Brain maturation is involved in the developments just described because all reactions begin in the brain (Johnson, 2011). Experience promotes specific connections between neurons and emotions.

Links between expressed emotions and brain growth are complex and thus difficult to assess and describe (Lewis, 2010). Compared with the emotions of adults, discrete emotions during early infancy are murky and unpredictable. The growth of synapses and dendrites is a likely explanation for the gradual refinement and expression of discrete emotions, the result of past experiences and ongoing maturation.

Many specific aspects of brain development support social emotions (Lloyd-Fox et al., 2009). For instance, the social smile and laughter appear as the cortex matures (Konner, 2010). The same is probably true for fear, self-awareness, and anger. The brains of 8-month-olds respond to other people who look or act afraid, and that probably enhances the infant’s own expression of fear (Missana et al., 2014).

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Cultural differences become encoded in the infant brain, called “a cultural sponge” by one group of scientists (Ambady & Bharucha, 2009, p. 342). It is difficult to measure exactly how infant brains are molded by their context, in part because parents are reluctant to allow the brains of their normally developing infants to be scanned.

One study (Zhu et al., 2007) of adults—half born in the United States and half in China—found that a particular area of the brain (the medial prefrontal cortex) was activated for both groups when the adults judged whether certain adjectives applied to them. However, when asked whether those adjectives applied to their mothers, that brain area was activated only for the Chinese participants.

Researchers consider this United States-versus-China finding to be “neuroimaging evidence that culture shapes the functional anatomy of self-representation” (Zhu et al., 2007, p. 1310). They speculate that brain activation occurs because the Chinese participants learned, as babies, that they are closely aligned with their mothers, whereas the Americans learned to be independent.

Beyond culture, an infant’s brain activity interacts with caregiver responses, probably in the first months of life as well as through the years of childhood (Nelson et al., 2014). Highly reactive 15-month-olds (who were quick to cry) with responsive caregivers (not hostile or neglectful) became less fearful, less angry, and so on. By age 4, they were able to regulate their emotions, presumably because they had developed neurological links between brain excitement and emotional response. In contrast, highly reactive toddlers whose caregivers were less responsive were often overwhelmed by emotions later on (Ursache, et al., 2013). Differential susceptibility is apparent: Innate reactions and caregiver actions together sculpt the brain.

The tentative social smile of 2-month-old infants to any face soon becomes a quicker and fuller smile when they see a familiar, loving caregiver. This occurs because, with repeated experience, the neurons that fire together become more closely and quickly connected to each other (via dendrites).

Social preferences form in the early months, connected not only with a person’s face, but also with voice, touch, and smell. Early social awareness is one reason adopted children ideally join their new parents in the first days of life (a marked change from 100 years ago, when adoptions began after age 1).

Social awareness is also a reason to respect an infant’s reaction to a babysitter: If a 6-month-old screams and clings to the parent when the sitter arrives, another caregiver probably needs to be found. (Do not confuse this reaction with separation anxiety at 12 months—a normal, expected reaction to everyone.)

Every experience that a person has—especially in the early days and months—activates and prunes neurons, such that the firing patterns from one axon to a dendrite reflect past learning. As illustrated in Visualizing Development in Chapter 5, p. 144, this was first shown dramatically with baby mice: Some were licked and nuzzled by their mothers almost constantly, and some were neglected. A mother mouse’s licking of her newborn babies reduced methylation of a gene (Nr3c1), which allowed more serotonin (a neurotransmitter) to be released by the hypothalamus (a region of the brain discussed in Chapter 8).

Serotonin not only increased momentary pleasure (mice love being licked) but also started a chain of epigenetic responses to reduce cortisol from many parts of the brain and body, including the adrenal glands. The effects on both brain and behavior are lifelong for mice and probably for humans as well.

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For many humans, social anxiety is stronger than any other anxieties. Certainly to some extent this is genetic. But epigenetic research finds environmental influences important as well. This was clearly shown in a longitudinal study of 1,300 adolescents (twins and siblings): Their genetic tendency toward anxiety was evident at every age, but life events affected how strong that anxiety was felt (Zavos et al., 2012).

A smaller study likewise found that if the infant of an anxious biological mother is raised by a responsive adoptive mother who is not anxious, the inherited anxiety does not materialize (Natsuaki et al., 2013). Parents need to be comforting (as with the nuzzled baby mice) but not overprotective. Fearful mothers tend to raise fearful children, but fathers who offer their infants exciting but not dangerous challenges (such as a game of chase, crawling on the floor) reduce later anxiety (Majdandžić et al., 2013).

