Our genes, when expressed in specific environments, influence our developmental differences. We are not “blank slates” (Kenrick et al., 2009). We are more like coloring books, with certain lines predisposed and experience filling in the full picture. We are formed by nature and nurture. But what are the most influential components of our nurture? How do our early experiences, our family and peer relationships, and all our other experiences guide our development and contribute to our diversity?

The formative nurture that conspires with nature begins at conception, with the prenatal environment in the womb, where embryos receive differing nutrition and varying levels of exposure to toxic agents. Nurture then continues outside the womb, where our early experiences foster brain development.

13-1 How do early experiences modify the brain?

Our genes dictate our overall brain architecture, but experience fills in the details. Developing neural connections prepare our brain for thought, language, and other later experiences. So how do early experiences leave their “fingerprints” in the brain? Mark Rosenzweig, David Krech, and their colleagues (1962) opened a window on that process when they raised some young rats in solitary confinement and others in a communal playground. When they later analyzed the rats’ brains, those who died with the most toys had won. The rats living in the enriched environment, which simulated a natural environment, usually developed a heavier and thicker brain cortex (FIGURE 13.1).

Rosenzweig was so surprised by this discovery that he repeated the experiment several times before publishing his findings (Renner & Rosenzweig, 1987; Rosenzweig, 1984). So great are the effects that, shown brief video clips of rats, you could tell from their activity and curiosity whether their environment had been impoverished or enriched (Renner & Renner, 1993). After 60 days in the enriched environment, the rats’ brain weights increased 7 to 10 percent and the number of synapses mushroomed by about 20 percent (Kolb & Whishaw, 1998).

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Such results have motivated improvements in environments for laboratory, farm, and zoo animals—and for children in institutions. Stimulation by touch or massage also benefits infant rats and premature babies (Field et al., 2007). “Handled” infants of both species develop faster neurologically and gain weight more rapidly. Preemies who have had skin-to-skin contact with their mothers sleep better, experience less stress, and show better cognitive development 10 years later (Feldman et al., 2014).

Nature and nurture interact to sculpt our synapses. Brain maturation provides us with an abundance of neural connections. Experiences trigger sights and smells, touches and tugs, and activate and strengthen connections. Unused neural pathways weaken. Like forest pathways, popular tracks are broadened and less-traveled ones gradually disappear. By puberty, this pruning process results in a massive loss of unemployed connections.

Here at the juncture of nurture and nature is the biological reality of early childhood learning. During early childhood—while excess connections are still on call—youngsters can most easily master such skills as the grammar and accent of another language. Lacking any exposure to language before adolescence, a person will never master any language. Likewise, lacking visual experience during the early years, a person whose vision is later restored by cataract removal will never achieve normal perceptions (Gregory, 1978; Wiesel, 1982). Without that early visual stimulation, the brain cells normally assigned to vision will die or be diverted to other uses. The maturing brain’s rule: Use it or lose it.

Although normal stimulation during the early years is critical, the brain’s development does not end with childhood. Thanks to the brain’s amazing plasticity, our neural tissue is ever changing and reorganizing in response to new experiences. New neurons are also born. If a monkey pushes a lever with the same finger many times a day, brain tissue controlling that finger will change to reflect the experience (FIGURE 13.2). Human brains work similarly. Whether learning to keyboard, skateboard, or navigate London’s streets, we perform with increasing skill as our brain incorporates the learning (Ambrose, 2010; Maguire et al., 2000).

13-2 In what ways do parents and peers shape children’s development?

In procreation, a woman and a man shuffle their gene decks and deal a life-forming hand to their child-to-be, who is then subjected to countless influences beyond their control. Parents, nonetheless, feel enormous satisfaction in their children’s successes or guilt and shame over their failures. They beam over the child who wins trophies and titles. They wonder where they went wrong with the child who is repeatedly in trouble. Freudian psychiatry and psychology encouraged such ideas by blaming problems from asthma to schizophrenia on “bad mothering.” Society has reinforced such parent blaming: Believing that parents shape their offspring as a potter molds clay, people readily praise parents for their children’s virtues and blame them for their children’s vices. Popular culture endlessly proclaims the psychological harm toxic parents inflict on their fragile children. No wonder having and raising children can seem so risky.

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But do parents really produce future adults with an inner wounded child by being (take your pick from the toxic-parenting lists) overbearing—or uninvolved? Pushy—or indecisive? Overprotective—or distant? Are children really so easily wounded? If so, should we then blame our parents for our failings, and ourselves for our children’s failings? Or does talk of wounding fragile children through normal parental mistakes trivialize the brutality of real abuse?

Parents do matter. But parenting wields its largest effects at the extremes: the abused children who become abusive, the neglected who become neglectful, the loved but firmly handled who become self-confident and socially competent. The power of the family environment also appears in the remarkable academic and vocational successes of children of people who fled war-torn Vietnam and Cambodia—successes attributed to close-knit, supportive, even demanding families (Caplan et al., 1992). Asian Americans and European Americans differ in their expectations for mothering. An Asian-American mother may push (or “nag,” as one study called it) her children to do well, but that pressure likely won’t strain their relationship (Fu & Markus, 2014). Having a supportive “Tiger Mother”—one who pushes them to do well and works alongside them (versus forcing them to work alone)—motivates Asian-American children to work harder. European Americans might view that kind of parenting as “smothering-mothering,” believing that it undermines children’s motivation (Deal, 2011).

Yet in personality measures, shared environmental influences from the womb onward typically account for less than 10 percent of children’s differences. In the words of behavior geneticists Robert Plomin and Denise Daniels (1987; Plomin, 2011), “Two children in the same family are [apart from their shared genes] as different from one another as are pairs of children selected randomly from the population.” To developmental psychologist Sandra Scarr (1993), this implied that “parents should be given less credit for kids who turn out great and blamed less for kids who don’t.” Knowing children are not easily sculpted by parental nurture, perhaps parents can relax a bit more and love their children for who they are.

As children mature, what other experiences do the work of nurturing? At all ages, but especially during childhood and adolescence, we seek to fit in with our groups (Harris, 1998, 2000):

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Howard Gardner (1998) has concluded that parents and peers are complementary:

Parents are more important when it comes to education, discipline, responsibility, orderliness, charitableness, and ways of interacting with authority figures. Peers are more important for learning cooperation, for finding the road to popularity, for inventing styles of interaction among people of the same age. Youngsters may find their peers more interesting, but they will look to their parents when contemplating their own futures. Moreover, parents [often] choose the neighborhoods and schools that supply the peers.

This power to select a child’s neighborhood and schools gives parents an ability to influence the culture that shapes the child’s peer group. And because neighborhood influences matter, parents may want to become involved in intervention programs that aim at a whole school or neighborhood. If the vapors of a toxic climate are seeping into a child’s life, that climate—not just the child—needs reforming. Even so, peers are but one medium of cultural influence. As an African proverb declares, “It takes a village to raise a child.”