The effects of time are the heartbeat of human development, the essence of what we study. As already explained, an earlier idea—that development advances gradually until about age 18, steadies, and then declines—has been refuted by life-span research. We now know, though, that aspects of change appear and disappear, with increases, decreases, and zigzags (see Figure 1.4).

The pace of change differs as well. Sometimes discontinuity is evident: Change can occur rapidly and dramatically, as when caterpillars become butterflies. Sometimes continuity is found: Growth can be gradual, as when redwoods add rings over hundreds of years. And sometimes no change seems to occur: Stability itself is remarkable.

Humans experience simple growth, radical transformation, improvement, and decline as well as stability, change, and continuity—day to day, year to year, and generation to generation.

No matter what the pattern, age is always significant, and that is the focus of our study. Is it normal for a boy to throw himself down, kicking and screaming, when he is frustrated? Yes, if he is 2 years old; no, if he is 12. Is it normal for a girl to be interested in boys? Yes, at age 16; no, at age 6. More broadly, children think, play, and learn differently depending partly on their age.

Some changes are sudden and profound because of a critical period, a time when something must occur for normal development or the only time when an abnormality can occur. For instance, the critical period for humans to grow arms and legs, hands and feet, fingers and toes, is between 28 and 54 days after conception. After that, it is too late: Unlike some insects, humans never grow replacement limbs or digits.

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We know the critical period for limb formation because of a tragic occurrence. Between 1957 and 1961, thousands of newly pregnant women in 30 nations took thalidomide, an anti-nausea drug. This change in nurture (via the mother’s bloodstream) disrupted nature (the embryo’s genetic program).

If an expectant woman ingested thalidomide during the 26 days of the critical period for limb formation, her newborn’s arms or legs were malformed or absent (Moore & Persaud, 2007). Whether all four limbs, or just arms, or only hands were missing depended on exactly when the drug was taken. If thalidomide was ingested only after day 54, the newborn had normal body structures.

Life has few critical periods. Often, however, a particular development occurs more easily—not exclusively—at a certain time. Such a time is called a sensitive period.

An example is found in language acquisition. If children do not communicate in their first language between ages 1 and 3, they might do so later (hence, these years are not critical), but their grammar is impaired (hence, these years are sensitive).

Similarly, childhood is a sensitive period for learning to pronounce a second or third language with a native accent. Many adults master a new language, but strangers might ask, “Where are you from?” Native speakers can detect an accent that reveals that the second language was learned after childhood.

Often in development, individual exceptions to general patterns occur. Sweeping generalizations, like those in the language example, do not hold true in every case. Accent-free speech usually must be learned before puberty, but exceptional nature and nurture (a person naturally adept at hearing and then immersed in a new language) can result in flawless second-language pronunciation (Birdsong, 2006; Muñoz & Singleton, 2011).

Because of sensitive periods, however, such exceptions are rare. According to a study of native Dutch speakers who become fluent in English, only 1 in 20 such adults masters native pronunciation (Schmid et al., 2014).

A leading developmentalist, Urie Bronfenbrenner (1917–2005), led the way to developmentalists considering all the contextual influences on the developing child as time goes on. Just as a naturalist studying an organism examines the ecology (the relationship between the organism and its environment) of a tiger, tree, or trout, Bronfenbrenner recommended that developmentalists take an ecological-systems approach (Bronfenbrenner & Morris, 2006) to understand humans.

The ecological-systems approach recognizes three nested levels of factors that surround individuals and affect them (see Figure 1.5). Most obvious are microsystems—each person’s immediate surroundings, such as family and peer group. The impact of each microsystem depends partly on timing. For instance, peers are far more important in adolescence than in infancy.

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Also important are exosystems (local institutions such as school and church) and macrosystems (the larger social setting, including cultural values, economic policies, and political processes).

Two more systems are related to these three. One is the mesosystem, consisting of the connections among the other systems. The other is the chronosystem (literally, “time system”); it is the historical context that allows developmentalists to focus specifically on the influence of changing historical circumstances.

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Bronfenbrenner believed that people need to be studied in their natural contexts repeatedly over time (Bronfenbrenner, 1977; 1986). He looked at children playing, or mothers putting babies to sleep, or nurses in hospitals—never asking people to come to a scientist’s laboratory for a contrived experiment.

Toward the end of his life, Bronfrenbrenner renamed his approach bioecological theory to highlight the role of biology, recognizing that systems within the body (e.g., the sexual-reproductive system, the cardiovascular system) affect the external systems as time goes on (Bronfenbrenner & Morris, 2006).

In studying development, it matters, of course, how old a child is, but it also matters what the political and medical conditions are when a child reaches a particular age. Both generational and cohort effects need to be considered. This needs some explanation.

All persons born within a few years of one another are called a cohort. Cohorts travel through life together, affected by the interaction between their chronological age and the values, events, technologies, and culture of the era. From the moment of birth, when parents decide the name of their baby, historical context is crucial (see Table 1.1).

Consider attitudes and data about marijuana. In the United States in the 1930s, marijuana was declared illegal, but enforcement was erratic. Then in the 1980s, marijuana was labeled a “gateway drug,” likely to lead to drug abuse and addiction (Kandel, 2002). People were arrested and jailed for possession of even a few ounces of the drug.

From 1990 on, attitudes gradually changed. By 2014, general marijuana use became legal in two states, and medical use was permitted in several others. According to a Gallup poll, more than half (54 percent) of Americans approve making marijuana use legal, as long as it is used by adults and not in public (Motel, 2014).

These historical shifts have had a notable effect on high school students (Johnston et al., 2014 and previous years). In 1978, only 12 percent thought experimental use of marijuana was harmful, and more than half had tried the drug. In 1991, when the gateway drug message became widespread, 80 percent thought there was “great risk” in regular use and only 21 percent ever smoked the drug. Since then, marijuana use has increased as attitudes have become more accepting: Approximately one-third of high school seniors used it in 2013.

Thus cohort, more than generation, influences attitudes and behavior toward marijuana. The same is true for adults’ attitudes and behavior: It matters more when an adult was born than how old that adult is. It is wiser, for example, to seek generalities about people born in 1950 rather than about people who are in their 60s, or 70s, or whatever (see Figure 1.6).

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The term plasticity denotes two complementary aspects of development: Human traits can be molded (as plastic can be), and yet people maintain a certain durability (as plastic does). This provides both hope and realism—hope because change is possible and realism because development builds on what has come before.

Plasticity is basic to our contemporary understanding of human development because it simultaneously incorporates two facts: People can change over time, and new behavior depends partly on what has already happened.

This is evident in one of the newer approaches to development, the dynamic-systems approach. The idea behind this approach is that human development is an ongoing, ever-changing interaction between the individual and all the systems described above. This approach began with nature:

[S]easons change in ordered measure, clouds assemble and disperse, trees grow to a certain shape and size, snowflakes form and melt, minute plants and animals pass through elaborate life cycles that are invisible to us, and social groups come together and disband.

[Thelen & Smith, 2006, p. 271]

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Note the word dynamic: Physical contexts, emotional influences, the passage of time, each person, and every aspect of the ecosystem are always interacting, always in flux, always in motion. For instance, a new approach to developing the motor skills of children with autism spectrum disorder stresses the dynamic systems that undergird movement—the changing aspects of the physical and social contexts (Lee & Porretta, 2013). Using those sys-tems may help the autistic child—not to make the autism disappear (plasticity implies that past conditions are always influential) but to make the child function more smoothly.

Plasticity is especially useful in understanding the potential of a particular person: Everyone is constrained by past circumstances but not confined by them. David is an example.

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