Emotions are connected to brain activity and hormones, but the connections are complicated; they are affected by genes, past experiences, hormones, and neurotransmitters not yet understood (Lewis, 2010). One link is clear: Excessive fear and stress harms the developing brain. The hypothalamus (discussed further in Chapter 8), in particular, grows more slowly if an infant is often frightened.

Brain scans of children who were maltreated in infancy show abnormal responses to stress, anger, and other emotions later on, including to photographs of frightened people. Some children seem resilient, but many areas of the brain (the hypothalamus, the amygdala, the HPA axis, the hippocampus, and the prefrontal cortex—all explained in later chapters) are affected by abuse, especially if the maltreatment begins in infancy (Bernard et al., 2014; Cicchetti, 2013a).

Since early caregiving affects the brain lifelong, caregivers need to be consistent and reassuring. This is not always easy. Remember that some infants cry inconsolably in the early weeks. As one researcher notes:

An infant’s crying has 2 possible consequences: it may elicit tenderness and desire to soothe, or helplessness and rage. It can be a signal that encourages attachment or one that jeopardizes the early relationship by triggering depression and, in some cases, even neglect or abuse.

[J. S. Kim, 2011, p. 229]

Sometimes mothers are blamed, or blame themselves, when an infant cries. This attitude is not helpful: A mother who feels guilty or incompetent may become angry at her baby, which leads to unresponsive parenting, an unhappy child, and then hostile interactions. Presumably the results would be the same if the father, or grandmother, were the primary caregiver, although extensive research on those infant-caregiver relationships has not yet been published. Years later, first-grade classmates and teachers are likely to consider children of unresponsive mothers disruptive and aggressive (Lorber & Egeland, 2011).

But the opposite may occur if early crying produces solicitous parenting. Then, when the baby outgrows the crying, the parent–child bond may be exceptionally strong.

Brain maturation may affect an infant’s ability to differentiate emotions—for instance, distinguishing between fear and joy. Some infants seem to cry at -everything. Early emotional confusion seems similar to synesthesia, a phenomenon in which one sense triggers another in the brain. For older children and adults, the most common form of synesthesia is when a number or letter evokes a vivid color. Among adults, synesthesia is unusual; often, it is partly genetic and indicates artistic creativity (K. J. Barnett et al., 2008).

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Synesthesia seems more common in infants because the boundaries between the sensory parts of the cortex are still forming (Walker et al., 2010). Textures seem associated with vision, sounds with smells, and the infant’s own body seems connected to the bodies of others.

These sensory connections allow cross-modal perception, when a sensation from one mode (such as a sound) is also experienced in another mode (such as sight). A caregiver’s smell and voice may evoke a vision of that person, for instance, or a sound may be connected with a shape. For example, when 4-month-old infants hear words that seem more staccato—such as “kiki”—they are more likely to look at an angular shape than a round one (Ozturk et al., 2013).

The tendency of one part of the brain to activate another may also occur for emotions. An infant’s cry can be triggered by pain, fear, tiredness, surprise, or excitement; laughter can turn to tears. Discrete emotions during early infancy are more difficult to recognize, differentiate, or predict than the same emotions in adulthood; infant emotions erupt, increase, or disappear for unknown reasons (Camras & Shutter, 2010). Brain immaturity is a likely explanation.

Temperament is defined as the “biologically based core of individual differences in style of approach and response to the environment that is stable across time and situations” (van den Akker et al., 2010, p. 485). “Biologically based” means that these traits originate with nature, not nurture.

Confirmation that temperament arises from the inborn brain comes from an analysis of the tone, duration, and intensity of infant cries after the first inoculation, before much experience outside the womb. Cry variations at this very early stage correlate with later temperament (Jong et al., 2010).

Temperament is not the same as personality, although temperamental inclinations may lead to personality differences. Generally, personality traits (e.g., honesty and humility) are learned, whereas temperamental traits (e.g., shyness and aggression) are genetic. Of course, for every trait, nature and nurture interact.

In laboratory studies of temperament, infants are exposed to events that are frightening or attractive. Four-month-olds might see spinning mobiles or hear unusual sounds. Older babies might confront a noisy, moving robot or a clown who quickly moves close to them. During such experiences, some children laugh, some cry, others are quiet, and still others exhibit some combination of these reactions that might be signs of one of four types of babies: easy (40%), difficult (10%), slow-to-warm-up (15%), and hard-to-classify (35%).

These categories originate from the New York Longitudinal Study (NYLS). Begun in the 1960s, the NYLS was the first large study to recognize that each newborn has distinct inborn traits (Thomas & Chess, 1977). According to the NYLS, by 3 months, infants manifest nine traits that cluster into the four categories just listed.

