmental change by asking whether it was nature or nurture. Due to advances in the discipline’s methodologies, findings, and theories, we now think in terms of nature and nurture, recognizing that most development has both genetic (nature) and environmental (nurture) influences. In fact, nearly every psychological characteristic is composed of a combination of biological genetic components and environmental influences. The amount of influence from genetics and environment varies depending on the characteristic. Today, psychologists work to discover exactly how much each source contributes. The extent to which the genetic component explains difference is called a heritability estimate. A good example of how nature and nurture both contribute is the timing of the onset of puberty. It is determined via the combined influence of nature and nurture, given that both biological determinants (nature) like genes, and environmental elements (nurture) like nutrition, can trigger its emergence.

Closely related to our thinking about the relative contributions of nature and nurture is the concept of reaction range (Figure 1.7). This is the idea that our genes likely set upper and lower levels for particular traits, behaviors, and abilities, as well as how sensitive these are to environmental forces. One example to consider is general intelligence (Sternberg, 2012). While scientists have yet to fully identify the specific genes responsible for intelligence, we know it’s not solely determined by genetics: environmental factors are of equal importance. However, it’s difficult to directly measure the impact of these environmental influences because researchers cannot ethically manipulate a child’s upbringing to observe the effects on their intelligence. Although the application of reaction range to intelligence remains largely theoretical, it serves as a useful way to conceptualize the complex interplay of genetics and environment in human development. It reminds us that both nature and nurture contribute to who we become.

Figure 1.7 Although how the contributing factors combine in complex ways not fully understood, researchers agree that both genes and environment play roles in the development of intelligence. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Windows of Opportunity for Development

In addition to studying what changes are expected and how they occur, developmental psychologists ask when we should expect certain skills to develop, and whether there are windows of opportunity for these that affect developmental outcomes. Is it possible to speed up development if we introduce an experience at just the right time? Is it possible to hinder or even prevent the development of a particular ability or characteristic altogether, such as speech? What are the impacts of highly enriching environments? What are the impacts of being deprived of certain experiences, such as human contact?

Scientists have learned that across nearly all psychological characteristics, humans are highly adaptable. They observe normative developmental outcomes, meaning those that are typical or expected, across a wide range of environmental conditions. Certainly, there are optimal environments for these developmental outcomes, but good outcomes occur even in suboptimal circumstances. This is central to resilience, an individual’s capacity for and “process of adapting well in the face of adversity, trauma, tragedy, threats or even significant sources of stress” (American Psychological Association [APA], 2014). It takes extreme deprivation to severely restrict a developing human’s potential, as well as such deprivation occurring at specific developmental times. In other words, resilience is common and lifespan development principles can be applied to increase the likelihood of resilience.

A critical period is the developmental age range in which certain experiences are required for a psychological or physical ability to develop (Colombo et al., 2019). For example, it appears that exposure to human speech is necessary in the early years of life for typical language development. Consider the 1970s case of Genie, a child who was severely neglected and isolated to the point that she was rarely spoken to. Upon her rescue at age thirteen years, Genie faced the monumental task of learning a language (a primary task of infants and toddlers) with an adolescent (post-pubescent) brain (Jones, 1995). Genie became the subject of intense study and remediation efforts by doctors, speech pathologists, and psychologists (Fromkin et al., 1974). Despite her extreme early deprivation, she made many improvements in language comprehension and speech production. However, her development of language differed markedly from what is normative. For example, language production and comprehension happen in the left hemisphere of the brain for the vast majority of humans, but Genie showed processing of language in her right cerebral hemisphere.

A sensitive period describes an age range during which the development of a characteristic is particularly amenable to inputs and influences from the environment (Colombo et al., 2019). For a critical period, certain experiences are necessary for normal development, whereas having certain experiences during a sensitive period makes an advantageous situation for ideal development (Pascalis, 2020). For example, a sports coach would want to know about normative periods for growth spurts in males and females, especially the typically wide variation around puberty. Not only could this awareness of timing help with training young athletes, but it could inform injury prevention practices. A faith-based youth leader working closely with a group of teenagers would want to understand that the adolescent period is a time of identity exploration and desire to fit in with peers, and plan activities accordingly. A social worker employed by an assisted living facility knows that residents might be concerned with life review and may develop a scrapbooking activity to support the process of reflection. Physical development, identity exploration, and reflections on past life can occur across the entire lifespan. What’s notable across these three examples is that each takes on more importance at a different stage in life, when the opportunities for impact on development are greatest. That is the concept of the sensitive period.

Variability and Diversity of Development

In this course, you’ll learn about numerous theories, topics, and research findings based on the developmental experiences of many thousands of people. It is sometimes easy to confuse or conflate the broad results of studies with individual life histories. Scientific ideas about development represent statistical averages for developmental outcomes across many individuals. But every individual is unique and has their own life history—one that may show varying degrees of similarity to and deviation from the findings presented as normative in this discipline.

For example, when you learn that two-word utterances typically appear around eighteen months, a reader might recall, “Well, my child did this when they were twelve months old.” The fact that this individual’s development doesn’t perfectly fit the finding doesn’t invalidate the overall research. What’s more important is the pattern or sequence from one-word utterances to two, and that this development occurs within a typical timeframe, the midpoint of thousands of examples with a range of variations on either side. So, while lifespan psychology attempts to uncover what is typical for all humans, psychologists recognize and allow for the inherent variability of a single individual’s development.

Variability in development can also occur on a cultural level. For example, research compared Dutch and Israeli parents and found cultural differences in how they promote their babies’ motor development (Oudgenoeg-Paz et al., 2020). Israeli parents encouraged more tummy time, believing it helps babies develop faster. Dutch parents were not as likely to encourage their infants spend time laying on their tummies. Interestingly, the Israeli babies who got more tummy time and stimulation from their parents showed better crawling skills than the Dutch babies. This study highlights that different cultures have different approaches to parenting, and these choices can influence how babies develop.