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Although the NYLS began a rich research endeavor, its nine dimensions have not held up in later studies. Generally, only three (not nine) dimensions of temperament are found (Hirvonen et al., 2013; van den Akker et al., 2010; Degnan et al., 2011), each of which affects later personality and school performance. The following three dimensions of temperament are apparent:

Each of these dimensions is associated with distinctive brain patterns as well as behavior, with the last of these (exuberance versus shyness) most strongly traced to genes (Wolfe et al., 2014).

Since these temperamental traits are apparent at birth, some developmentalists seek to discover which alleles affect specific emotions (M. H. Johnson & Fearon, 2011). For example, researchers have found that the 7-repeat allele of the DRD4 VNTR gene, when combined with the 5-HTTLPR genotype, results in 6-month-olds who are difficult—they cry often, are hard to distract, and are slow to laugh (Holmboe et al., 2011). Infants with a particular allele of the MOA gene are quick to anger. You need not remember the letters of these alleles, but remember that infant emotions vary, partly for genetic reasons.

One longitudinal study analyzed temperament in the same children at 4, 9, 14, 24, and 48 months and in middle childhood, adolescence, and adulthood. The scientists designed laboratory experiments with specifics appropriate for the age of the children; collected detailed reports from the mothers and later from the participants themselves; and gathered observational data and physiological evidence, including brain scans. Past data were reevaluated each time, and cross-sectional and international studies were considered (Fox et al., 2001, 2005, 2013; Hane et al., 2008; L. R. Williams et al., 2010; Jarcho et al., 2013).

Half of the participants did not change much over time, reacting the same way and having similar brain-wave patterns when confronted with frightening experiences. Curiously, the participants most likely to change from infancy to age 4 were the inhibited, fearful ones. Least likely to change were the exuberant babies (see Figure 7.1). Apparently, adults coax frightened infants to be brave but let exuberant children stay happy.

The researchers found unexpected gender differences. As teenagers, the formerly inhibited boys were more likely than the average adolescent to use drugs, but the inhibited girls were less likely to do so (L. R. Williams et al., 2010). The most likely explanation is cultural: Shy boys seek to become less anxious by drinking alcohol, smoking marijuana, or snorting cocaine, but shy girls may be more accepted as they are, or more likely to obey their parents.

Examination of these children in adulthood found intriguing differences between brain and behavior. Those who were inhibited in childhood still showed, in brain scans, evidence of their infant temperament. That confirms that biology affected their traits. However, learning (specifically cognitive control) was evident: Their behavior was similar to those with a more outgoing temperament, unless other factors caused serious emotional problems. Apparently, most of them had learned to override their initial temperamental reactions—not to erase their innate impulses, but to keep them from impairing adult action (Jarcho et al., 2013).

Continuity and change were also evident in another study that found that angry infants were likely to make their mothers hostile toward them, and, if that happened, such infants became antisocial children. However, if the mothers were loving and patient, despite the difficult temperament of the children, hostile traits were not evident later on (Pickles et al., 2013).

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Other studies confirm that difficult infants often become easier—if their parents provide excellent, patient care (Belsky & Pluess, 2009). How could this be? Some scientists suggest that, since fussy and scared children often come to the parents for comfort or reassurance, they are particularly likely to flourish with responsive parenting, but they wither if their parents are rejecting (Stupica et al., 2011). This is differential susceptibility again.

The two patterns evident in all these studies—continuity and improvement—have been replicated in many longitudinal studies of infant temperament, especially for antisocial personality traits. Difficult babies tend to become difficult children, but family and culture sometimes mitigate negative outcomes.

All the research finds that traces of childhood temperament endure, blossoming into adult personality, but all the research also confirms that innate tendencies are only part of the story. Context always shapes behavior. Ideally, parents find a goodness of fit—that is, meshing infant temperament and parent personality to allow smooth infant–caregiver interactions. With a good fit, parents of difficult babies build a close and affectionate relationship; parents of exuberant infants learn to protect them from harm; parents of slow-to-warm-up toddlers encourage them while giving them time to adjust.

However, not only does infant temperament vary, parental temperament varies as well. One study found that parents who claimed goodness of fit between themselves and their infant were likely to be better parents, even when such obvious correlates as education were taken into account (Seifer et al., 2014). Obviously, adults need to adjust to babies more than vice-versa, but even in infancy, a person’s temperament affects how people treat him or her.

Childhood temperament is linked to the parents’ genes and their personalities, which often are assessed using five dimensions, called the Big Five—openness, conscientiousness, extroversion, agreeableness, and neuroticism. Adults who are high in extroversion (surgency), high in agreeableness (effortful control), and low in neuroticism (negative mood) tend to be warmer and more competent parents (de Haan et al., 2009).

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